Agrometeorology

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Measurement of water potential using thermocouple hygrometers.
(1982) Savage, Michael John.; Crass, Albert.; De Jager, James M.
Theory predicts that the time dependent voltage curve of a thermocouple psychrometer where there is no change in output voltage with time during the evaporation cycle defines the wet bulb temperature T[w] corresponding to the water potential. In practice, a change in voltage with time does occur and it is convenient to define the voltage corresponding to the water potential as the maximum point of- inflection voltage. A predictive model based on calibration data at a few tempertures is used to obtain the psychrometer calibration slope at any temperature. Use of this model indicates that psychrometers differ from each other and therefore must be individually calibrated if accuracy better than ±5 % in the measurement of water potential is required. Dewpoint hygrometers are shown to be less temperature sensitive than psychrometers and have the added advantage of a voltage sensitivity nearly twice that of psychrometers, typically -7,0 x 10¯³ μV/kPa compared to -3,7 x 10¯³ μV/kPa at 25 °C. The accurate temperature correction of hygrometer calibration curve slopes is a necessity if field measurements are undertaken using either psychrometric or dewpoint techniques. In the case of thermocouple psychrometers, two temperature correction models are proposed, each based on measurement of the thermojunction radius and calculation of the theoretical voltage sensitivity to changes in water potential. The first model relies on calibration at a single temperature and the second at two temperatures. Both these models were more accurate than the temperature correction models currently in use for four leaf psychrometers calibrated over a range of temperatures (15 to 38°C). The model based on calibration at two temperatures is superior to that based on only one calibration. The model proposed for dewpoint hygrometers is similar to that for psychrometers. It is based on the theoretical voltage sensitivity to changes in water potential. Comparison with empirical data from three dewpoint hygrometers calibrated at four different temperatures indicates that these instruments need only be calibrated at, say 25°C, if the calibration slopes are corrected for temperature. A model is presented for the calculation of the error in measured thermocouple hygrometric water potential for individual hygrometers used in the dewpoint or psychrometric mode. The model is based on calculation of the relative standard error in measured thermocouple psychrometric water potential as a function of temperature. Sources of error in the psychrometric mode were in calibration of the instrument as a function of water potential and temperature and in voltage (due to electronic noise and zero offsets) and temperature measurement in the field. Total error increased as temperature decreased, approaching a value usually determined by the shape of the thermocouple junction, electronic noise (at low voltages less than 1 μV) and errors in temperature measurement. At higher temperatures, error was a combination of calibration errors, electronic noise and zero offset voltage. Field calibration data for a number of leaf psychrometers contained total errors that ranged between 6 (at a °C) and 2 %(at 45 °C) for the better psychrometers and between 11 (at 0° C) and 5 % (at 45 C) for the worst assuming that the zero offset was 0,5 μV. Zero offset values were less than 0,7 μV at all times. The dewpoint errors arose from calibration of the dewpoint hygrometer as a function of water potential, extrapolation of the calibration slope to other temperatures, setting the dewpoint coefficient and errors in voltage and temperature measurement. The total error also increased as temperature decreased, because of the differences in temperature sensitivity between dewpoint and psychrometric calibration constants. Consequently, the major source of error in the dewpoint mode arose from the difficulty in determining the dewpoint coefficient. This error, which is temperature dependent, contains three subcomponent errors; the temperature dependence, random variation associated with determining the temperature dependence and error in setting the correct value. Calibration and extrapolation errors were smaller than those of the psychrometric technique. Typically, the error in a dewpoint measurement varied between about 6 and 2 % for the best hygrometer and between 10 and 3 % for the worst for temperatures between 0 and 45 °C respectively. At low temperatures, the dewpoint technique often has no advantage over the psychrometric technique, in terms of measurement errors. In a comparative laboratory study, leaf water potentials were measured using the Scholander pressure chamber, psychrometers and hydraulic press. Newly mature trifoliates cut from field grown soybean (Glycine max (L) Merr. cv. Dribi) were turgidified and, after different degrees of dehydration, leaf water potential measured. One leaflet from the trifoliate was used for the thermocouple psychrometer and another for the press while the central leaflet with its petiolule was retained for use in the pressure chamber. Significant correlations between measurements using these instruments were obtained but the slopes for hydraulic press vs psychrometer measurement curve and hydraulic press vs pressure chamber were 0,742 and 0,775 respectively. Plots of pressure-volume curves indicate that the point of incipient plasmolysis was the same (statistically) for the thermocouple psychrometer and the pressure chamber, but much larger for the hydnaulic press. The above-mentioned differences between the three instruments emphasize the need for calib rating the endpoint defined us i ng the press against one or more of the standard techniques, and, limi ting the use of the press to one person. Cuticular resistance to water vapour diffusion between the substomatal cavity and the sensing psychrometer junction is a problem unique to leaf psychrometry and dewpoint hygrometry; this resistance is not encountered in soil or solution psychrometry. The cuticular resistance may introduce error in the leaf water potential measurement. The effect of abraiding the cuticle of Citrus jambhiri to reduce its resistance, on the measured leaf water potential was investigated. Psychrometric measurements of leaf water potential were compared with simultaneous measurements on nearby leaves using the Scholander pressure chamber, in a field situation. Leaf surface damage, due to abrasion, was investigated using scanning electron microscopy. Thermocouple psychrometers are the only instruments which can measure the in situ water potential of intact leaves, and which may be suitable for continuous, non-destructive monitoring of water potential. Unfortunately, their usefulness is limited by a number of difficulties, among them fluctuating temperatures and temperature gradients within the psychrometer, sealing of the psychrometer chamber to the leaf, shading of the leaf by the psychrometer and resistance to water vapour diffusion by the cuticle when the stomates are closed. Using Citrus jambhiri, several psychrometer designs and operational modifications were tested. In situ psychrometric measurements compared favourably with simultaneous Scholander pressure chamber measurements on neighbouring leaves, corrected for the osmotic potential and the apparent effect of "xylem tension relaxation" following petiole excision. It is generally assumed that enclosure of a leaf by an in situ thermocouple psychrometer substantially modifies the leaf environment, possibly altering leaf water potential, the quantity to be measured. Furthermore, the time response of leaf psychrometers to sudden leaf water potential changes has not been tested under field conditions. In a laboratory investigation, we found good linear correlation between in situ leaf psychrometer (sealed over abraided area) and Scholander pressure chamber measurements (using adjacent leaves) of leaf water potential, 2 to 200 minutes after excision of citrus leaves. A field investigation involved psychrometric measurement prior to petiole excision, and 1 min after excision, simultaneous pressure chamber measurements on adjacent citrus leaves immediately prior to the time of excision and then on the psychrometer leaf about 2 min after excision. Statistical comparisons indicated that within the first two minutes after excision, psychrometer measurements compared favourably with pressure chamber measurements. There was no evidence for a psychrometer leaf water potential time lag. For the high evaporative demand conditions, water potential decreased after excision by as much as 700 kPa in the first minute. Psychrometer field measurements indicated that within the first 5 min of leaf petiole excision, the decrease in leaf water potential with time was linear but that within the first 15 s, there was a temporary increase of the order of a few tens of kilopascal. The thermocouple psychrometer can be used to measure dynamic changes in leaf water potential non-destructively, with an accuracy that compares favourably with that of the pressure chamber. Using in situ thermocouple leaf hygrometers (dewpoint and psychrometric techniques employed) attached to Citrus jambhiri leaves, an increase in measured water potential immediately following petiole excision was observed. The increase ranged between 20 to 80 kPa and occurred 30 s after petiole excision and 100 s after midrib excisions. No relationship between the actual leaf water potential and the increase in water potential due to excision, was found.
Comparative evaporation measurements above commercial forestry and sugarcane canopies in the KwaZulu-Natal Midlands.
(1999) Burger, Caren.; Savage, Michael John.
An understanding of the water use of different crops commonly grown in an area is essential for the implementation of integrated catchment management in South Africa. With increasing pressure on water resources, mainly due to the recent changes in the Water Act, it has become important to determine the actual water demands of agricultural and other crops. Policy makers require knowledge of whether forestry canopies use more water than grassland and other agricultural crops. The Bowen ratio and Penman-Monteith methods were used in a comparative study of the evaporation from Saccharum, Acacia and Eucalyptus. All of the research was conducted at marginal sites located in the KwaZulu-Natal Midlands of South Africa over a period of two years. The Bowen ratio energy balance (BREB) technique combines the Bowen ratio (J3) (the ratio between the sensible, H and latent heat flux density, XE), with the net irradiance (Rn) and soil heat flux densities (G) to calculate evaporation. A comparative study of the sitespecific energy balance components (Rn, G, H and AE), general climatic conditions (rainfall, solar irradiance and air temperature) and other site-specific parameters (leaf area index and average canopy height) was conducted on Saccharum and young commercial forests consisting of Acacia and Eucalyptus. The energy balance highlighted important differences in the energy balance components between the different canopies. The differences between the reflection coefficients at the three sites contributed mainly to the differences in the evaporation rates. The low reflection coefficients of the forest canopies (Acacia and Eucalyptus) (0.1 and 0.08 respectively) were smaller than of the sugarcane canopy (0.2). This resulted in more energy available (« 6 %) for partitioning between the sensible and latent heat flux densities and higher evaporation rates for the forestry canopies. Where low leaf area indices existed (Acacia and Eucalyptus sites) (LAI < 2), the soil heat flux density contributed up to 40 % of the net irradiance (G = 0.4 Rn). The evaporation rates for Saccharum, Acacia and Eucalyptus averaged 2 mm day"1 in winter and 5 mm day"1 in summer. The slightly higher summer evaporation rate for Eucalyptus (5.6 mm day"1), compared to Acacia (4.9 mm day"1), resulted from the lower reflection coefficients and canopy resistance (rc) for Eucalyptus (ocs = 0.08, rc = 35 s m"1) compared to Acacia (ocs = 0.1, rc = 45 s m"1). Automatic weather station data (solar irradiance, air temperature, water vapour pressure and windspeed) were applied to site-specific Penman-Monteith equations to predict evaporation for all three sites. Statistically significant relationships (slope, m « 1, r2 > 0.8) were found between the measured (Bowen ratio) and simulated (site-specific Penman-Monteith) evaporation estimates. The current study has demonstrated the effectiveness of applying the Penman-Monteith equation to forest and sugarcane canopies to predict evaporation, provided accurate net irradiance, soil heat flux densities and canopy resistances are used.
Varying levels of incident solar irradiance and microclimatic variations on banana (Musa spp) growth and productivity.
(2001) Kizito, Fred.; Savage, Michael John.
