Browsing by Author "Savage, Michael John."

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Analysis and integration of regional scale temperature datasets into a seasonal crop monitoring system.
(2018) Magadzire, Tinomutenda Tamuka.; Savage, Michael John.; Funk, Christopher C.
Populations in excess of 20 million people in southern Africa annually face food insecurity. This number increases appreciably when detrimental seasonal climate conditions lead to widespread reductions in crop harvests. This situation has led to the development of regionalscale crop monitoring systems that incorporate crop-specific water balance (CSWB) models for early detection and warning of impending weather-related crop production shortfalls. Early warning of anticipated reductions in crop harvests facilitates early action in responding to potential crises. One such system, used by the Famine Early Warning Systems Network (FEWS NET) in southern Africa for crop monitoring, calculates the water requirements satisfaction index (WRSI) using a CSWB model. Operationally, CSWB models for calculating WRSI have used a static length of growing period (LGP) to bracket the period over which rainfall and evapotranspiration variations can affect crop yields. In the long term, concerns have been raised by some studies on the impact of rising air temperatures on crop production. There is therefore a need to incorporate the impacts of air temperature on crops directly into food security monitoring systems, in order to improve the accuracy of these monitoring systems in identifying and locating weather-related crop production shortfalls. This study sought to assess the potential improvements that can be introduced to the crop monitoring system in general and the WRSI in particular, by incorporating air temperature data into the CSWB model. To address this objective, daily maximum and minimum air temperature grids derived from a general circulation model reanalysis were used to generate thermal time estimates, expressed as growing degree day (GDD) grids for a maize crop. The GDDs were used to estimate the LGP of maize for each pixel of each summer season (which typically runs between around October and March) from 1982/1983 to 2016/2017 in southern Africa. The variable, temperature-driven LGP estimates compared favourably with LGP values obtained from literature for a few sample locations. The variable LGP was used to calculate the WRSI for 35 seasons, and the resultant WRSI showed improved correlation with historical yield estimates compared to the static-LGP WRSI, particularly after the farming practice of planting on multiple dates was taken into consideration. Various expressions of WRSI were considered in the analysis, including WRSI calculated assuming planting at the onset of rains, WRSI aggregated from varying number of separate planting dates, including three and six planting dates as test examples, and WRSI calculated using a modified soil water holding capacity to better capture local soil management practices. Historical maize yields for sub-national administrative units from seven southern African countries were correlated with the various WRSI expressions. Gridded GDD data that reflect the accumulated severity of extreme warm temperatures experienced during the crop growth period, referred to as extreme growing degree days, or eGDD, were also noted to have significant correlations with historical maize yields in several southern African countries. A number of variants of the eGDDs were tested, including eGDDs accumulated throughout the crop’s growth period, eGDDs that only occurred simultaneously with periods of crop water deficit, eGDDs that occurred during the crop flowering stage, and eGDDs scaled by the severity of crop water deficit. In several areas, the various eGDD expressions indicated higher correlations with yield than any of the WRSI variants indicated. The eGDD parameter showed strong correlations with WRSI, suggesting that the accumulated high temperatures were a reflection of the influence of low rainfall and low soil moisture during episodes of high temperature. More work is required to calibrate and refine the temperature-based monitoring parameters that were developed in this study, at local, sub-national scales. In particular, assumptions of the linearity of maize yield response for the various parameters should be tested. Potential improvements of the combined eGDD-water deficit parameter through the incorporation of prediction coefficients and constants should also be tested. A secondary aim of the study was to explore how readily available temperature-related datasets can be utilized to derive air-temperature metrics. To this end, satellite-derived thermal infrared (TIR) brightness temperature data were analysed, and a method was developed for identifying cloud cover, while simultaneously estimating cloud-free diurnal brightness temperature curves, using a single TIR satellite channel. The diurnal brightness temperature curves were developed using a sinusoidal and exponential model for daytime and nighttime respectively, utilizing modifications that enabled the curves to be estimated from two known temperatures at any two given times with cloud-free brightness temperature scenes. Comparison of the cloud mask developed in this study with an existing operational cloud mask based on a methodology developed by the EUMETSAT Satellite Applications Facility for Nowcasting gave an accuracy of 85.4%, when the operational method was considered as truth in a confusion matrix analysis. Situations were identified in which the different cloud detection methods showed superior performance, and could therefore complement each other. A statistical method was also developed for calibrating the cloud-free brightness temperatures to station-observed 2-m air temperatures using relationships between the means and diurnal temperature ranges of the two datasets. This enabled the identification of periods of occurrence of extreme warm air temperatures with a coefficient of determination of 0.91, and demonstrated the potential for the usage of TIR data for generating estimates of useful air temperature metrics. The efficiency of the algorithms that were used for simultaneous cloud masking and generation of cloud-free brightness temperature should be improved, in order to enable the methodology to be scaled up to a regional or global gridded level of analysis. Further work for improving operational gridded air temperature datasets by combining station-observed temperature data, modelled data from global circulation models, satellite-derived modelled cloud-free brightness temperature data and cloud masks is recommended.
