No description available.

Thesis (Ph. D.)--University of Washington, 1998. / Vita. Includes bibliographical references (p. [116]-122).

A two-level diagnostic baroclinic model is constructed for the purpose of examining the role of latent heat release in the production of vertical motion and rainfall. Incorporated in the model are terrain and frictional inflow effects and a variable level of non-divergence. The computations are carried out on a 300 point standard rectangular grid covering the North American continent. [...]

Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2008. / Full text also available online. Scroll down for electronic link.

Thesis (Ph.D.) - University of KwaZulu-Natal, Pietermaritzburg, 2010. / Full text also available online. Scroll down for electronic link.

Thesis (Ph.D.) - University of KwaZulu-Natal, Pietermaritzburg, 2010. / Full text also available online. Scroll down for electronic link.

No description available.

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. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2007.

The high population growth rate and rapid urbanization that the world is experiencing today has aggravated the competition for the already scarce resource ¡V water ¡V between the agricultural sector and the other economic sectors. Moreover, within the agricultural sector, water is increasingly being used for commercial plantations as opposed to growing food crops, threatening food security. Therefore, it is very important that this scarce resource is managed in an efficient and sustainable manner, for now and future use. This requires understanding the process of evaporation for accurate determination of water-use from agricultural lands. In the past, direct measurements of evaporation have proven difficult because of the cost and complexity of the available equipments, and level of expertise involved. This justifies a quest for relatively simple, accurate and inexpensive methods of determining evaporation for routine field applications. Estimation of sensible heat flux (H) from high frequency air temperature measurements and then calculating latent energy flux (ƒÜE) and hence evaporation as a residual of the shortened surface energy balance equation, assuming that closure is met, is appealing in this sense. Concurrent net irradiance (Rn) and soil heat flux (G) measurements can be conducted with relative ease for use in the energy balance equation. Alternately, evaporation can also be mathematically modelled, using single- or multi-layer models depending on vegetation cover, from less expensive routine meteorological observations. Therefore, the ultimate objective of this study is to estimate and model H and ƒÜE, and thereby evaporation, accurately over sparsely vegetated agricultural lands at low cost and effort. Temperature-variance (TV) and surface renewal (SR) methods, which use high-frequency (typically 2 to 10 Hz) air temperature measurements, are employed for estimation of H. The TV method is based on the Monin and Obukhov Similarity Theory (MOST) and uses statistical measures of the high frequency air temperature to estimate H, including adjustments for stability. The SR method is based on the principle that an air parcel near the surface is renewed by an air parcel from above and, to determine H, it uses higher order air temperature differences between two consecutive sample measurements lagged by a certain time interval. Single- and double-layer models that are based on energy and resistance combination theory were also used to estimate evaporation and H from sparse vegetation. Single- and double-layer models that were extended to include inputs of radiometric temperature in order to estimate H were also used. The transmission of solar irradiance to the soil beneath in sparse canopies is variable and depends on the vegetation density, cover and apparent position of the sun. A three-dimensional radiation interception model was developed to estimate this transmission of solar irradiance and was used as a sub-module in the double-layer models. Estimations of H from the TV (HTV), SR (HSR) and double-layer models were compared against H obtained from eddy covariance (HEC), and the modelled ƒÜE (single- and double-layer) were compared with that obtained from the shortened energy balance involving HEC. Besides, long-term ƒÜE calculated from the shortened energy balance using HTV and HSR were compared with those calculated using HEC. Unshielded and naturally-ventilated fine-wire chromel-constantan thermocouples (TCs), 75 ƒÝm in diameter, at different heights above the ground over sparse Jatropha curcas trees, mixed grassland community and bare fallow land were used to measure air temperature. A three-dimensional sonic anemometer mounted at a certain height above the ground surface was also used to measure virtual temperature and wind speed at all three sites. All measurements were done differentially at 10-Hz frequency. Additional measurements of Rn, G and soil water content (upper 60 mm) were also made. The Jatropha trees were planted in a 3-m plant and inter-row spacing in a 50 m ¡Ñ 60 m plot with the surrounding plots planted to a mixture of Jatropha trees and Kikuyu grass. Average tree height and leaf area index measurements were taken on monthly and bimonthly basis respectively. An automatic weather station about 10 m away from the edge of the Jatropha plot was also used to obtain solar irradiance, air temperature and relative humidity, wind speed and direction and precipitation data. Soil water content was measured to a depth of 1000 mm from the surface at 200 mm intervals. Soil and foliage surface temperatures were measured using two nadir-looking infrared thermometers with one mounted directly above bare soil and the other above the trees. The three-dimensional solar irradiance interception model was validated using measurements conducted on different trees and planting patterns. Solar irradiance above and below tree canopies was measured using LI-200 pyranometer and tube solarimeters respectively. Leaf area density (LAD) was estimated from LAI, canopy shape and volume measurements. It was also determined by scanning leaves using either destructive sampling or tracing method. The performance of the TV method over sparse vegetation of J. curcas, mixed grassland community and fallow land was