A field experiment was conducted at Inselele, KwaZulu-Natal South Coast, South Africa, in 1999/2000, to assess the influence of shading as related to varying levels of incident solar irradiance as well as microclimatic variations on banana (Musa spp) growth, phenology trends, morphology and productivity. The trial was established in August 1999 on a ratoon plantation. The experimental site, 0.655 ha in extent, comprised of three replications with four treatments having varying levels of incident solar irradiance levels of 100 %, 70 %, 40 % and 20 % under a planting density of 1666 plants ha. The irradiance levels were derived from black shade cloth, erected 1 m above the banana canopy level. Banana plantations have vegetation that does not completely cover the underlying ground surface. For such a canopy, there are basically two distinct and interacting surface components, the overstorey/canopy and the understorey/soil. Independent investigations and measurements of the solar energy fluxes for each of these two components forms a vital step to comprehend the factors that control the overhead energy fluxes in the plantation. In this study, evaluation of flux components in the understorey of the canopy using plastic microlysimeters was conducted. Considering normal variations in field measurements, the agreement and consistency among the different measurements with previous findings was adequate. Microlysimeter measurements of daytime soil evaporation were generally less than 1 mm, with an average of 0.45 mm. The study did not include flux measurement at the two level approach (Bowen ratio and Eddy correlation methods) which could have been compared with the single level approach and microlysimeter results. Two methods of deriving/measurement of energy fluxes were used and the differences between them are discussed. A reassessment of the microlysimeter technique is suggested. For the understorey, the sum of sensible and latent heat fluxes derived from spreadsheet computation was equal to the available energy. Mean soil temperatures at a depth of 20 to 60 mm ranged between 13 and 16 degrees C. This study illustrated that energy flux measurement and interpretation in within the experimental area. Good agreement was found in the patterns of wind speed profile measurements, with the 1.5 m profile depicting a mean difference of 52 % compared to the 3 m profile between day of year (DOY) 230 and 248 inclusive. This suggested that wind speed attenuation was strongly correlated to increment in height within the plantation due to canopy roughness. Differential canopy temperatures (measured with infrared thermometry) were more sensitive to the vapour pressure deficit than to wind speed. The most dense shade affected banana productivity indicated by a bunch weight of 22.69 and 33.65 kg under the 20 % and 100% irradiance treatments respectively. The bunch mass reduction was 32 %. Flowering dates were delayed by 8 days, 13 days and 21 days with incident irradiances of 70 %, 40 % and 20 % of the unshaded control (lOO % irradiance), respectively. The phenological responses in this study appear to be a result ofa contribution or interaction of both seasonal responses and shading treatments and this is further evidenced by the high levels of correlation (98.4%) reported between these two variables. Shading resulted in diminished leaf emergence rates (LER), pseudostem circumference and pseudostem height. However, just before flowering, no significant differences were observed in the pseudostem circumferences. There was a progressive increment in pseudostem height for all the treatments, with the 20 % irradiance treatment depicting the least heights registered compared to the rest of the treatments. Evident seasonal differences were registered in the LER and emergence to harvest (EH) interval. Comparison of LER and mean air temperature trends revealed a similar curve pattern and depicted a highly positive correlation of90.4%. The highest LER of3.8 was recorded in the month of January at a peak mean monthly air temperature of24.3°C while the lowest LER of 1.2 was registered in July which had the lowest mean air temperature of 14.7 cc. The EH intervals measured between September and December flowering varied from 125 days to 112 days respectively before harvest yet the April to May flowering had an EH duration of186 to 195 days respectively. The strong seasonal influence on phenological responses is further confirmed by these EH trends. Reductions in LAI observed with time were principally due to leaf senescence.
Modeling variety differences in canopy growth and development of sugarcane (Saccharum officinarum L.) using Canegro.
(2003) Zhou, Marvellous Mabeza.; Savage, Michael John.
Crop models have great potential as research tools, for crop system management and policy analysis. One of the most promising future uses of crop models is in crop improvement. The limitation in the use of models for crop improvement has been the inability of crop models to predict variety differences. Currently, the CANEGRO model, a sugarcane crop model developed the South African Sugar Association Experiment Station (Inman-Bamber, 1991a) can only model the performance of the NC0376 variety. Experiments were undertaken in the South East Lowveld of Zimbabwe, which is a hot and dry environment where sugarcane is grown under irrigation, to examine the canopy growth and development of four commercial varieties, ZN6, ZN7, N14 and NC0376. The study aimed at determining variety differences in canopy (tillers and leaves) development, develop parameters that can be used to model variety differences and test the improved CANEGRO canopy model for its ability to predict variety differences in canopy growth and development. For the late season, the numbers of leaves and tillers produced by each variety were counted every fortnight throughout the crop cycle. The total leaf area of the varieties and the individual leaf area on a stalk were determined using a Delta-T leaf area meter every fortnight. The date of emergence of successive leaves on a stalk was recorded daily. The leaf angles of each variety were measured every fortnight. The amount of photosynthetically active radiation (PAR) intercepted by the varieties was measured using a SunScan Ceptometer. Tillering and tiller senescence rates, phyllochron intervals, extinction coefficients and base temperatures were determined for the growth and development processes of varieties ZN6, ZN7, N14 and NC0376. Tiller and leaf population development was varietal. Tillering and leaf emergence were highly correlated to thermal time while tiller and leaf senescence were less correlated to thermal time. The poor correlation of the senescence phases to thermal time could mean that tiller and leaf senescence was driven by other factors other than thermal time. PAR interception could be one of these factors. The data showed that PAR interception could be a trigger of tiller senescence. The study showed that the tiller and leaf population development could be approximated by two linear equations. Tillering will be the first linear phase and tiller senescence the second linear phase. The first linear phase is driven by thermal time. While the second linear phase is triggered by PAR interception, the major driving factors need to be determined. This study proposed the use of two linear equations to model tiller and leaf population development as opposed to the polynomial equations used in the current CANEGRO model. Polynomial equations assume the factors driving tillering and tiller senescence are the same. The green leaf numbers per plant showed that all varieties experienced a decline in green leaf numbers with crop age. Varieties NC0376 and ZN7 had the greatest decline in green leaf numbers per plant while varieties N14 and ZN6 had the least decline. Variety ZN7 had the highest number of green leaves per plant while NC0376 had the least. The tiller growth and development was divided into three phases: the exponential phase during the initiation of stalks, the first linear phase during a period of rapid stalk elongation and the second linear phase during sucrose accumulation and maturation. The first two phases of development were driven by thermal time while the sucrose accumulation was not. There were variety differences in tiller growth and development. There were variety differences in base temperature for the development of various components of the canopy. Internode formation occurred at lower air temperatures than stalk elongation and tillering while canopy heights were correlated with higher air temperatures. This implies that internode formation could occur under conditions unsuitable for stalk elongation and may explain the short internodes frequently observed in stalks exposed to winter during rapid stalk elongation. The basic requirements for physiological parameters are that they should be stable across different environments, have significant differences between varieties and have physiological meaning. The parameters studied were thermal time requirement for shoot emergence, leaf appearance, to reach peak tiller population and to start of stalk elongation; surface area of the youngest biggest leaf, leaf number of the youngest biggest leaf, PAR transmission at the start of tiller senescence, extinction coefficients, and peak and mature tiller population. The difference between varieties in thermal time to shoot emergence was least using a base temperature of 16 QC compared to using 10 QC and therefore 16 QC could be a more appropriate base temperature for shoot emergence. The accumulated soil temperatures were less variable than accumulated air temperature and could therefore be a more reliable driver of shoot emergence. However, the limitations in the use of soil temperature are that it is not a readily available measurement and that it is not easy to measure. The gradual increase in phyllochron intervals appeared to be a better method of predicting leaf appearance compared to using a broken stick model. The phyllochron gradient was proposed, as it is likely to be a more robust way of modelling leaf appearance. The varieties had different phyllochron gradients. Variety ZN7 had highest rate of leaf appearance and produced the highest number of leaves per stalk while NC0376 had the lowest rate and produced the least number of leaves. There were statistically significant differences between varieties (P = 0,05) in PAR transmission at the start of tiller senescence and a base temperature of 16 QC was best at determining accumulated thermal time to the start of tiller senescence. Varieties with higher peak tiller population had higher final tiller population, lower thermal time per tiller and a higher ratio of final to peak tiller population. There were differences between varieties in the youngest leaf number attaining maximum leaf area and the leaf area of the youngest biggest leaf. Variety Nl4 had the biggest leaves and NC0376 had the smallest. Variety Nl4 had the highest leaf area index (LAI) while ZN7 had the lowest. There were significant differences (P = 0,01) in PAR intercepted by the varieties but there were no significant differences in extinction coefficients. Extinction coefficients increased with crop age. The varieties had significantly different (P = 0,01) leaf angles and ellipsoidal leaf angle distribution parameters. The measurement of LAI using SunScan ceptometer provided a better estimate of extinction coefficients than LAI measured using Delta-T leaf area meter. Model evaluation showed that CANEGRO canopy model version 2 was improved compared to than version 1. The model (version 2) was accurate in predicting tiller heights and dead leaf numbers per stalk. It was fairly accurate in predicting green leaf numbers per plant, stalk population and intercepted PAR but was poor in predicting LA!. Version 2 has proved to be a substantial improvement over version 1 in predicting stalk population. Generally, the version 2 model overestimated tiller heights early and underestimated later, overestimated the tiller population and LAI after peak, underestimated green leaf numbers per stalk for varieties ZN6, ZN7 and N14 and overestimated dead leaf numbers per stalk and intercepted PAR. The version 2 model predicted a constant green leaf numbers per plant and LAI from peak to harvest while observed data showed that green leaf numbers per stalk and LAI decreased towards harvest. Version 2 model predicted the tiller population of NC0376 closely but underestimated tiller senescence in N14 and also underestimated final tiller population in varieties ZN6 and ZN7. Future model refinements may need to focus on the prediction of the sigmoid pattern of tiller heights. The model may need to be calibrated to predict the green leaf numbers per stalk accurately, which should possibly improve the prediction of LAI that in turn could improve the prediction of intercepted PAR. The improvement in the timing and rate of tiller senescence should improve the prediction of tiller population particularly in varieties ZN6, ZN7 and N14. The study showed that the broken stick method IS superior in explaining leaf and tiller population development compared to using polynomial equations. The development of variety parameters helped improve the prediction of variety differences in canopy growth and development. A major weakness of most crop models is modelling variety differences in canopy growth and development. The inability of crop models to predict variety differences has limited their use in plant breeding. This study has resulted in an improved version of CANEGRO version 1 that is an initial attempt at modelling variety differences of sugarcane.
Estimation of reference evaporation and comparison with ET-gage evaporimeter
(2003) Abezghi, Tekeste Weldegabrial.; Savage, Michael John.