Application of crop models in assessing impact of climate change and identifying adaptation options in a tropical environment in Ethiopia.
(2021) Feleke, Hirut Getachew.; Savage, Michael John.; Tesfaye, Kindie.
Abstract available in PDF.
Assessment of vulnerability of cattle farming to climate variability and change in South Africa.
(2021) Masemola, Makgethwa Jillie.; Savage, Michael John.; Tongwane, Mphethe Isaac.
Livestock are dependent upon weather for their comfort and food supplies. Sometimes, adverse weather conditions can cause production losses, especially if experienced during critical stages of growth. Heat stress is a major cause of production losses in the dairy and beef industries. Heat stress occurs when the temperature of the environment increases above the comfort zone of cattle as a result of solar irradiance. Heat stress decreases grazing and feed intake in cattle, while drought can limit pasture availability for grazing cattle. The temperature-humidity index (THI), a combination of air temperature and relative humidity, was used to determine the influence of heat stress on the productivity of cattle. The aim of the study was to investigate the air temperature and relative humidity conditions over South Africa accountable for high THI values for cattle farming for the period 1985 to 2015. The standard precipitation index (SPI) and normalised difference vegetation index (NDVI) at three months were computed to assess the soil moisture conditions and vegetation greenness for the season with high THI averages. The THI data analysis was performed seasonally, using a 15-year average and daily values from 75 weather stations in South Africa. Monthly rainfall data from 192 weather stations were used to compute SPI at three months. The NDVI used MODIS satellite information to create vegetation images for the three summer months. Results indicated summer as a season when cattle are vulnerable to heat stress. The periods (2005/06, 2007/08, 2012/13, 2013/14) experienced high seasonal averages (THI > 80) compared to the remaining years. Daily THI extremes were prevalent in February in South Africa. The SPI results indicated that the North West, the western Free State and east of the Eastern Cape provinces were vulnerable to dry conditions for the four summer periods. The NDVI results indicated that the eastern and coastal parts of South Africa were areas of high vegetation activity and greenness for all the summers. The Northern Cape, Limpopo, North West and Free State provinces had vegetation that was vulnerable to dry conditions. Heat stress and a healthy vegetation activity were a problem for the Northern Cape and Limpopo provinces, while KwaZulu-Natal and Mpumalanga had heat stress as their only challenge. The northern and eastern parts of South Africa were heat stress areas with air temperature as the main driver in THI. New smaller areas were developing as heat stress zones, and areas that were identified as heat stress zones in the past were increasing in size for South Africa. Natural grazing areas indicated no significant relationship with THI but showed a relationship with SPI. Cattle farmers situated in the heat stressed regions of South Africa and new heat stress developing areas need to take the necessary precautions to make profitable decisions for their cattle production.
Bowen ratio and surface temperature techniques for measuring evaporation from cabbages.
(1997) Lukangu, Gastao.; Savage, Michael John.; Johnston, M. A.
