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Doctoral Degrees (Agrometeorology)

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Browsing Doctoral Degrees (Agrometeorology) by Subject "Evaporation (Meteorology)."

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    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.
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    Sensible heat flux for estimating evaporation.
    (2010) Savage, Michael John.
    The focus of the research is on investigations of various methods for obtaining sensible heat flux (H) for estimating evaporation. The key for this approach is the application of the shortened energy balance equation, and in the case of methods based on the Monin-Obukhov similarity theory (MOST), such as surface-layer scintillometrv and temperature variance with adjusted for stability using air temperature skewness, and surface renewal (SR), die iterative procedures. The application of the shortened energy balance requires that errors associated with measurement of net irradiance (Rnet) and soil heat flux (S) are kept to a minimum To this end. methodology for the calibration of net radiometers for both the infrared and short wave irradiances receive attention. A field study attempts to quantify the error in soil heat flux measurement for a mesic grassland. A standard, convenient and accurate method for calibrating net radiometers would assist in unravelling reasons for the perplexing lack of surface energy balance closure when employing the eddy covariance (EC) flux estimation method as well as improve on the accuracy of the energy balance residual method for estimating evaporation. A relatively inexpensive, accurate and quick laboratory method, based on physical theory, for non-steady radiative conditions above a large water-heated or water-cooled radiator containing circulated water, with surface-embedded thermocouples is used to obtain reproducible net radiometer calibration factor's for the infrared waveband for a wide range in net irradiance. When applied, the method would reduce error m the most important term of the shortened energy balance and assist in energy balance closure aspects of EC measurements. The SLS method, reliant on MOST, is used for estimating a really-averaged H for a mesic grassland for a 30-month period. Comparisons with EC measurements feature prominently in this unique study. These comparisons include using different MOST procedures and the influence of the Bowen ratio on SLS measurement: of if is investigated. Furthermore, since there are reports in the literature that the EC method may underestimate H and or latent energy flux (LE), resulting in the shortened energy balance not being closed, effort is devoted to this aspect. Other methods used for comparison purposes are the traditional Bowen ratio energy balance (BREB), SR, TV and ETo (grass reference) methods. The TV and SLS and/or EC measurements of H are compared above three contrasting canopy surfaces. It is shown that other high frequency air temperature-based methods, for example, for the first time the TV method with adjustment for skewness, may pave the way for evaporation stations from which real-time and sub-hourly estimates may be obtained relatively inexpensively. Another area of research that receives attention is the placement height of EC instruments above short-canopy surfaces and a spectral analysis of the vertical wind speed and some temperature measurement: for close-canopv placement heights. The SR method is used to estimate, for the first time, open-water evaporation. The ideal SR method applied above canopies is the most inexpensive micrometeorological method for estimating H, but the SR weighting factor a needs to be determined using EC and for this reason, the TV method with adjustment for skewness was investigated. Finally, a unique implementation of SR uses an iterative method for calculating H. A similar iterative procedure is applied for MOST and ETo calculations.