Accurate estimation of reference evaporation is necessary for the estimation of actual evaporation for irrigation and water resource management purposes. Estimation of reference evaporati~n using the Penman-Monteith method using automatic weather station (AWS)measurements requires the available energy to be accurately estimated. The available energy of short grass of 0.12 m was measured using a component net radiometer and soil heat flux plate measurements at the Faculty of Sciences and Agricultural (Agrometeorological station, University of Natal, Pietermaritzburg, latitude ~29.79 oS, longitude ~ 30.95 °E, altitude ~ 650 m). In an attempt to evaluate the accuracy of commonly used procedures of estimating available energy, estimates of net irradiance (from net long wave irradiance and reflection coefficient estimate) and soil heat flux density were compared to the actual measurements. The linear approximation of atmosphere minus crop surface emittance based on air temperature was compared with measured net long wave irradiance and similar empirical formulations. The underestimation of the measured net long wave irradiance was observed using the linear approach. Furthermore, a plot of measured clear sky net long wave irradiance and air temperature showed a logarithmic relation. The estimated reflected solar irradiance was overestimated for the reference crop. The measured soil heat flux density was observed to vary not only with net irradiance but also with cloudiness, wind speed and soil water content. The soil heat flux density measured with plates was noticed to follow the measured net irradiance. The sensitivity of Penman-Monteith latent heat estimate was investigated for the use of estimated reflection coefficient and soil heat flux density as well as ignored soil heat flux density. Results showed the latent heat estimate to be greater when soil heat flux density was ignored. Reduced set assumptions of Penman-Monteith were assessed usmg the microclimatic measurements. The grass reference evaporation estimate using estimated water vapour pressure from the pervious day minimum air temperature and approximated wind speed were found to be seasonal and procedure dependent. The hourly-reduced set estimate of reference evaporation was in good agreement with the grass Penman-Monteith estimate. The estimated daily water vapour pressure underestimated the daily grass Penman-Monteith estimate. The sensitivity of the reduced set reference evaporation estimate was compared for the two values of approximated wind speeds. The assumption of 2 m S-1 wind speed gave a relatively better result. The sensitivity of the surface temperature energy balance (STEB) estimate of reference evaporation was investigated using two different atmospheric stability procedures. The evaporation estimate agreement and performance of the technique were found to vary depending on the stability correction procedure. The Monteith (1973) correction procedure was observed to be more sensitive to a higher surface-air temperature difference. The Monteith (1973) procedure was found to underestimate the reference evaporation and this resulted in a lower correlation coefficient. The uncorrected and Campbell and Norman (1998) stability corrected procedure of STEB estimate overestimated the reference evaporation but resulted in good agreement with actual reference evaporation. The use of estimated available energy using the STEB method resulted in a 7 % overestimate of measured available energy. Different designs of atmometers have been used to measure evaporation. The less expensive and simple ET_gageR (Model A and E) atmometer for daily evaporation measures were compared to grass-based and alfalfa-based Penman-Monteith and STEB estimate of reference evaporation. Two different evaporation surface covers used with the device allowed for the comparison to be made. Measurements using the canvas 30 ET-gage cover for grass reference evaporation were compared to grass based Penman-Monteith and STEB reference evaporation estimates. Correlation between the canvas 30 measures and Penman-Monteith estimates were good compared to the STEB estimate. The ET-gage canvas 54 measures were in a good agreement with alfalfa based Penman-Monteith reference evaporation estimate. There was, however, a slight time lag in ET-gage evaporation with ET-gage evaporation continuing accumulation when the reference evaporation was zero. The sensitivity of the ET-gage for microclimate variation was tested using the measurements made for two levels and three different microclimates. A shade measurement of reference evaporation was overestimated. The response of the ET-gage to one and two meter microclimate measures was similar to the short grass measurement. Furthermore, the ET-gage surface evaporation estimate using the STEB method showed equal response to the ET-gage surface for the microclimate measure and explained the possible cause of the lag of the ET-gage response. Accurate microclimate measurements is a requirement for the performance of the PenmanMonteith approach for the estimation of reference evaporation. The investment cost required for an AWS set up is high. Alternative options for gathering information of the microc1imate measurements required for calculating reference evaporation were assessed in terms of cost saving, accuracy and other advantages. A weather station system using a Hobo H8 logger (internal relative humidity and air temperature sensor and two external channels, one which was used for solar irradiance measurements) was found to be a cost-effective method for calculating the necessary microclimatic information for calculating reference evaporation. With this system reference evaporation was estimated with reasonable accuracy, at 16 % of the cost of normal AWS system. The use of an Event Hobo logger and an ET-gage was found to provide a reasonable estimate of reference evaporation. The use of the reduced set evaporation weather station was found to be unreasonable in terms of cost and accuracy. Air temperature and relative humidity were measured from different design of radiation shields and Stevenson screens. The use of home-made seven-plate plastic radiation shields provided a similar shield to radiation and ventilation compared to manufactured shields. At a low solar angle when wind speed was very low, all the radiation shields including the small Stevenson screens showed a higher air temperature difference relative to the standard Stevenson screen. The highest average difference of air temperature measurement was measured within the small Stevenson screen and metal-radiation shield. The home-made plastic radiation shield showed similar averages of air temperature and water vapour pressure difference compared to the six- and twelve-plate Gill radiation shields. The home-made metal radiation shield showed relatively higher deviation from the mean being cold at night time and hot during the day. More research is needed to explore the efficiency of the ET-gage evaporation from variety of microclimates to establish the cause of the overestimate under shade, to develop better relation of clear day net long wave irradiance and air temperature and the use of a wind speed sensor with Hobo H8 weather station system.
Estimating solar radiation for water-use and yield simulations under present and projected future climate using Cropsyst.
(2003) Abraha, Michael Ghebrekristos.; Savage, Michael John.
Agricultural scientists are faced with the challenge of producing enough food for the increasing world population. Hence the need to develop tools for managing soil and plant systems to increase food production in order to meet the world food demand in the future. Crop simulation models have become promising tools in predicting yield and related components fi'om a set of weather, soil, plant and management data inputs. This study describes the estimation of solar radiant density, a crucial input in crop simulation models; calibration and validation of a soil-plant growth simulator, CropSyst, for management purposes; and generation of weather data for assessment of crop production under possible climate changes in the future. Daily solar radiant density, an input required by most crop simulation models, is infiequently observed in many stations. This may prevent application of crop simulation models for specific locations. Long-term data records of daily minimum and maximum air temperatures, precipitation, sunshine hours and/or solar radiant density were obtained for Cedara, Durban, Seven Oaks and Ukulinga in KwaZulu-Natal, South Africa. Solar radiant density was estimated fi'om sunshine hours using the Angstrom equation and ten other models that involved daily minimum and maximum air temperatures and/or precipitation along with extratelTestrial radiant density. Coefficients for the Angstrom equation and one of the other ten models were specifically developed for South African conditions; the remaining models required fitting coefficients using the available data for all locations. The models were evaluated using (i) conventional statistics that involved, root mean square elTor (RMSE) along with its systematic and unsystematic components, slope, intercept, index of agreement (d), and coefficient of determination (R\ and (ii) a fuzzy expert system that involved a single modular indicator (Ira d) aggregated from the modules of accuracy (aggregation of the indices relative RMSE, model efficiency and I-student probability), con'elation (Pearson's correlation coefficient) and pattem (aggregation of pattem index vs day of year and pattem index vs minimum air temperature). For each index, two functions describing membership to the fuzzy subsets Favourable (F) and Unfavourable (V) were defined. The expelt system calculates the modules according to both the degree of membership and a set of decision rules. Solar radiant density estimated from sunshine hours for the Durban station resulted in R2 , RMSE (MJ m,2) and d index of 0.90, 2.32 and 0.97 respectively. In the absence of observed solar radiant density data, estimations from sunshine hours were used for derivation of coefficients as well as evaluation of the models. For Durban, the performance of the models was generally poor. For Cedara, Seven Oakes and Ukulinga two of the models resulted in a high d index and smallest systematic RMSE. The solar radiant density estimated from each model was also used as an input to simulate maize grain yields using the soil-plant growth simulator, CropSyst. The models were ranked according to their ability to simulate grain yields that match those obtained from using the observed solar radiant density. The rankings according to crop simulation, conventional statistics and expert system were compared. The CropSyst model was also evaluated for its ability to simulate crop water-use of fallow and cropped (oats, Italian ryegrass, rye and maize) plots at Cedara, KwaZulu-Natal, South Africa. Soil characteristics, initial soil water conditions, irrigation and weather data were inputted to CropSyst. Crop input parameters for oats, Italian ryegrass and rye were used, with little modifications, as determined from field experiments conducted at Kromdraai open cast mine, Mpumalanga province, South Africa. Crop input parameters for maIze were either determined fi'om field experiments or taken from CropSyst crop input parameters documentation and adjusted within a narrow specification range of values as dictated by CropSyst. The findings indicated that CropSyst was generally able to simulate reasonably well the water-use of fallow and cropped (oats, Italian ryegI°ass, rye and maize) plots; leaf area index and crop evapotranspiration of rye; and grain yield and developmental stages of maize. The validated CropSyst model was also used to simulate timing and amount of irrigation water, and investigate incipient water stress in oats, Italian ryegrass and rye. The CropSyst model was used to investigate potential effects of future climate changes on the productivity of maize grain yields at Cedara, KwaZulu-Natal, South Africa. The effect of planting date (local planting date, a fortnight earlier and a fortnight later) was also included in the study. A 30-year baseline weather data input series were generated by a stochastic weather generator, ClimGen, using 30 years of observed weather data (l971 to 2000). The generated weather data series was compared with the observed for its distributions of daily rainfall and wet and dry series, monthly total rainfall and its variances, daily and monthly mean and variance of precipitation, minimum and maximum air temperature, and solar radiant density. Four months of the year failed to reproduce distributions of wet and dry series, daily precipitation, and monthly variances of precipitation of the observed weather data series. In addition, Penman-Monteith reference evaporation (ETa) was calculated using the observed and generated data series. Cumulative probability function of ETa calculated using the generated weather data series followed the observed distribution well. Moreover, maize grain yields were simulated using the generated and observed weather data series with local, a fortnight earlier and a fortnight later planting dates. The mean simulated grain yields for the respective planting dates were not statistically different from each other; the grain yields simulated using the generated weather data had significantly smaller variance than the grain yields simulated using the observed weather data series. When the generated weather data series was used an input, the early planting date as compared to the locally practiced and late planting dates resulted in significantly greater simulated grain yields. The grain yields simulated using the observed weather data for the early and local planting dates were not statistically different from each other. The baseline period was modified by synthesized climate projections to create future climatic scenarios. The climate changes considered corresponded to doubling of [C02] from 350 to 700 ~t1 ,-I without air temperature and water regime changes, and doubling of [C02] accompanied by increases in mean air temperature and precipitation changes of 2 (lC and 10%, 2 (le and 20%>, 4 °c and 10%, and 4 (lC and 20% respectively. Solar radiant density was also estimated from daily air temperature range for all scenarios that involved change in mean air temperature. In addition, input crop parameters of radiation-use and biomass transpiration efficiencies were modified for maize, in CropSyst, to accommodate changes in elevated levels of [C02]. Equivalent doubling of [C02], without air temperature or water regime changes, resulted in increased simulated grain yields as compared to the baseline period. Adding 2 QC to the mean daily temperature and 10% to the daily precipitation of a [C02] elevated atmosphere reduced the grain yield but still kept it above the level of the baseline period grain yield. Adding 4 QC to the mean daily temperature and 10% to the daily precipitation fLllther decreased the yield. Increasing the daily precipitation by 20% instead of 10% did not change the simulated grain yield as compared to the 10% increments. Early planting date, for all scenarios, also resulted in higher yields, but the relative increment in grain yield was higher for the late planting dates with scenarios that involved increment in mean air temperature. In general, this study confi1l11ed that doubling of [C02] increases yield but the accompanied increase in mean air temperature reduces yield.
Frequency domain reflectometry for irrigation scheduling of cover crops.
(2003) Gebregiorgis, Mussie Fessehaye.; Savage, Michael John.