Good irrigation water management requires accurate, automated, non-destructive and simple techniques to measure crop water consumption. The actual evaporation from a cabbage crop was measured using the Bowen ratio energy balance technique (BREB), the surface temperature technique and the Penman-monteith method. All models used the shortened energy balance equation to estimate latent heat in which the advected energy is assumed to be negligible. Four irrigations were applied and 17 rainfall events were recorded during the experiment. The soil at the experimental field was a clay loam. An attempt to detect and reduce measurement error that could result from using inaccurate sensors was performed by calibrating the sensors. Data from inaccurate sensors were not used to compute the latent heat. Error and sensitivity analyse were performed, and the integrity of the weather data using the estimates of weather data from an appropriate model were checked. In addition, a comparative study showed that, for daily totals, there was a very small error in the latent heat calculations when fixed "constants" (density of air, specific heat capacity of air, psychrometric constant, slope of the saturation water vapour pressure vs temperature relationship and specific heat capacity of soil) were used instead of calculated ones. The Bowen ratio (β), a fundamental input of the BREB technique, was estimated accepting the Similarity Principle and excluding nighttime data. However, an error in β was also observed during the daytime measurement of the profiles entities because the sensors were wet and the stability condition was different from neutral conditions under which the Similarity Principle could not be observed. Negative values of β were observed when there were strong winds advecting sensible heat into the field under study. Data were rejected during mornings, and during strong advection periods. Data were also rejected when the sensors were wet because of rain or irrigation. In this experiment, only 35 % of data were valid for determining latent and sensible heat estimated using the BREB technique. Comparative analysis showed that the BREB technique overestimated the latent heat by 17 % compared to the Penman-Monteith method. However, both the Penman-Monteith method and BREB technique could not be trusted because of the presence of advection, a component of the energy balance equation normally assumed to be negligible. Either the surface to air temperature differential or the aerodynamic resistance, or both, were the source of overestimation of latent heat using the surface temperature technique. The surface to air temperature differential was large in magnitude when there were high wind speeds and drier conditions in the upwind field . It was small with lighter wind speeds and wetter surface conditions. An error of less than 5 % was attributed to the use of fixed air density and specific heat capacity and acceptance of 2 % and 20 % error in measuring the net irradiance and soil heat flux density, respectively. A comparative study showed that the surface temperature latent heat was overestimated in relation to the Penman-Monteith and BREB latent heat. Generally, the technique has been reported to overestimate evaporation, although to a lesser extent than the 57 % error reported in this experiment when compared to the BREB technique. An analysis of the energy balance closure, taking the Penman-Monteith and BREB as standards, suggested that the surface temperature technique overestimated the consumption of sensible heat from the air. This observation was also confirmed when the eddy correlation technique was used to compare sensible heat estimated using the surface temperature technique. The effect of placement height of air temperature sensors suggested that the consumption of sensible heat would be overestimated if the sensor was placed far from the crop surface. This overestimation in consumption of sensible heat resulted in an overestimation of latent heat. Irrigation water management was analysed using the crop water stress index (CWSI). The CWSI was calculated using the actual to potential evaporation ratio estimated using the Penman-Monteith method and the surface temperature techniques. The estimated and measured actual surface to air temperature differential, and the estimated potential and non-transpiring surface to air temperature differential were also used to estimate the CWSI using the Penman-Monteith method, the surface temperature technique and empirical method. The estimates of the CWSI using these techniques were inaccurate because of the poor correlation between the surface to air temperature differential and the water vapour pressure deficit (or water vapour pressure deficit and net irradiance). However, use of the CWSI estimated using the actual to potential evaporation ratio (CWSI = 1 - λ(a)/ λE(p) compared well to the standard CWSI determined using the Penman-Monteith approach. The actual canopy resistance was estimated using an empirical equation based on the potential canopy resistance, solar irradiance, soil water content and the shelter factor. A value of 50 s m(-1) was estimated for potential (minimum) canopy resistance of the cabbage crop. The soil water content was poorly correlated to CWSI, while the canopy resistance was well correlated. Comparative analysis showed that the estimated soil water content using the soil water balance equation was underestimated in relation to the soil water content measured using the ThetaProbe (frequency domain reflectometry technique) when the evaporation component was overestimated, and vice versa. Soil water content was underestimated throughout the experiment when evaporation from the surface temperature technique was used. There was an underestimation of soil water content in the early stages and overestimation in later stages of the experiment when the BREB and Penman-Monteith evaporation were used. Use of the estimated soil water content using the soil water balance with the overestimated evaporation would result in an early date of irrigation application, an unnecessarily large irrigation amount and frequent irrigations. More research is needed to find the cause of overestimation of evaporation using the surface temperature technique. The robustness of the equipment allowed a long period of measurement without frequent maintenance, as was required when using the BREB technique. The technique can monitor evaporation and irrigation management aspects at a regional scale. A combination of the Penman-Monteith, surface temperature and empirical method can assist the estimation of the crop water requirement by determining the CWSI. Future research would focus on quantification of sensible and latent heat advection, and analysis of additional resistances to water vapour flow from the surface to the atmosphere. The equipment for the BREB should be refined so that it measures actual latent heat under adverse weather conditions for a protracted period. A precise use of the soil water balance equation for water management should take into consideration runoff, vertical flow of soil water through a profile, intercepted water on plant surfaces and an accurately determined evaporation.