A well-managed irrigation scheduling system needs a rapid, preCIse, simple, costeffective and non-destructive soil water content sensor. The PRl profile probe and Diviner 2000 were used to determine the timing and amount of irrigation of three cover crops (Avena sativa L., Secale cereale L. and Lolium multiflonlm Lam.), which were planted at Cedara, KwaZulu-Natal. The PRl profile probe was first calibrated in the field and also compared with the Diviner 2000. For the calibration of the PRl profile probe the factory-supplied parameters (aJ = 8.4 and ao = 1.6) showed good correlation· compared to the soil-estimated parameters (aJ = 11.04 and ao = 1.02). The factorysupplied parameters gave a linear regression coefficient (r2 ) of 0.822 and root mean square error (RMSE) of 0.062. The soil-estimated parameter showed a linear regression coefficient of 0.820 with RMSE of 0.085. The comparison between the soil water content measured using the PR1 profile probe and Diviner 2000 showed a linear regression coefficient of 0.947 to 0.964 with a range of RMSE of 0.070 to 0.109 respectively for the first 100 to 300 mm soil depths. The deeper depths (400, 600 and 1000 mm) showed a linear regression coefficient ofO.716to 0.810 with a range of 0.058 to 0.150 RMSE. These differences between the shallow and deeper depths could be due to soil variability or lack of good contact between the access tube and the surrounding soil. To undertake irrigation scheduling using the PRl profile probe and Diviner 2000, the soil water content limits were determined using field, laboratory and regression equations. The field method was done by measuring simultaneously the soil water content using the PR1 profile probe and soil water potential using a Watermark sensor and tensiometers at three depths (100, 300 and 600 mm) from a 1 m2 bare plot, while the soil dries after being completely saturated. The retentivity function was developed from these measurements and the drained upper limit was estimated to be 0.355 m3 m-3 when the drainage from the pre-wetted surface was negligible. The lower limit was calculated at -1500 kPa and it was estimated to be 0.316 m3m,3. The available soil water content, which is the difference between the upper and lower limit, was equal to 0.039 m3 m,3. In the laboratory the soil water content and matric potential were measured from the undisturbed soil samples taken from the edge of the 1 m2 bare plot before the sensors were installed. Undisturbed soil samples were taken using a core sampler from 100 to 1000 mm soil depth in three replications in 100 mm increments. These undisturbed soil samples were saturated and subjected to different matric potentials between -1 to -1500 kPa. In the laboratory, the pressure was increased after the cores attained equilibrium and weighed before being subjecting to the next matric potential. The retentivity function was then developed from these measurements. The laboratory method moved the drained upper limit to be 0.390 m3 m,3 at -33 kPa and the lower limit be 0.312 m3m-3 at -1500 kPa. The regression equation, which uses the bulk density, clay and silt percentage to calculate the soil water content at a given soil water potential, estimated the drained upper limit to be 0.295 m3m-3at -33 kPa and the lower limit 0.210 m3 m,3 at -1500 kPa. Comparison was made between the three methods using the soil water content measured at the same soil water potential. The fieldmeasured soil water content was not statistically the same with the laboratory and estimated soil water content. This was shown from the paired-t test, where the probability level (P) for the laboratory and estimated methods were 0.011 and 0.0005 respectively at 95 % level of significance. However, it showed a linear regression coefficient of 0.975 with RMSE of 0.064 when the field method was compared with the laboratory method. The field method showed a linear regression coefficient of 0.995 with RMSE of 0.035 when compared with the estimated method. The timing and amount of irrigation was determined using the PR1 profile probe and Diviner 2000. The laboratory measured retentivity function was used to define the fill (0.39 m3 m-3 ) and high refill point (0.34 m3 m-3 ). The soil water content was measured using both sensors two to three times per week starting from May 29 (149 day of year, 2002) 50 days after planting until September 20 (263 day of year) 11 days before harvesting. There were five irrigations and twenty rainfall events. The next date of irrigation was predicted graphically using, the PRl profile probe measurements, to be on 3 September (246 day of year) after the last rainfall event on 29 August (241 day of year) with 8 mm. When the Diviner 2000 was used, it predicted two days after the PRl profile probe predicted date. This difference appeared since the Diviner 2000-measured soil water content at the rooting depth was slightly higher than the PRl profile probe measurements. The amount of irrigation was estimated using two comparable methods (graphic and mathematical method). The amount of irrigation that should have been applied on 20, September (263 day of year) to bring the soil water content to field capacity was estimated to be 4.5 hand 23 mm graphically and 5.23 hand 20 mm mathematically. The difference between these two methods was caused due to the error encountered while plotting the correct line to represent the average variation in soil water content and cumulative irrigation as a function of time. More research is needed to find the cause for the very low soil water content measurements of the PRI profile probe at some depths. The research should be focused on the factors, which could affect the measurement of the PRl profile probe and Diviner 2000 like salinity, temperature, bulk density and electrical conductivity. Further research is also needed to extend the non-linear relationship between the electrical resistance of the sensor and soil water potential up to -200 kPa. This non-linear equation of the Watermark is only applicable within the range of soil water potential between -10 and -100 kPa.
The use of infrared thermometry for irrigation scheduling of cereal rye (Secale cereale L.) and annual ryegrass (Lolium multiflorum Lam.)
(2003) Mengistu, Michael Ghebrekidan.; Savage, Michael John.; Everson, Colin Stuart.
Limited water supplies are available to satisfy the increasing demands of crop production. It is therefore very important to conserve the water, which comes as rainfall, and water, which is used in irrigation. A proper irrigation water management system requires accurate, simple, automated, non-destructive method to schedule irrigations. Utilization of infrared thermometry to assess plant water stress provides a rapid, nondestructive, reliable estimate of plant water status which would be amenable to larger scale applications and would over-reach some of the sampling problems associated with point measurements. Several indices have been developed to time irrigation. The most useful is the crop water stress index (CWSI), which normalizes canopy to aIr temperature differential measurements, to atmospheric water vapour pressure deficit. A field experiment was conducted at Cedara, KwaZulu-Natal, South Africa, to determine the non-water-stressed baselines, and CWSI of cereal rye (Secale cereale L.) from 22 July to 26 September 2002, and aImual (Italian) ryegrass (Lolium multiflorum Lam.) from October 8 to December 4, 2002, when the crops completely covered the soil. An accurate measurement of canopy to air temperature differential is crucial for the determination of CWSI using the empirical (Idso et al., 1981) and theoretical (Jackson et al., 1981) methods. Calibrations of infrared thermometers, a Vaisala CS500 air temperature and relative humidity sensor and thermocouples were performed, and the reliability of the measured weather data were analysed. The Everest and Apogee infrared thermometers require correction for temperatures less than 15 QC and greater than 35 QC. Although the calibration relationships were highly linearly significant the slopes and intercepts should be corrected for greater accuracy. Since the slopes of the thermocouples and Vaisala CS500 air temperature sensor were statistically different from 1, multipliers were used to correct the readings. The relative humidity sensor needs to be calibrated for RH values less than 25 % and greater than 75 %. The integrity of weather data showed that solar irradiance, net irradiance, wind speed and vapour pressure deficit were measured accurately. Calculated soil heat flux was underestimated and the calculated surface temperature was underestimated for most of the experimental period compared to measured canopy temperature. The CWSI was determined using the empirical and theoretical methods. An investigation was made to determine if the CWSI could be used to schedule irrigation in cereal rye and annual rye grass to prevent water stress. Both the empirical and theoretical methods require an estimate or measurement of the canopy to air temperature differential, the non-waterstressed baseline, and the non-transpiring canopy to air temperature differential. The upper (stressed) and lower (non- stressed) baselines were calculated to quantify and monitor crop water stress for cereal rye and annual ryegrass. The non-water-stressed baselines were described by the linear equations Te - Ta = 2.0404 - 2.0424 * VPD for cereal rye and Te - Ta = 2.7377 - 1.2524 * VP D for annual ryegrass. The theoretical CWSI was greater than the empirical CWSI for most of the experimental days for both cereal rye and annual ryegrass. Variability of empirical (CWSI)E and theoretical (CWSI)T values followed soil water content as would be expected. The CWSI values responded predictably to rainfall and irrigation. CWSI values of 0.24 for cereal rye and 0.29 for annual ryegrass were found from this study, which can be used for timing irrigations to alleviate water stress and avoid excess irrigation water. The non-water-stressed baseline can also be used alone if the aim of the irrigator is to obtain maximum yields. However the non-water-stressed baseline determined using the empirical method cannot be applied to another location and is only valid for clear sky conditions. And the non-water-stressed baseline determined using theoretical method requires computation of aerodynamic resistance and canopy resistances, as the knowledge of canopy resistance, however the values it can assume throughout the day is still scarce. The baseline was then determined using a new method by Alves and Pereira (2000), which overcomes these problems. This method evaluated the infrared surface temperature as a wet bulb temperature for cereal rye and annual ryegrass. From this study, it is concluded that the infrared surface temperature of fully irrigated cereal rye and annual ryegrass can be regarded as a surface wet bulb temperature. The value of infrared surface temperature can be computed from measured or estimated values of net irradiance, aerodynamic resistance and air temperature. The non-water-stressed baseline is a useful concept that can effectively guide the irrigator to obtain maximum yields and to schedule irrigation. Surface temperature can be used to monitor the crop water status at any time of the day even on cloudy days, which may greatly ease the task of the irrigator.
Modelling the soil water balance and applications using a decision support system (DSSAT v3.5).
(2003) Ghebreab, Tesfalidet Alem.; Savage, Michael John.
Water is a scarce resource used by various stakeholders. Agriculture is one of the users of this resource especially for growing plants. Plants need to take up carbon dioxide to prepare their own food. For this purpose plants have stomatal openings. These same openings are used for transpiration. Quantifying transpiration is important for efficient water resource management and crop production because it is closely related to dry matter production. Transpiration could be measured using a number of methods or calculated indirectly through quantification of the soil water balance components using environmental instruments. The use of models such as the Decision Support System for Agrotechnology Transfer (DSSAT v3.5) is, however, much easier than environmental instruments. Nowadays, with increased capabilities of computers, the use of crop simulation modelling has become a common practice for various applications. But it is important that models, such as DSSAT v3.5, be calibrated and verified before being used for various applications such as long-term risk assessment, evaluation of cultural practices and other applications. In this study the model inputs have been collected first Then the model was calibrated and verified. Next sensitivitY analysis was carried to observe the model behavior to changes in inputs. Finally the model has been applied for long-term risk assessment and evaluation of cultural practices. In this study, the data collected formed the basis forthe minimum dataset needed for running the DSSAT v3.5 model. In addition, the factory given transmission of shading material over a tomato crop was compared to actual measurements. Missing weather data (solar irradiance, minimum and maximum air temperature and rainfall) were completed after checking that it was homogeneous to measurements from nearby automatic weather station. It was found that factory-given transmission value of 0.7 of the shade cloth was different from the actual one of 0.765. So this value was used for conversion of solar irradiance measured outside the shade cloth to solar irradiance inside the shade cloth. Conventional laboratory procedures were used for the analysis of soil physical and chemical properties. Soil water content limits were determined using texture and bulk density regression based equations. Other model inputs were calculated using the DSSAT model. Crop management inputs were also documented for creation of the experimental details file. The DSSATv3.5 soil water balance model was calibrated for soil, plant and weather conditions at Ukulinga by modifying some of its inputs and then simulations of the soil water balance components were evaluated against actual measurements. For this purpose half of the data available was used for calibration and the other half for verification. Model simulations of soil water content (150 to 300 mm and 450 to 600 mm) improved significantly after calibration. In addition, simulations of leaf area index (LA!) were satisfactory. Simulated evapotranspiration (E1) had certain deviations from the measured ET because the latter calculated ET by multiplying the potential ET with constant crop multiplier so-called the crop coefficient. Sensitivity analysis and long-term risk assessments for yield, runoff and drainage and other model outputs were carried out for soil, plant and weather conditions at Ukulinga. For this purpose, some of the input parameters were varied individually to determine the effect on seven model output parameters. In addition, long-term weather data was used to simulate yield, biomass at harvest, runoff and drainage for various initial soil water content values. The sensitivity analysis gave results that conform to the current understanding of the soil-plant atmosphere system. The long-term assessment showed that it is risky to grow tomatoes during the winter season at Ukulinga irrespective of the initial soil water content unless certain measures are taken such as the use of mulching to protect the plants from frost. The CROPGRO-Soya bean model was used to evaluate the soil water balance and gro'W1:h routines for soil, plant and weather conditions at Cedara. In addition, cultural practices such as row spacing, seeding rate and cultivars were also evaluated using longterm weather data. Simulations of soil water content were unsatisfactory even after calibration of some of the model parameters. Other model parameters such as LAI, yield and flowering date had satisfactory agreement with observed values. Results from this study suggest that the model is sensitive to weather and cultural practices such as seeding rates, row spacing and cultivar maturity groups. The general use of decision support systems is limited by various factors. Some of the factors are: unclear definition of clients/end users; no end user input prior to or during the development of the DSS; DSS does not solve the problems that the client is experiencing; DSS do not match their decision-making style; producers see no reason to change the current management practices; DSS does not provide benefit over current decision-making system; limited computer ownership amongst producers; lack of field testing; producers do not trust the output due to the lack of understanding of the underlying theories of the models utilized; cannot access the necessary data inputs; lack of technical support; lack of training in the development ofDSS software; marketing and support constraints; institutional resistances; short shelf-life of DSS software; technical constraints, user constraints and other constraints. For successful use of DSS, the abovementioned constraints have to be solved before their useful impacts on farming systems could be realized. This study has shown that the DSSAT v3.5 model simulations of the soil water balance components such as evapotranspiration and soil water content were unsatisfactory while simulations of plant parameters such as leaf area index, yield and phonological stages were simulate to a satisfactory standard. Sensitivity analysis gave results that conform to the current understanding of the soil-plant -atmosphere system. Model outputs such as yield and phonological stages were found to sensitive to weather and cultural practices such as seeding rates, row spacing and cultivar maturity groups. It ha been further investigated that the model could be used for risk assessment in various crop management practices and evaluation of cultural practices. However, before farmers can use DSSAT v3.5, several constraints have to be solved.
Potential for using trees to limit the ingress of water into mine workings : a comparison of total evaporation and soil water relations for eucalyptus and grassland .
(2003) Jarmain, Caren.; Savage, Michael John.
Current mining methods used to extract coal from underground mine workings disturb the natural environment and the existing stable geological structures. As a result, the ingress of water into the mines increases and the quality of the water passing through the mine workings deteriorates, irrespective of the operational status of the mines. Water ingress is generated by regional aquifers, local aquifers, recharge from the surface through rainfall, natural drainage paths on the surface, and surface water bodies. The quality of water in the mines deteriorates as a result of contact with the remaining coal in the mine workings. Mining can therefore cause an increased influx of water into a mine and the degradation of this water. The solution to reducing the impact of mines on the environment is to prevent, or at least reduce, the amount of water entering the mines, and to manage this water to prevent further degradation in water quality. This study focused on afforestation with Eucalyptus viminalis trees to manage or inhibit ingress of water into underground mine workings. The hypothesis of this study was that a change in vegetation, from grassland to fast-growing and potentially high water-using trees like Eucalyptus. could possibly reduce the drainage of water below the root-zone and into the mine workings. The hypothesis was tested by estimating the components of the soil water balance for a grassland site and a Eucalyptus tree site. The research site was situated in Mpumalanga, (260 36' Sand 290 08' E, 1650 m a.m.s.l.), one of South Africa's major coal bearing areas. Although the Secunda area is a treeless environment and conditions are not optimal for forestry, some Eucalyptus species are suited for conditions (frost and periodic droughts) encountered in this area. The soil water balance of grassland and E. viminalis trees were studied through a field experiment and a long-term (30 years) modelling exercise. Total evaporation of the grassland site was estimated using the Bowen ratio energy balance technique. The transpiration of six representative E. viminalis trees were estimated using the heat pulse velocity technique. The soil water storage changes at both sites were determined from the soil water content, estimated using water content reflectometers. Measurements were performed in a smectic clay soil which resulted in measurements difficulties. Vertical cracks were formed under soil drying. To establish the importance of climate and plant growth on the drainage beyond the root-zone, the soil water balance of a grassland and an E. viminalis site were simulated over a 30-year period with the Soil Water Atmosphere Plant (SWAP) model. It was concluded from the comparative field experiment and modelling, that a change in vegetation from grassland to E. viminalis will reduce the drainage of water below the root-zone, especially under above-average rainfall conditions. The reduction in drainage beyond the root-zone at the E. viminalis sites, compared to the grassland site, was demonstrated in the modelling exercise and can be deduced from the total evaporation and soil water storage estimated at both sites. The results from the field experiment confirmed the modelling results and showed that usually there were higher transpiration rates for the E. viminalis tree site, compared to the grassland site. The higher transpiration rates for E. viminalis trees resulted in lower relative saturation of soil layers and lower profile soil water contents at the E. viminalis site, and higher daily soil water storage changes at the E. viminalis site compared to the grassland site. These differences were more pronounced during winter when the grassland was dormant. The results from the modelling exercise showed that an E. viminalis tree stand, with a closed canopy, reduced drainage below the root-zone compared to a grassland. The drainage at the grassland site contributed to up to 54 % of the rainfall, compared to the 43 % at the E. viminalis site. However, under below-average rainfall conditions the annual drainage at both sites, were similar. Further, the absolute magnitude of the drainage was similar to the total evaporation at the grassland site under certain conditions. The results not only suggest that a change in vegetation, from grassland to E. viminalis trees, would reduce the drainage beyond the root-zone, but that it may delay the onset of drainage. Under above-average rainfall conditions, the modelled drainage at the E. viminalis site only exceeded 20 mm, a month later than at the grassland site. The simulation results also showed that under conditions of aboveaverage rainfall, drainage occurs whenever the rainfall exceeds the long-term average rainfall, irrespective of the existing vegetation. However, when the rainfall is belowaverage drainage at both sites are limited to large rainfall events. This simulation showed that over a period of eight years, E. viminalis trees could potentially reduce the drainage by 1235 mm more than grassland, which is equivalent to 1540 m3 ha- I a-I, or 1.54 Me ha- I a-I. The annual average reduction in drainage below the root-zone caused by E. viminalis trees (1.79 Mf ha-1 a-\ is a small reduction when compared to the influx of water into mineworkings. E.g. the influx of water into a bord-and-pillar mine range between 0.5 and 4 Mt d-I per area mined and up to 17000 Mt d-I per area mined under high extraction mining (Hodgson and Krantz, 1998; Hodgson et aI., 2001). This work gave a comprehensive account of the differences in the soil water relations of grassland and E. viminalis trees overlying coal mine working. Few other studies in South Africa compared the total evaporation and soil water relations of grassland and E. viminalis trees in so much detail. State of the art monitoring techniques were used and produced valuable comparison of their use in expansive clay profiles. The work should contribute to management decisions focussed on limiting ingress of water into mine workings.
The long-term measurement of total evaporation over Acacia mearnsii using large aperture scintillometry
(2007) Clulow, Alistair David.; Savage, Michael John.
A large aperture scintillometer (LAS) was operated continuously over a distance of 575 m from 19 August 2006 to 29 September 2007 in the South African KwaZulu-Natal midlands mistbelt area over Wattle (Acacia mearnsii). The LAS measurements of the structure parameter of the refractive index of air ( 2 n C ), were used to calculate the sensible heat flux. The shortened energy balance equation was used to estimate the latent energy flux as a residual from which the total evaporation (ET) was calculated. The LAS estimates of sensible heat flux during the short transition period (1 hour) between stable and unstable conditions were on occasion erroneous and required verification. Advection was also found to affect sensible heat flux estimates. Long-term operation of the LAS was however found to be possible even at remote sites, producing reliable and continuous results. The LAS estimates of sensible heat are sensitive to zero-plane displacement height and wind speed data impact and these should be derived as accurately as possible. Tree heights were measured at monthly intervals and a zero-plane displacement and effective height were calculated every two weeks. The sensible heat flux was thus processed in two week blocks of data corresponding to progressive effective heights. The tree growth rate was consistent over time and was not affected by seasonality, indicating that reduced air temperatures, rainfall and solar irradiance in winter are not limiting growth. The average growth rate was 0.37 m per month or 4.5 m per year. The LAS ET was compared to the American Society of Civil Engineers - Environmental and Water Resources Institute (ASCE-EWRI) short grass reference evaporation (ETsz) for a seven-month period and was found to compare favourably (R2 = 0.78) with outliers caused by advection and rainfall events. Calculations of grass reference evaporation at hourly and daily intervals provide different results. The daily estimates are lower than the hourly estimates by 17 % on average. Where hourly data is summed to calculate a daily ETsz, night-time values should be included. The LAS ET measurements were validated against the Priestley and Taylor (1972) method of estimating ET and found to be in good agreement (R2=0.94). The Priestley and Taylor daily total latent energy flux, from 22 August 2006 to 29 September 2007, was 9 % higher than the LAS results on average. The Bowen ratio for the entire period is less than 1, indicating that the latent energy flux dominates at the site. The ET over the period of measurement (13 months) is 1250 mm and the rainfall is 750 mm. This confirms previous results at the site using the Bowen ratio energy balance method showing that the ET exceeds the rainfall by 45 % and justifies further research into soil water, ground water and root interactions in the deep soil profile.
Long-term measurements of spatially-averaged sensible heat flux for a mixed grassland community, using surface layer scintillometry.
(2007) Odhiambo, George O.; Savage, Michael John.
Evapotransipration by vegetation cover is an important component of the water budget and energy balance in any ecosystem. A key to more improved water management therefore lies in improving our understanding of evapotranspiration, the process that drives water use by plants. Estimations of the turbulent fluxes are required for various applications in micrometeorology, hydrology, environmental studies and agriculture. Numerous methods for estimation of turbulent fluxes have been developed and tested. Direct measurements of fluxes are usually achieved by the eddy covariance (EC) method, which is considered as the most reliable. However, the application of the EC method is often problematic. The necessary sensors for wind, temperature and humidity must respond very fast (resolution of 10 Hz or better) and at the same time must not show noticeable drift. This makes them delicate, expensive and difficult to calibrate among other problems associated with the method. Due to their ability to integrate atmospheric processes along a path length that may range between a few hundred metres to a few kilometres, optical methods based on the analysis of scintillation appear to be an alternative and possible supplement to classical micrometeorological methods such as the EC method, which may provide local fluxes typically at the scale of 100 m. The use of the scintillometry technique in surface flux measurements is therefore gaining in popularity. The accuracy of the measurements obtained by one method is judged by comparison of the measurements obtained by those of another method considered as the standard. For turbulent flux measurements, the EC method is taken as the standard method for the determination of sensible heat fluxes. This research presents the measurement of sensible heat fluxes using the surface layer scintillometer (SLS). The SLS system used has a dual-beam and a recommended path length of between 50 and 250 m. The method was tested against the EC method for different Bowen ratio (f3) values, as required by the theory, under different atmospheric stability conditions, as well as for different wind directions relative to the SLS beam path and slanting beam path orientation. Also presented is an analysis of the different forms of the Monin-Obukhov Similarity (MOST) functions used in micrometeorology and suggested by various authors, done by comparing the resulting sensible heat flux measured by the SLS method with the ones calculated through an iterative determination of the Monin-Obukhov parameters. A comparison of the structure function parameter of temperature (Ci ) corrected for fJ and those measured (using SLS) was carried out, with the results showing very good correspondence between the corrected and uncorrected ci values, indicating that not correcting for fJ for SLS measured ci does not result in significant error in the resulting ci values, and hence sensible heat flux estimates. A comparison of the sensible heat flux Fh obtained using EC and SLS methods for fJ < 0.6 and fJ > 0.6 followed and the results also show good correspondence between the values obtained using the EC and SLS methods, although the agreement is slightly improved for cases when fJ > 0.6. A sensitivity analysis indicates that both the ECand SLS-measurements of Fh are influenced by fJ values. A sensitivity analysis on the influence of fJ on Fh measurements by both the EC and SLS methods further indicates that the influence of fJ on Fh measurements is not large enough to warrant correcting Fh measurements for fJ . The F" measurements by the EC method appears to be influenced more by fJ especially for fJ values less than 0.74. A comparison of the various methods for computing the empirical similarity functions used by MOST was also carried out and the results show a significant difference in the Fh computed following the various methods suggested by different researchers. As for the agreement between the EC and SLS methods determination of Fh for the different atmospheric stability conditions, there seems to be a better agreement in the Fh measurements as noted by correlation coefficients closer to 1 and greater tvalues obtained during unstable atmospheric conditions in the colder months of June and August while reduced agreement in the values is recorded in the warmer summer period from November to December. Also noted is a slight difference in the EC measurements compared to the SLS measurement of F". The difference in the measurements is noticed for unstable atmospheric conditions. Also noted is that EC and SLS measurements of Fh differ slightly when the atmospheric condition is nearneutral. However the agreement between the Fh values measured by the two measurement methods is still good. was set up in an inclined position, with the receiver set at 0.68 m above the ground level and transmitter at 1.68 m, resulting in an effective height difference of 1.00 m. There was generally good agreement in the 2-min measurements of F" by the two methods for the SLS set up in inclined position, with the 30-min data resulting in even better agreements. The findings confirm that the SLS set up does not impair its performance in measuring sensible heat fluxes. This also shows that the SLS would also work well in non-ideal (heterogeneous) conditions which the inclined optical beam path mimics. For those days when wind direction was mainly approximately perpendicular to the beam, the F" values obtained by SLS and EC methods are more in agreement than when the wind direction was either irregular or parallel to the SLS beam path. Wind speed also seems to influence the F" estimates by the two methods since the agreement in the Fh values obtained by the two methods is greater when wind speed is higher compared to times of the day when the wind speed is reduced. The atmospheric stability influences the peak position of footprint with the peak footprint position being further from the measurement point when the atmospheric stability condition is closer to stable as denoted by the Obukhov length of -5 and closer to the measurement point for convectively unstable atmospheric conditions as shown by the Obukhov length of -30. Also shown is that a larger fetch is required when the atmosphere is convectively unstable as indicated by the contours plotted on top of the footprint plots. In general, there seems to be very good agreement in the sensible heat flux values obtained by the two methods, especially since SLS offers areal-averaged sensible heat flux measurements compared to the EC method which basically provides a point measurement. The SLS method therefore offers a better alternative for obtaining sensible heat flux from larger and heterogeneous area - although to a limit of250 m since beyond 250 m, the method suffers from a saturation problem.