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.
Detection and early warning of lightning and extreme storm events in KwaZulu-Natal, South Africa.
(2020) Mahomed, Maqsooda.; Clulow, Alistair David.; Mabhaudhi, Tafadzwanashe.; Savage, Michael John.; Chetty, Kershani Tinisha.; Strydom, Sheldon.
Abstract available in pdf.
The effect of shadenetting on ‘3-29-5’ avocado production under subtropical conditions.
(2016) Mazhawu, Evidence.; Savage, Michael John.; Blakey, Robert John.; Tesfay, Samson Zeray.
Abstract available in PDF file.
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.
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.
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.
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.
Incorporating the Canegro sugarcane model into the DSSAT V4 cropping system model framework.
(2013) Jones, Matthew Robert.; Savage, Michael John.
Canegro is a leading sugarcane crop simulation model and has been used extensively in agronomic research and management. The model has been under development since the late 1980s at the South African Sugarcane Research Institute (SASRI). The Decision Support System for Agrotechnology Transfer (DSSAT) is a software package containing models for a wide range of field crops, and utilities for processing, storing and analysing model inputs and outputs. Canegro was included as part of version 3.1 of DSSAT in the mid-1990s. The SASRI Canegro model was subsequently developed further, but these changes were never integrated, nor incorporated, into DSSAT. DSSAT has also developed substantially, and as of version 4 adopted a modular Cropping System Model (CSM) structure, providing numerous scientific and practical advantages over previous non-modular versions. The DSSAT-Canegro v.3 model was not modified to use this modular structure. Following recognition of the advantages offered by DSSAT and its modular CSM, a project was initiated to incorporate the Canegro model into the DSSAT CSM. The project entailed: (i) restructuring and integrating the current Canegro plant growth and development code into the DSSAT v4 CSM modular framework, making use of its generic modules for management, soil, weather and the energy balance; (ii) verification of DSSAT CSM Canegro model results against the current SASRI version of Canegro to ensure that the new model produced similar results to the original model, for a set of simulated situations; and (iii) evaluation of the new DSSAT CSM Canegro model against experimental datasets. The new DSSAT v4 CSM Canegro model has been verified to behave identically to the SASRI Canegro model when the water balance is not modelled and growth can occur at climatic potential rates. When the water balance is simulated but where the crop is not stressed, near identical output is produced by both models. Under water-stressed conditions, some discrepancies appear between the two models, due to differences in the calculation of reference evaporation, soil surface evaporation and runoff. Validation of the new model against data from 16 experimental crops produced root mean squared errors of 6.62 t ha-1 for stalk dry mass and 3.59 t ha-1 for sucrose mass – very similar to published values for Canegro. This project has yielded a functional, well-documented, maintainable and user-friendly version of the Canegro model, which is available for universal use via the official release of the DSSAT v4.5.
Influence of land cover degradation on the water balance of the Northern Drakensberg high altitude mesic grasslands, South Africa.
(2022) Gray, Byron Andrew.; Toucher, Michele Lynn.; Clulow, Alistair David.; Savage, Michael John.