Heat and energy exchange above different surfaces using surface renewal.
(2008) Mengistu, Michael Ghebrekidan.; Savage, Michael John.
The demand for the world’s increasingly scarce water supply is rising rapidly, challenging its availability for agriculture and other environmental uses, especially in water scarce countries, such as South Africa, with mean annual rainfall is well below the world’s average. The implementation of effective and sustainable water resources management strategies is then imperative, to meet these increasingly growing demands for water. Accurate assessment of evaporation is therefore crucial in agriculture and water resources management. Evaporation may be estimated using different micrometeorological methods, such as eddy covariance (EC), Bowen ratio energy balance (BR), surface renewal (SR), flux variance (FV), and surface layer scintillometry (SLS) methods. Despite the availability of different methods for estimating evaporation, each method has advantages and disadvantages, in terms of accuracy, simplicity, spatial representation, robustness, fetch, and cost. Invoking the shortened surface energy balance equation for which advection and stored canopy heat fluxes are neglected, the measurement of net irradiance, soil heat flux, and sensible heat flux allows the latent energy flux and hence the total evaporation amount to be estimated. The SR method for estimating sensible heat, latent energy, and other scalars has the advantage over other micrometeorological methods since it requires only measurement of the scalar of interest at one point. The SR analysis for estimating sensible heat flux from canopies involves high frequency air temperature measurements (typically 2 to 10 Hz) using 25 to 75 ìm diameter fine-wire thermocouples. The SR method is based on the idea that parcel of air near a surface is renewed by an air parcel from above. The SR method uses the square, cube, and fifth order of two consecutive air temperature differences from different time lags to determine sensible heat flux. Currently, there are three SR analysis approaches: an ideal SR analysis model based on structure function analysis; an SR analysis model with finite micro-front period; and an empirical SR analysis model based on similarity theory. The SR method based on structure function analysis must be calibrated against another standard method, such as the eddy covariance method to determine a weighting factor á which accounts for unequal heating of air parcels below the air temperature sensor height. The SR analysis model based on the finite micro-front time and the empirical SR analysis model based on similarity theory need the additional measurement of wind speed to estimate friction velocity. The weighting factor á depends on measurement height, canopy structure, thermocouple size, and the structure function air temperature lag. For this study, á for various canopy surfaces is determined by plotting the SR sensible heat flux SR H against eddy covariance EC H estimates with a linear fit forced through the origin. This study presents the use of the SR method, previously untested in South Africa, to estimate sensible heat flux density over a variety of surfaces: grassland; Triffid weed (Chromolaena odorata); Outeniqua Yellow wood (Podocarpus Falcatus) forest; heterogeneous surface (Jatropha curcas); and open water surface. The sensible heat flux estimates from the SR method are compared with measurements of sensible heat flux obtained using eddy covariance, Bowen ratio, flux variance, and surface layer scintillometer methods, to investigate the accuracy of the estimates. For all methods used except the Bowen ratio method, evaporation is estimated as a residual using the shortened energy balance from the measured sensible heat and from the additional measurements of net irradiance and soil heat flux density. Sensible heat flux SR H estimated using the SR analysis method based on air temperature structure functions at a height of 0.5 m above a grass canopy with a time lag r = 0.5 s, and á =1 showed very good agreement with the eddy covariance EC H , surface layer scintillometer SLS H , and Bowen ratio BR H estimates. The half-hourly latent energy flux estimates obtained using the SR method SR ë E at 0.5 m above the grass canopy for a time lag r = 0.5 s also showed very good agreement with EC ë E and SLS ë E . The 20-minute averages of SR ë E compared well with Bowen ratio BR ë E estimates. Sensible heat and latent energy fluxes over an alien invasive plant, Triffid weed (C. odorata) were estimated using SR , EC , FV and SLS methods. The performance of the three SR analysis approaches were evaluated for unstable conditions using four time lags r = 0.1, 0.4, 0.5, and 1.0 s. The best results were obtained using the empirical SR method with regression slopes of 0.89 and root mean square error (RMSE) values less than 30 W m-2 at measurement height z = 2.85 and 3.60 m above the soil surface for time lag r = 1.0 s. Half-hourly SR H estimates using r = 1.0 s showed very good agreement with the FV and SLS estimates. The SR latent energy flux, estimated as a residual of the energy balance ë ESR , using time lag r = 1.0 s provided good estimates of EC ë E , FV ë E , and SLS ë E for z = 2.85 and 3.60 m. The performance of the three SR analysis approaches for estimating sensible heat flux above an Outeniqua Yellow wood stand, were evaluated for stable and unstable conditions. Under stable conditions, the SR analysis approach using the micro-front time produced more accurate estimates of SR H than the other two SR analysis approaches. For unstable conditions, the SR analysis approach based on structure functions, corrected for á using EC comparisons produced superior estimates of SR H . An average value of 0.60 is found for á for this study for measurements made in the roughness sublayer. The SR latent energy flux density estimates SR ë E using SR H based on structure function analysis gave very good estimates compared with eddy covariance ( EC ë E ) estimates, with slopes near 1.0 and RMSE values in the range of 30 W m-2. The SR ë E estimates computed using the SR analysis approach using the micro-front time also gave good estimates comparable to EC ë E . The SR and EC methods were used to estimate long-term sensible heat and latent energy flux over a fetch-limited heterogeneous surface (J. curcas). The results show that it is possible to estimate long-term sensible heat and latent energy fluxes using the SR and EC methods over J. curcas. Continuous measurements of canopy height and leaf area index measurements are needed to determine á . The weighting factor á was approximately 1 for placement heights between 0.2 and 0.6 m above the Jatropha tree canopy. The daily sensible heat and latent energy flux estimates using the SR analysis gave excellent estimates of daily EC sensible heat and latent energy fluxes. Measurements of sensible heat and estimates of the latent energy fluxes were made for a small reservoir, using the SR and EC methods. The SR sensible heat flux SR H estimates were evaluated using two air temperature time lags r = 0.4 and 0.8 s at 1.0, 1.3, 1.9, 2.5 m above the water surface. An average á value of 0.175 for time lag r = 0.4 s and 0.188 for r = 0.8 s was obtained. The SR H and EC H estimates were small (-40 to 40 W m-2). The heat stored in water was larger in magnitude (-200 to 200 W m-2) compared to the sensible heat flux. The SR and EC latent energy fluxes were almost the same in magnitude as the available energy, due to the small values of the sensible heat fluxes. The daily evaporation rate ranged between 2.0 and 3.5 mm during the measurement period. The SR method can be used for routine estimation of sensible heat and latent energy fluxes with a reliable accuracy, over a variety of surfaces: short canopies, tall canopies, heterogeneous surface, and open water surface, if the weighting factor á is determined. Alternatively, the SR method can be used to estimate sensible heat flux which is exempt from calibration using the other two SR analysis approaches, with additional measurement of wind speed for estimating friction velocity iteratively. The advantages of the SR method over other micrometeorological methods are the relatively low cost, easy installation and maintenance, relatively low cost for replicate measurements. These investigations may pave the way for the creation of evaporation stations from which real-time and sub-hourly estimates of total evaporation may be obtained relatively inexpensively.
Sensible heat flux for estimating evaporation.
(2010) Savage, Michael John.
The focus of the research is on investigations of various methods for obtaining sensible heat flux (H) for estimating evaporation. The key for this approach is the application of the shortened energy balance equation, and in the case of methods based on the Monin-Obukhov similarity theory (MOST), such as surface-layer scintillometrv and temperature variance with adjusted for stability using air temperature skewness, and surface renewal (SR), die iterative procedures. The application of the shortened energy balance requires that errors associated with measurement of net irradiance (Rnet) and soil heat flux (S) are kept to a minimum To this end. methodology for the calibration of net radiometers for both the infrared and short wave irradiances receive attention. A field study attempts to quantify the error in soil heat flux measurement for a mesic grassland. A standard, convenient and accurate method for calibrating net radiometers would assist in unravelling reasons for the perplexing lack of surface energy balance closure when employing the eddy covariance (EC) flux estimation method as well as improve on the accuracy of the energy balance residual method for estimating evaporation. A relatively inexpensive, accurate and quick laboratory method, based on physical theory, for non-steady radiative conditions above a large water-heated or water-cooled radiator containing circulated water, with surface-embedded thermocouples is used to obtain reproducible net radiometer calibration factor's for the infrared waveband for a wide range in net irradiance. When applied, the method would reduce error m the most important term of the shortened energy balance and assist in energy balance closure aspects of EC measurements. The SLS method, reliant on MOST, is used for estimating a really-averaged H for a mesic grassland for a 30-month period. Comparisons with EC measurements feature prominently in this unique study. These comparisons include using different MOST procedures and the influence of the Bowen ratio on SLS measurement: of if is investigated. Furthermore, since there are reports in the literature that the EC method may underestimate H and or latent energy flux (LE), resulting in the shortened energy balance not being closed, effort is devoted to this aspect. Other methods used for comparison purposes are the traditional Bowen ratio energy balance (BREB), SR, TV and ETo (grass reference) methods. The TV and SLS and/or EC measurements of H are compared above three contrasting canopy surfaces. It is shown that other high frequency air temperature-based methods, for example, for the first time the TV method with adjustment for skewness, may pave the way for evaporation stations from which real-time and sub-hourly estimates may be obtained relatively inexpensively. Another area of research that receives attention is the placement height of EC instruments above short-canopy surfaces and a spectral analysis of the vertical wind speed and some temperature measurement: for close-canopv placement heights. The SR method is used to estimate, for the first time, open-water evaporation. The ideal SR method applied above canopies is the most inexpensive micrometeorological method for estimating H, but the SR weighting factor a needs to be determined using EC and for this reason, the TV method with adjustment for skewness was investigated. Finally, a unique implementation of SR uses an iterative method for calculating H. A similar iterative procedure is applied for MOST and ETo calculations.
Sensible heat flux under unstable conditions for sugarcane using temperature variance and surface renewal.
(2010) Nile, Eltayeb Sulieman.; Savage, Michael John.
Increased pressure on the available limited water resources for agricultural production has a significant impact on sugarcane production. Routine monitoring of evaporation with reliable accuracy is essential for irrigation scheduling, for more efficient use of the available water resources and for management purposes. An indirect method for estimating evaporation involves measuring the sensible heat flux (H) from which latent energy flux and hence total evaporation can be calculated, as a residual using the shortened energy balance from measurements of net irradiance and soil heat flux. Various methods for measuring H may include Bowen ratio energy balance, eddy covariance (EC), flux variance (FV), optical scintillation, surface renewal (SR) and temperature variance (TV). Each method has its own advantages and disadvantages, in terms of method theoretical assumptions, accuracy, complexity, cost, fetch requirements and power consumption. The TV and SR methods are inexpensive and reasonably simple with a reduced power requirement compared to other methods since they require high frequency air temperature data which is obtained by using an unshielded naturally-ventilated type-E fine-wire thermocouple at a single point above the canopy surface. The TV method is based on the Monin-Obukhov similarity theory (MOST) and uses the mean and standard deviation of the air temperature for each averaging period. Currently, there are two TV methods used for estimating sensible heat flux (HTV) at sub-hourly time intervals, one includes adjustment for stability, and a second that includes adjustment for air temperature skewness. Another method used to estimate sensible heat flux from the mean and standard deviation of air temperature is based on MOST and uses spatial second-order air temperature structure function. For the TV method adjusted for stability and the method based on MOST that uses a spatial second-order air temperature structure function, the Monin-Obukhov atmospheric stability parameter () is needed. The parameter  can be estimated from EC measurements or alternatively estimated independently using an iteration process using horizontal wind speed measurements. The TV method including adjustment for air temperature skewness requires the mean and standard deviation of the air temperature and air temperature skewness for each averaging time period as the only input. The SR method is based on the coherent structure concept. Currently, there are various SR models method for estimating sensible heat flux. These include an ideal SR analysis model method based on an air temperature structure function analysis, the SR analysis model with a finite micro-front period, combined SR with K-theory and combined SR model method based on MOST. The ideal SR analysis model based on an air temperature structure function analysis should be calibrated to determine the SR weighting factor (). The other SR approaches require additional measurements such as crop height and horizontal wind speed measurements. In all of the SR approaches, air temperature time lags are used when calculating the air temperature structure functions. In this study, the performance of TV and SR methods were evaluated for estimation of sensible heat and latent energy fluxes at different heights for air temperature time lags of 0.4 and 0.8 s for daytime unstable conditions against EC above a sugarcane canopy at the Baynesfield Estate in KwaZulu-Natal, South Africa. For all methods, latent energy flux (LE) and hence evaporation was estimated as a residual from the shortened energy balance equation using H estimates and net irradiance and soil heat flux density measurements. The ideal SR analysis model method based on an air temperature structure function analysis approach was calibrated and validated against the EC method above the sugarcane canopy using non-overlapping data sets for daytime unstable conditions during 2008. During the calibration period, the SR weighting factor was determined for each height and air temperature time lag. The magnitude of ranged from 0.66 to 0.55 for all measurement heights and an air temperature time lag of 0.8 s. The value increased with a decrease in measurement height and an increase in air temperature time lag. For the validation data set, the SR sensible heat flux (HSR) estimates corresponded well with EC sensible heat flux (HEC) for all heights and both air temperature time lags. The agreement between HSR and HEC improved with a decrease in measurement height for the air temperature time lag of 0.8 s. The best HSR vs HEC comparisons were obtained at a height of 0.20 m above the crop canopy using = 0.66 for an air temperature time lag of 0.8 s. The residual estimates of latent energy flux by SR and EC methods were in good agreement. The LESR at a height of 0.20 m above the canopy yielded the best comparisons with LEEC estimated as a residual. The performance of the TV method, including adjustment for stability, and
Sensible heat flux and evaporation for sparse vegetation using temperature-variance and a dual-source model.
(2010) Abraha, Michael Ghebrekristos.; Savage, Michael John.
The high population growth rate and rapid urbanization that the world is experiencing today has aggravated the competition for the already scarce resource ¡V water ¡V between the agricultural sector and the other economic sectors. Moreover, within the agricultural sector, water is increasingly being used for commercial plantations as opposed to growing food crops, threatening food security. Therefore, it is very important that this scarce resource is managed in an efficient and sustainable manner, for now and future use. This requires understanding the process of evaporation for accurate determination of water-use from agricultural lands. In the past, direct measurements of evaporation have proven difficult because of the cost and complexity of the available equipments, and level of expertise involved. This justifies a quest for relatively simple, accurate and inexpensive methods of determining evaporation for routine field applications. Estimation of sensible heat flux (H) from high frequency air temperature measurements and then calculating latent energy flux (ƒÜE) and hence evaporation as a residual of the shortened surface energy balance equation, assuming that closure is met, is appealing in this sense. Concurrent net irradiance (Rn) and soil heat flux (G) measurements can be conducted with relative ease for use in the energy balance equation. Alternately, evaporation can also be mathematically modelled, using single- or multi-layer models depending on vegetation cover, from less expensive routine meteorological observations. Therefore, the ultimate objective of this study is to estimate and model H and ƒÜE, and thereby evaporation, accurately over sparsely vegetated agricultural lands at low cost and effort. Temperature-variance (TV) and surface renewal (SR) methods, which use high-frequency (typically 2 to 10 Hz) air temperature measurements, are employed for estimation of H. The TV method is based on the Monin and Obukhov Similarity Theory (MOST) and uses statistical measures of the high frequency air temperature to estimate H, including adjustments for stability. The SR method is based on the principle that an air parcel near the surface is renewed by an air parcel from above and, to determine H, it uses higher order air temperature differences between two consecutive sample measurements lagged by a certain time interval. Single- and double-layer models that are based on energy and resistance combination theory were also used to estimate evaporation and H from sparse vegetation. Single- and double-layer models that were extended to include inputs of radiometric temperature in order to estimate H were also used. The transmission of solar irradiance to the soil beneath in sparse canopies is variable and depends on the vegetation density, cover and apparent position of the sun. A three-dimensional radiation interception model was developed to estimate this transmission of solar irradiance and was used as a sub-module in the double-layer models. Estimations of H from the TV (HTV), SR (HSR) and double-layer models were compared against H obtained from eddy covariance (HEC), and the modelled ƒÜE (single- and double-layer) were compared with that obtained from the shortened energy balance involving HEC. Besides, long-term ƒÜE calculated from the shortened energy balance using HTV and HSR were compared with those calculated using HEC. Unshielded and naturally-ventilated fine-wire chromel-constantan thermocouples (TCs), 75 ƒÝm in diameter, at different heights above the ground over sparse Jatropha curcas trees, mixed grassland community and bare fallow land were used to measure air temperature. A three-dimensional sonic anemometer mounted at a certain height above the ground surface was also used to measure virtual temperature and wind speed at all three sites. All measurements were done differentially at 10-Hz frequency. Additional measurements of Rn, G and soil water content (upper 60 mm) were also made. The Jatropha trees were planted in a 3-m plant and inter-row spacing in a 50 m ¡Ñ 60 m plot with the surrounding plots planted to a mixture of Jatropha trees and Kikuyu grass. Average tree height and leaf area index measurements were taken on monthly and bimonthly basis respectively. An automatic weather station about 10 m away from the edge of the Jatropha plot was also used to obtain solar irradiance, air temperature and relative humidity, wind speed and direction and precipitation data. Soil water content was measured to a depth of 1000 mm from the surface at 200 mm intervals. Soil and foliage surface temperatures were measured using two nadir-looking infrared thermometers with one mounted directly above bare soil and the other above the trees. The three-dimensional solar irradiance interception model was validated using measurements conducted on different trees and planting patterns. Solar irradiance above and below tree canopies was measured using LI-200 pyranometer and tube solarimeters respectively. Leaf area density (LAD) was estimated from LAI, canopy shape and volume measurements. It was also determined by scanning leaves using either destructive sampling or tracing method. The performance of the TV method over sparse vegetation of J. curcas, mixed grassland community and fallow land was evaluated against HEC. Atmospheric stability conditions were identified using (i) sensor height (z) and Obukhov length (L) obtained from EC and (ii) air temperature difference between two thermocouple measurement heights. The HTV estimations, adjusted and not adjusted for skewness (actual and estimated) of air temperature (sk), for unstable conditions only and for all stability conditions were used. An improved agreement in terms of slope, coefficient of determination (r2) and root mean square error (RMSE), almost over all surfaces, was obtained when the temperature difference rather than the z/L means of identifying stability conditions was used. The agreement between the HTV and HEC was improved for estimations adjusted for actual sk than not adjusted for sk. Improved agreement was also noted when HTV was adjusted using estimated sk compared to not adjusting for sk over J. curcas. The TV method could be used to estimate H for surfaces with varying homogeneity with reasonable accuracy. Long-term water-use of a fetch-limited sparse vegetation of J. curcas was determined as a residual of the shortened surface energy balance involving HTV and HSR and compared with those estimated using HEC. Concurrent measurements of Rn and G were also performed. The long-term water-use of J. curcas trees calculated from the shortened surface energy balance involving HTV and HSR agreed very well when compared with those obtained from HEC. The seasonal HTV and HSR also agreed very well when compared with HEC. Changes in structure of the canopy and environmental conditions appeared to influence partitioning of the available energy into H and ƒÜE. The seasonal total evaporation for the EC, TV and SR methods amounted to 626, 640 and 674 mm respectively with a total rainfall of 690 mm. Footprint analysis also revealed that greater than 80% of the measured flux during the day originates from within the surface of interest. The TV and SR methods, therefore, offer a relatively low-cost means for long-term estimation of H, and ƒÜE, hence the total evaporation, using the shortened surface energy balance along with measurements of Rn and G. Evaporation and biomass production estimations from tree crops requires accurate representation of solar irradiance transmission through the canopy. A relatively simple three-dimensional, hourly time-step tree-canopy radiation interception model was developed and validated using measurements conducted on isolated trees, hedgerows and tree canopies arranged in tramline mode. Measurements were obtained using tube solarimeters placed 0.5 m from each other starting from the base of a tree trunk in four directions, along and perpendicular to the row up to mid-way between trees and rows. Model-simulations of hourly radiant transmittance were in good agreement with measurements with an overall r2 of 0.91; Willmott.s index of agreement of 0.96; and general absolute standard deviation of 17.66%. Agreement between model-estimations and measurements, however, was influenced by distance and direction of the node from the tree trunk, sky conditions, symmetry of the canopy, and uniformity of the stand and leaf distribution of the canopy. The model could be useful in planning and management applications for a wide range of tree crops. Penman-Monteith (PM) equation and the Shuttleworth and Wallace (SW) model, representing single- and dual-source models respectively, were used to determine the total evaporation over a sparse vegetation of J. curcas from routine automatic weather station observations, resistance parameters and vegetation indices. The three-dimensional solar irradiance interception model was used as a sub-module in the SW model. The total evaporation from the sparse vegetation was also determined as a residual of the shortened surface energy balance using measurements of Rn, G and HEC. The PM equation failed to reproduce the .measured. daily total evaporation during periods of low LAI, with improved agreement with increased LAI. The SW model, however, produced total evaporation estimates that agreed very well with the .measured. with a slope of 0.96, r2 of 0.91 and RMSE of 0.45 mm for a LAI ranging from 0 (no leaves) to 1.83 m2 m-2. The SW model also estimated soil evaporation and plant transpiration separately, and about 66 % of the cumulative evaporation was attributed to soil evaporation. These findings suggest that the PM equation should be replaced by the SW model for surfaces that assume a range of LAI values during the growing season. The H was estimated using (i) SW model that was further developed to include surface radiometric temperature measurements; (ii) one-layer model, but linked with a two-layer model for estimation of excess resistance, that uses surface radiometric temperature; and (iii) the SW model (unmodified). The agreement between modelled and measured H, using 10-min data, was in general reasonably good with RMSE (W m-2) of 45.11, 43.77 and 39.86 for the three models respectively. The comparative results that were achieved from (iii) were not translated into the daily data as all models appeared to have a tendency to underestimate H. The resulting RMSEs for the daily H data for the three models were (MJ m-2) 1.16, 1.17 and 1.18 respectively. It appears that similar or better agreement between measured and estimated H can be forged without the need for surface radiometric temperature measurements. The study showed, in general, that high frequency air temperature measurements can be used to estimate H with reasonable accuracy using the simple and relatively low-cost TV and SR methods. Moreover, these methods can be used to calculate ƒÜE, hence ET, as a residual of the shortened surface energy balance equation along with measurements of Rn and G assuming that energy balance closure is met. The simple and low-cost nature of these methods makes replication of measurements easier and their robust nature allows long-term measurements of energy fluxes. The study also showed that H and ƒÜE can be modeled using energy and resistance combination equations with reasonable accuracy. It also reiterated that the SW-type models, which treat the plant canopy and soil components separately, are more appropriate for estimation of H and ƒÜE over sparse vegetation as opposed to the PM-type models.
Seasonal variation of surface energy fluxes above a mixed species and spatially homogeneous grassland.
(2011) Moyo, Nicholas C.; Savage, Michael John.