Mountainous regions provide vital ecosystem services, such as water provisions to low land areas. However, these regions are also considered sensitive to the effects of environmental change, due to their high levels of endemism and biodiversity. Thus, environmental change within these regions could have significant consequences beyond the extent of the region itself. An important implication of environmental change is the impact a change in land cover could have on the water balance of a catchment, especially within the headwater catchments. The Drakensberg mountains in South Africa, is such a mountain region, which is vital for its provision of ecosystem services and generation of water resources. This mountainous region is under threat from anthropogenic environmental change. The Drakensberg mountain range has been identified within South Africa as a strategic water source area (SWSA) and within the Northern-Drakensberg SWSA exists the Maloti-Drakensberg Park, which is a protected area managed by Ezemvelo KZN Wildlife and a World Heritage site. The Northern-Drakensberg mountain range natural grasslands are under threat from two forms of land cover change degradation, that of woody encroachment and following disturbance, invasion of bracken fern. Woody encroachment occurs within the natural grasslands following the removal of fire, which is of concern within the protected areas, where fire is a current management tool to maintain the natural grassland cover. Outside the protected area, disturbances due to human activities such as overgrazing and poor land management have led to a substantial level of degradation occurring, providing ideal conditions for the invasion of bracken fern. To safeguard and maintain the assurance of supply from an important SWSA of South Africa, there is the need for improved understanding on the impacts of land cover change and degradation within the Northern-Drakensberg mountain range on the water balance of the region. Thus, the overall aim of this thesis is to understand the influence of land cover change related degradation on the water balance of the Northern-Drakensberg high-altitude mesic grasslands and what this in turn means for the water supply generated from this SWSA of South Africa. To achieve the main aim, the research was scaled from a point measurement to a basin scale where management decisions are made. To improve our understanding of the impact of degradation related land cover change on the water balance an observational approach is required. Located within the Maloti-Drakensberg Park is the long-term Cathedral Peak research catchments which provided the platform for the observation component of this research. Of interest in the Cathedral Peak research catchments were three hydrologically individual catchments. Catchment III which consists of a degraded bracken fern land cover following the historical research experiments within the catchment on the impacts of commercial Pinus patula plantation on streamflow. Despite rehabilitation efforts following the removal of the plantation, the catchment transitioned to a degraded state and is currently near-completely invaded with bracken fern. Catchment VI which is under natural grassland condition, is managed with a bi-annual spring burn as proven best practice within the Drakensberg region. This catchment formed the baseline catchment for this research. Catchment IX is a woody encroached land cover following the protection of fire since the 1950s as part of research into the implications of removing fire from the natural grasslands. These three catchments provided the ideal platform from which the change in water balance components under each land cover could be monitored and investigated. The process of evapotranspiration (ET), which forms the connection between the energy and water balances, is well understood to be one of the most affected components of the water balance following a change of land cover. Therefore, ET was the focus of the initial point observation research. Due to the cost and stringency of the prominent method for ET measurement of eddy covariance (EC), an alternative more financially feasible method of surface renewal (SR) was tested over both Leucosidea sericea (woody) and Pteridium aquilinum (bracken) vegetation in comparison to EC. It was determined that the SR method, and in particular the SR dissipation theory (SRDT) method, which is independent of EC, was the best alternative and cheapest method to EC. The SR method is now used within the Cathedral Peak research catchments for long-term estimations of ET over both vegetation canopies. During this research, calibration factors (α) for the surface renewal 1 (SR1) method were determined for both the Leucosidea sericea (woody) and Pteridium aquilinum (bracken) vegetation types for winter and summer. Having confirmed the SRDT ability as an alternative to EC, the seasonal ET of Leucosidea sericea (woody) and Pteridium aquilinum (bracken) was determined. Providing first insight into the seasonality of ET over these vegetation types. Both vegetations followed a similar seasonal ET cycle to the natural grasslands, with the largest difference occurring in winter when the grasslands become dormant. It was also found that the energy balance was altered under the degraded land covers, with both forms increasing available energy and latent heat flux. Following the understanding of the seasonal change in ET through point measurements, the research focus was scaled up to the research catchment. A comparison of catchment VI and IX was conducted to identify changes in the headwater catchment water balance between the natural grassland and woody encroachment land covers. Long-term data sets of precipitation and streamflow from the Cathedral Peak research catchments, in combination with the seasonal ET observations showed that over time, as woody encroachment increased, the catchment rainfall:runoff response ratio decreased, as did streamflow under woody encroachment compared to natural grassland. Having gained an understanding of the impacts on a water balance of a headwater catchment, hydrological modelling in combination with scenario analysis was used to understand the impacts on water supply from the upper-uThukela catchment were both land cover degradation threats left unmanaged. Land cover parameters were unavailable for Leucosidea sericea (woody) and Pteridium aquilinum (bracken), and therefore were derived from understanding gained during observation. The ACRU agrohydrological model was utilised and confirmed to simulate current land cover conditions within the upper-uThukela satisfactorily at both the headwater and catchment levels. It was identified that both forms of land cover change resulted in a reduction in streamflow. This was largest for woody encroachment. The most affected flows were the low flows and winter dry period flows. Headwater catchments were also identified as the most impacted by land cover change. The key conclusions of the research were: • that the surface renewal methodology is a viable alternative for obtaining estimates of evapotranspiration over indigenous vegetation types; • that the energy balance and ET of woody vegetation in comparison to the natural grassland was significantly altered; • the importance of fire, as not only a management tool for the maintenance of the natural grasslands, but also to ensure the sustainability of the vital water resources and ensuring water security; • that there is an evident lag between the onset of degradation in the form of woody encroachment and the resultant impacts on streamflow; • the disproportionally large impact degradation related land cover change within these headwater catchments has on the downstream water balance relative to low land catchments. Following the analysis and with the understanding gained, it is recommended that the natural grasslands continue to be managed using fire, and continued protection of the natural grasslands needs to be maintained. Observation within these headwaters is key to improving the understanding of change and to drive decision making, allowing for the optimal management of the important SWSA.
Investigating possible impact of climate change on sugarcane production in KwaZulu-Natal, South Africa.
(2016) Sithole, Bonga Benson.; Savage, Michael John.
The KwaZulu-Natal Department of Agriculture, Environmental Affairs and Rural Development in 2010 undertook to investigate the vulnerability of KwaZulu-Natal to climate change which identified various sectors of the economy in the region that are impacted by climate change. The aim of the current study was to investigate the possible impact of climate change on sugarcane production in KwaZulu-Natal. The main objective of the study was to identify and synthesize current knowledge, scientific literature and data relating to specific aspects of climate change in KwaZulu-Natal, South Africa. In order to achieve the objectives of the study, a questionnaire was developed to ascertain sugarcane farmers’ awareness about climate change. Based on questionnaire data, the study reveals that sugarcane farmers are aware of the effect of climate change on sugarcane production. Cane growers are also aware of their activities that contribute to climate change. Climatological data for the region were collected and analyzed through the Decision Support System Agro-tech Transfer (DSSAT) daily crop model to assess possible climate change impacts on sugarcane production. Daily rainfall, solar radiation, wind-speed, air temperature (minimum and maximum) and dew point temperature data for various sugarcane mill supply areas were collected from the South African Sugar Research Institute. RClimDex (1.0) software was used to determine if climate change did occur for the period 1966 to 2016. The climate data sets were positive to climate change with respect to daily maximum air temperature, daily minimum air temperature, daily rainfall, daily dew point temperature and daily solar radiation. The impact of climate change on sugarcane production in KwaZulu-Natal has been recognized as the main cause for yield reduction. No major decline in sugarcane production has been noted in KwaZulu-Natal for those farmers practicing irrigation and improved management. The study also demonstrates an increase in the amount of trash on sugarcane in the latter years of the study period. In general, the approach presented in this study encompassed and assessed the effect of climate change on sugarcane production with inclusion farmers perception can be considered as a strategic issue on existing climate change concern in KwaZulu-Natal province. Further research about what measures have been implemented by other countries in addressing climate change is recommended.
Investigation of microclimate for human comfort in the natural environment and in a car parked in the open.
(2016) Luthuli, Sithandiwe Ignatia.; Savage, Michael John.
Abstract available in PDF file.
Irrigation control system with a web-based interface for the management of Eucalyptus planting stock.
(2015) Kaptein, Nkosinathi David.; Savage, Michael John.