The increasing human population, industrialization, urbanisation and climate change challenges have resulted in an increased demand for already scarce water resources. This has left the agricultural sector with less water for production. Sustainable water management strategies would therefore require accurate determination of water-use. In agriculture, water-use can best be determined from total evaporation which is the loss of water from soil and vegetation to the atmosphere. Accurate quantification of total evaporation from vegetation would require a thorough understanding of water transport processes between vegetation and the atmosphere, especially in a water-scarce country like South Africa. Several methods for estimating total evaporation have been developed and are in use today. Some of the common methods used today are: the Bowen ratio energy balance, eddy covariance, scintillometry, flux variance and surface renewal. However, various methods have advantages and disadvantages. Considerations include the cost of equipment and level of skill required for use of some of the methods. A number of methods involve indirect or direct estimation of sensible heat flux then calculating latent energy flux and hence total evaporation as a residual of the shortened energy balance equation. The main objective of this study is to determine the effects of grassland management practices on the energy balance components as well as on the surface radiation balance. Eddy covariance and surface renewal methods were employed to investigate the effects of grassland management practices (mowing and burning) on the micrometeorology of naturally occurring grassland. A 4.5-ha grassland site (Ukulinga, Pietermaritzburg, South Africa) was divided into two halves: one area was initially mowed (cut-grass site) to a height of 0.1 m while the other was not mowed (tall-grass site). The tall-grass site was later treated by burning and hence referred to as the burnt-grass site. Two eddy covariance systems were deployed, one at each of the cut-grass and the tall-grass sites. The systems each comprised a three-dimensional sonic anemometer to measure high frequency sonic temperature, orthogonal wind speeds and directions and the eddy covariance sensible heat flux (W m-2). Latent energy flux, from which total evaporation was then determined, was calculated as a residual from the shortened energy balance equation from measurements of sensible heat flux, net irradiance and soil heat flux assuming closure is met. Other microclimatic measurements of soil water content, soil temperature, surface reflection coefficient and reflected solar irradiance were performed, the latter with a four-component net radiometer. An automatic weather station was also set up at the research site for continuous measurements of solar irradiance, air temperature, relative humidity, wind speed and direction and rainfall. Water vapour pressure and grass reference evaporation were also determined online. Energy fluxes from the tall-grass site were measured from March to June 2008. Greater total evaporation rates (2.27 mm day-1) were observed at the beginning of the experiment (March). As winter approached most of the energy balance components showed a constant decreasing trend and the average total evaporation rates for May and June were 1.03 and 0.62 mm day-1, respectively. The tall-grass site had consistently lower soil temperatures that changed diurnally when compared to the cut-grass site. The soil water content at both sites showed no significant differences. Most of the energy balance components were similar between the two sites and changed diurnally. Although there were small differences observed between other energy balance components, for example, latent energy flux was slightly greater for the tall-grass site than for the cut-grass site. The tall-grass site had more basal cover and this may have contributed to the differences in temperature regimes observed between the two sites. However, the plants growing at the cut-grass site showed more vigour than the ones at the tall-grass site as spring approached. Burning of a mixed grassland surface caused significant changes to most of the optical properties and energy fluxes of the surface. Following burning, the soil temperature was elevated to noticeable levels due to removal of basal cover by burning. The surface reflection coefficient measured before and after the burn also presented a remarkable change. The surface reflection coefficient was significantly reduced after the burn but a progressive increase was observed as the burnt grass recovered after the spell of spring rains. The energy fluxes: net irradiance, latent energy flux and soil heat flux also increased following the burn but the latent energy flux was reduced as transpiration was effectively eliminated by the burning of all actively transpiring leaves. As a result, the main process that contributed towards latent energy flux was soil evaporation. An ideal surface renewal analysis model based on two air temperature structure functions was used to estimate sensible heat flux over natural grassland treated by mowing. Two air temperature lag times r (0.4 and 0.8 s) were used when computing the air temperature structure functions online. The surface renewal sensible heat fluxes were computed using an iteration process in Excel. The fluxes, obtained using an iterative procedure, were calibrated to determine the surface renewal weighting factor (a) and then validated against the eddy covariance method using different data sets for unstable conditions during 2008. The latent energy flux was computed as a residual from the shortened energy balance equation. The surface renewal weighting factor was determined for each of the two heights and two lag times for each measurement height (z) above the soil surface. The a values obtained during the surface renewal calibration period (day of year 223 to 242, 2008) ranged from 1.90 to 2.26 for measurement height 0.7 m and r = 0.4 and 0.8 s. For a measurement height of 1.2 m and r = 0.4 and 0.8 s, a values of 0.71 and 1.01 were obtained, respectively. Good agreement between surface renewal sensible heat flux and eddy covariance sensible heat flux was obtained at a height of 1.2 m using a = 0.71 and a lag time of 0.4 s. Total evaporation for the surface renewal method was compared against the eddy covariance method. The surface renewal method, for a height of 1.2 m and a lag time of 0.4 s, yielded 1.67 mm while the eddy covariance method yielded 1.57 mm for a typical cloudless day. For the same day for a measurement height of 1.2 m and a lag time of 0.8 s, eddy covariance and surface renewal methods yielded 1.57 and 1.10 mm, respectively. For a lag time of 0.4 s, the surface renewal method overestimated total evaporation by 0.10 mm while for a lag time of 0.8 s, the total evaporation was underestimated by 0.47 mm. As a result, the surface renewal method performed better for z = 1.2 m and a lag time of 0.4 s. The eddy covariance method gave reliable sensible heat fluxes throughout the experiment and this allowed a comparison of fluxes across all treatment areas to be achieved. The short-term analysis of the surface renewal method also gave reliable energy fluxes after calibration. Compared to the eddy covariance method, the surface renewal method is more attractive in the sense that it is easy to operate and use and it is relatively cheap. However, the surface renewal method requires calibration and validation against a standard method such as the eddy covariance method. This study showed that grassland management practices had a considerable effect on surface radiation and energy balance of the mowed and burnt treatment sites. Total evaporation was mainly controlled by the available energy flux, rainfall and grassland surface structure. High total evaporation values were observed during summer when net irradiance was at its highest and grass growth at its peak. Low total evaporation values were observed in winter (dry atmospheric conditions) when net irradiance was at its lowest and most vegetation was dormant.
Monitoring fire danger in near real-time using field-based agrometeorological measurement systems.
(2014) Strydom, Sheldon.; Savage, Michael John.
Africa has been termed the ‘Fire Continent’ due to its high annual fire frequency. Wildfires are considered one of the most common disasters in South Africa resulting in a high number of human fatalities and financial loss on an annual basis. It is believed that increased population growth, as well as more concentrated settlement planning is likely to result in increased fire disasters and increased human fatalities as a direct result of wildfires. The high number of human fatalities and high financial loss associated with wildfires served as the main motivation for the research throughout all studies. While wildfires may provide beneficial environmental service, increased wildfire activity can result in a number of adverse effects on the environment, for example the removal of vegetation, fascilitating/aggravating floods and soil erosion but do bring with them positive effects such as nutrient recycling and removal of alien species. In order to better understand the spatial and temporal variations and characteristics of wildfires in South Africa an 11-year dataset of MODIS-derived Active Fire Hotspots was analysed using an open source geographic information system. The study included the mapping of national fire frequency over the 11-year period. Results indicate that the north-eastern regions of South Africa experience the greatest fire frequency, in particular the mountainous regions of KwaZulu-Natal, Mpumalanga and the Western Cape. Increasing trends in provincial fire frequency was observed in eight out of the nine provinces with Mpumalanga being the only province where a decrease in annual fire frequency over the study period was observed. Temporally, fires have been observed in all months for all provinces although distinct fire seasons were observed, largely driven by rainfall seasons. The South-Western regions of South Africa (winter rainfall patterns) experienced higher fire frequencies during the summer months with the rest of the country (summer rainfall) experiencing higher fire frequencies during the winter months. Regions which experience bi-modal rainfall seasons did not display distinct fire seasons. The study included an investigation into the likely effects of climate change on South African fire frequency. Three of the 11 years were identified as being climatologically anomalous. Fire frequencies in 2005 and 2010 (two of the warmest years in recent history) were significantly greater than normal years. Observed fire frequencies in 2008 were also significantly greater. The increased fire frequency was attributed to a severe La Niña event which may have resulted in increased vegetation growth prior to the dry season. A current issue with the mitigation of wildfires is the lack of proper real-time monitoring and measurement systems which can aid decision makers in the timing of controlled fires. The development of a system for improved monitoring of meteorological conditions conducive to fire was investigated. The traditionally used nomogram and lookup table used by Lowveld fire danger index (LFDI) system was replaced by mathematical functions which were then programmed into an automatic weather station datalogger. Near real-time results of the calculated LFDI were displayed in a web-based teaching, learning and research system found at: http://agromet.ukzn.ac.za:5355/?command=RTMC&screen=Fire%20danger%20index. Warm, dry mountain winds known as Berg winds have a direct link to fire weather, enhancing the danger of uncontrollable fires. Berg winds are associated with periods of increased air temperature, decreased relative humidity and increased wind speeds. This increases the potential for the development and spread of wildfires. The effects of Berg winds on the microclimate and fire danger were quantified. For this purpose, historic hourly meteorological data, local and international, were used together with a fuzzy logic system for determining Berg wind conditions in near real-time. This included the use of modelled diurnal sinusoidal functions for solar irradiance, air temperature, relative humidity, wind speed and direction, for various locations. Application of the system demonstrated that out of four sites in the KwaZulu-Natal Midlands, South Africa, Baynesfield was the most vulnerable to Berg wind occurrences, followed by Ukulinga (near Pietermaritzburg) and lastly Cedara. Boulder in Wyoming, USA, experienced a larger frequency of Berg wind events compared to Sidney in Montana, USA, despite being located at a higher altitude. The McArthur Fire Danger Meters (FDM) have been used to monitor and measure fire danger in Australia since the 1950s when they were first developed following severe wildfires. The McArthur FDM has traditionally made use of nomograms to calculate grassland and forest fire danger. In the 1980s mathematical descriptions of the McArthur FDM were developed. These then allowed for the sub-daily and near real-time measurement of grassland and forest fire danger. While commercial forestry contributes only a small percentage of South Africa’s GDP, forest fires have been identified as a common occurrence in commercial forestry plantations and natural forests, which can at times spread to neighbouring grasslands or non-forest vegetation. Forest fire disasters in South Africa are well documented by the mainstream media and it is believed that proper monitoring of forest fire danger, along with grassland fire danger, may mitigate potential wildfire disasters and limit human fatalities and financial loss. The McArthur FDM also provides opportunity to model fire behaviour – a component of wildfire mitigation lacking in the Lowveld fire danger index. The study utilized the equations developed to investigate the use of the McArthur FDM in South Africa by applying the equations to historical hourly meteorological data recorded at four locations in the KwaZulu-Natal Midlands, South Africa. Modifications to the McArthur grassland fire danger index (GFDI) were needed and resulted in the development of a method to calculate vegetation curing as a function of micrometeorological variables. Datasets were analysed to investigate seasonal curing at the four locations with results indicating high monthly curing averages in all months, with the greatest range of curing averages experienced in the winter months. Frequency analysis of both the GFDI and the forest fire danger index (FFDI) indicate that lower fire danger ratings are more common than high danger ratings as one would expect. The GFDI and FFDI displayed acceptable sensitivity to changes in the microclimate as observed through conducting sensitivity analysis and by plotting diurnal variations of GFDI and FFDI during Berg wind events. The fire behaviour modelling component of the McArthur FDM does not yield realistic results on steep topography but does provide a baseline on fire characteristics on gentler slopes
Modelling the impacts of increased air temperature on maize yields in selected areas of the South African highveld using the cropsyst model.
(2014) Pasi, Jonathan M.; Savage, Michael John.
Abstract available in PDF file.
Web-based teaching, learning and research using real-time data from field-based agrometeorological measurement systems.
(2014) Savage, Michael John.; Everson, Colin Stuart.
Abstract available in PDF file.

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