Commercial forestry nurseries use large quantities of water to irrigate the planting stock to meet the annual forestry industry planting demands. However, South Africa is a water scarce country and there is high competition for this limited resource with other sectors. Thus, sustainable water management strategies should be put in place to preserve this precious resource. In commercial forestry nurseries, sustainable water use may be achieved by carefully managing irrigation schedules, such as accurately measuring growing media water content and then replenishing the depleted soil water. Improved nursery irrigation management may not only save water, but also has the potential to reduce the prevalence of pests and diseases, excessive leaching of nutrients, irrigation costs and may produce robust planting stock that is better suited to adapt to field conditions. Growing media water content can be directly measured by the gravimetric method. However, this method is laborious, time consuming, costly and does not allow for near real-time monitoring and control. Several indirect methods for estimating soil water content have been developed and are in use today. Some common methods are: frequent domain reflectometry, time domain reflectometry, time domain transmission and the dual-needle heat pulse method. However, each method has advantages and disadvantages. Considerations for choosing the most appropriate method are the ability to automate, accuracy and precision, skills required to use and the costs to purchase. The main objective of this study was to calibrate the low cost commercially available Decagon EC-5 soil water content sensors using nursery growing media. The calibration equation was then used to measure and control irrigation for Eucalyptus grandis x Eucalyptus urophylla and Eucalyptus dunnii planting stock grown in seedling trays in a greenhouse. A web-based data and information system was utilised to share measurements and display greenhouse environmental conditions in near real-time. The data could be viewed or downloaded using the internet1. Decagon EC-5 soil water content sensors were laboratory-calibrated, using nursery growing media, against the standard gravimetric method. Four nursery growing media were used for calibrations: coir/perlite mix (CP), coir/pine bark/vermiculite mix (CPBV), pine bark (PB) and sandy soil. The calibration relationships between gravimetric water content and sensor output for each growing media were established, and the manufacturer supplied calibration equation was evaluated against laboratory calibration equations. The appropriate laboratory calibration equation was programmed in the datalogger to measure and control irrigation. Greenhouse microclimate measurements of air temperature, relative humidity and solar irradiance were conducted. Hourly grass reference evaporation (ETo) was calculated by the datalogger. The greenhouse microclimate measurements were compared against an open area automatic weather station at the University of KwaZulu-Natal Agrometeorology Instrumentation Mast (UKZN AIM) system measurements. The EC-5 soil water content sensors were used to schedule irrigation for E. grandis x E. urophylla hybrid clones (GxU) (Experiment 1) and E. dunnii seedlings (Experiment 2) grown in seedling trays inside a fully air temperature controlled greenhouse. Irrigation was controlled in three treatments: low, medium and high watering. All treatments were treated the same except for differing irrigation application. Total daily irrigation and drainage were measured per treatment. Irrigation scheduling for GxU was programmed at a set point and E. dunnii seedlings at lower and drained upper limits. Seedling measurements conducted were root collar diameter (RCD), heights, stomatal conductance (gs), root-to-shoot ratio and total leaf area. Total drainage and its electrical conductivity (EC) was also measured. The calibration relationship showed a linear relationship between gravimetric water content and sensor output for all four growing media with an R2 greater than 0.92. The manufacturer supplied calibration equation poorly estimated growing media water content compared to the laboratory calibration. Poor estimation exceeded the 5% error specified by the manufacturer. Air temperature was consistently less than 25°C inside the greenhouse. The external air temperature, as measured by the UKZN AIM system, fluctuated and reached a maximum of 36.6°C during the study period. Solar irradiance inside the greenhouse was 60% lower than that measured by the UKZN AIM system. The relative humidity was consistently higher inside the greenhouse compared to that measured by the UKZN AIM system. Greenhouse grass reference evaporation was consistently lower than the UKZN AIM system due to low air temperature and high RH inside the greenhouse. For Experiment 1, the GxU clones were irrigated too frequently for short periods. This led to over- and under- irrigation in high and low watering treatments, respectively. However, these challenges were addressed in Experiment 2 using E. dunnii seedlings irrigated at lower and drained upper irrigation limits. In Experiment 2, variability in sensor measurements within each treatment were observed at drained upper limit and decreased at lower limit. This was likely caused by a change in the pore space volume from dry to wet growing media. The web-based system was successfully used as an early warning system to monitor soil water content measurements and greenhouse microclimate, averting experimental failure due to lack of irrigation on one occasion. Seedlings in the high watering treatment had the highest RCD, heights and gs followed by the medium and low watering treatments. Although the low watering treatment had the lowest growth rates and gs these seedlings were more robust, hardy and resistant to water stress. The root-to-shoot ratio showed no statistically significant differences between treatments. However, seedlings in the high watering treatment had slightly greater root volume. This was probably due to the increased total seedling leaf area for this treatment which facilitated increased photosynthetic activity and carbohydrates production, enabling increased root growth. The highest EC measurements were recorded in the low watering treatment. This was likely due to low irrigation and therefore nutrients were not washed off the growing media. Medium watering treatment EC was 30% lower than the low watering treatment whilst high watering EC was almost equivalent to the irrigation water. The analysis of economics showed that implementing the fully automated system could be costly. However, there are many potential benefits that may be offered by this system such as reduction in water use, pumping costs and management time. The early warning offered by this system could potentially help avoid the loss of planting stock if there is a problem with the irrigation system. The study showed that irrigation may be automatically scheduled for nursery seedlings trays using low cost Decagon EC-5 soil water content sensors with reasonable accuracy. However, medium-specific calibration is important to improve the soil water content measurement accuracy. The study also showed that reducing irrigation may result in reduced growth rates of seedlings. However, other benefits such as seedling resistance to water stress, robust seedlings, irrigation water savings and a reduction in washing off nutrients may be achieved.
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.
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.
Microclimate modification to improve productivity of ‘Carmen®-Hass’ avocado orchards using shadenet under subtropical conditions of Limpopo Province, South Africa.
(2016) Malapana, Clarance Kgethego.; Savage, Michael John.
The agricultural environment is a complex and dynamic system. Microclimate, the crop, biosphere, and management practices interact to determine the best yield production. South Africa is a water-scarce country, with high variability in annual rainfall. Thus, water quality and quantity are major limiting factors in agriculture. Hence, shadenetting can be used to modify the orchard microclimate to make the environment more conducive for fruit production. The South African avocado industry is export-oriented, so there is a commercial need to optimise the exportable percentage of avocado fruit. Sunburn, wind and hail damage and small fruit size as a result of water stress are the major cull factors for the industry. It is believed that shadenetting, with changes in management practices, can counter these limiting factors. There is no literature on growing avocado fruit under shadenetting. Therefore, the aim of the research was to determine the effects of a 20% white shadenet on ‘Carmen®-Hass’ avocado orchards and productivity. The long term objective is to improve avocado fruit quality and profitability in the Mooketsi Valley, Limpopo province, South Africa, a subtropical environment by reducing abiotic stress, particularly, solar irradiance, heat and wind. The trial was conducted at Goedgelegen Estate in the Mooketsi Valley on ‘Carmen®-Hass’ trees planted in 2007/8 season. A 1-ha shadenet structure (6 m high) was used, with 20% white shadenet over the roof and 40% green shadenet on the sides. Air and canopy temperature, relative humidity, wind speed, solar irradiance and leaf wetness duration (LWD) and sap flow were monitored at a sub-hourly rate. Evapotranspiration was calculated from the above mentioned parameters. Irrigation was monitored five times per week using tensiometers at 300- and 600-mm soil depths. The comparison between open and shadenet leaf areas showed that leaves in the open treatment were reduced as a result of the abiotic stress. Fruit water content under the shadenet compared to the open was greater, such enabled fruit under the shadenet to reach maturity two weeks earlier when compared with open treatment. Air and canopy temperature and relative humidity were slightly reduced under the shadenet, with the greatest difference occurring during the flowering period in mid-winter. The modification in air temperature and relative humidity was beneficial for bee activity and pollination in 2015 compared to the 2014 season. ‘Carmen®-Hass’ flowers in mid-winter when temperature conditions are not conducive for pollination. Canopy temperature was also reduced under shadenetting compared to the open treatment. The reduction was due to differences in tree density and the role that shadenetting plays. The infrared thermometer measurements were uniform with dense canopies compared to sparse tree canopies. The midday incoming solar irradiance was reduced by 18% under the shadenet compared to the open treatment. Calm conditions were experienced under the shadenet. Hence, windspeed was reduced to negligible levels. Also, the shadenet resist air flow to a certain height compared to the open treatment. LWD was extended by 12% under shadenet. An infestation of the insect pest citrus leaf roller (Archips occidentalis) caused severe damage to the fruit during the 2014/15 season due to the high plant density used. Significant results were that evapotranspiration was reduced by 14 and 29% less water was applied under the shadenet to maintain an adequate soil water content compared to the open treatment. Fruit reached minimum maturity two weeks earlier under shadenet compared to the open treatment. Fruit quality and pack-out were improved under the shadenet due to reduction in sunburn, wind damage and small fruit. But poor yields were experienced during the 2014 season due to poor bee activity, pollination and fruit size distribution were reduced under the shadenet compared to the open treatment. But following the improved bee activity in 2015, the 2016 normal season yield is likely to be improved under the shadenet than in the open treatment. Data collected in the Mooketsi Valley showed that 20% white shadenet has modified the microclimate and improved fruit quality. The water use under the shadenet was improved compared to the open treatment. But a thorough investigation on bee management under shadenet is required to optimise pollination in order to obtain greater yields under the shadenet.