evaluated against HEC. Atmospheric stability conditions were identified using (i) sensor height (z) and Obukhov length (L) obtained from EC and (ii) air temperature difference between two thermocouple measurement heights. The HTV estimations, adjusted and not adjusted for skewness (actual and estimated) of air temperature (sk), for unstable conditions only and for all stability conditions were used. An improved agreement in terms of slope, coefficient of determination (r2) and root mean square error (RMSE), almost over all surfaces, was obtained when the temperature difference rather than the z/L means of identifying stability conditions was used. The agreement between the HTV and HEC was improved for estimations adjusted for actual sk than not adjusted for sk. Improved agreement was also noted when HTV was adjusted using estimated sk compared to not adjusting for sk over J. curcas. The TV method could be used to estimate H for surfaces with varying homogeneity with reasonable accuracy. Long-term water-use of a fetch-limited sparse vegetation of J. curcas was determined as a residual of the shortened surface energy balance involving HTV and HSR and compared with those estimated using HEC. Concurrent measurements of Rn and G were also performed. The long-term water-use of J. curcas trees calculated from the shortened surface energy balance involving HTV and HSR agreed very well when compared with those obtained from HEC. The seasonal HTV and HSR also agreed very well when compared with HEC. Changes in structure of the canopy and environmental conditions appeared to influence partitioning of the available energy into H and ƒÜE. The seasonal total evaporation for the EC, TV and SR methods amounted to 626, 640 and 674 mm respectively with a total rainfall of 690 mm. Footprint analysis also revealed that greater than 80% of the measured flux during the day originates from within the surface of interest. The TV and SR methods, therefore, offer a relatively low-cost means for long-term estimation of H, and ƒÜE, hence the total evaporation, using the shortened surface energy balance along with measurements of Rn and G. Evaporation and biomass production estimations from tree crops requires accurate representation of solar irradiance transmission through the canopy. A relatively simple three-dimensional, hourly time-step tree-canopy radiation interception model was developed and validated using measurements conducted on isolated trees, hedgerows and tree canopies arranged in tramline mode. Measurements were obtained using tube solarimeters placed 0.5 m from each other starting from the base of a tree trunk in four directions, along and perpendicular to the row up to mid-way between trees and rows. Model-simulations of hourly radiant transmittance were in good agreement with measurements with an overall r2 of 0.91; Willmott.s index of agreement of 0.96; and general absolute standard deviation of 17.66%. Agreement between model-estimations and measurements, however, was influenced by distance and direction of the node from the tree trunk, sky conditions, symmetry of the canopy, and uniformity of the stand and leaf distribution of the canopy. The model could be useful in planning and management applications for a wide range of tree crops. Penman-Monteith (PM) equation and the Shuttleworth and Wallace (SW) model, representing single- and dual-source models respectively, were used to determine the total evaporation over a sparse vegetation of J. curcas from routine automatic weather station observations, resistance parameters and vegetation indices. The three-dimensional solar irradiance interception model was used as a sub-module in the SW model. The total evaporation from the sparse vegetation was also determined as a residual of the shortened surface energy balance using measurements of Rn, G and HEC. The PM equation failed to reproduce the .measured. daily total evaporation during periods of low LAI, with improved agreement with increased LAI. The SW model, however, produced total evaporation estimates that agreed very well with the .measured. with a slope of 0.96, r2 of 0.91 and RMSE of 0.45 mm for a LAI ranging from 0 (no leaves) to 1.83 m2 m-2. The SW model also estimated soil evaporation and plant transpiration separately, and about 66 % of the cumulative evaporation was attributed to soil evaporation. These findings suggest that the PM equation should be replaced by the SW model for surfaces that assume a range of LAI values during the growing season. The H was estimated using (i) SW model that was further developed to include surface radiometric temperature measurements; (ii) one-layer model, but linked with a two-layer model for estimation of excess resistance, that uses surface radiometric temperature; and (iii) the SW model (unmodified). The agreement between modelled and measured H, using 10-min data, was in general reasonably good with RMSE (W m-2) of 45.11, 43.77 and 39.86 for the three models respectively. The comparative results that were achieved from (iii) were not translated into the daily data as all models appeared to have a tendency to underestimate H. The resulting RMSEs for the daily H data for the three models were (MJ m-2) 1.16, 1.17 and 1.18 respectively. It appears that similar or better agreement between measured and estimated H can be forged without the need for surface radiometric temperature measurements. The study showed, in general, that high frequency air temperature measurements can be used to estimate H with reasonable accuracy using the simple and relatively low-cost TV and SR methods. Moreover, these methods can be used to calculate ƒÜE, hence ET, as a residual of the shortened surface energy balance equation along with measurements of Rn and G assuming that energy balance closure is met. The simple and low-cost nature of these methods makes replication of measurements easier and their robust nature allows long-term measurements of energy fluxes. The study also showed that H and ƒÜE can be modeled using energy and resistance combination equations with reasonable accuracy. It also reiterated that the SW-type models, which treat the plant canopy and soil components separately, are more appropriate for estimation of H and ƒÜE over sparse vegetation as opposed to the PM-type models. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.

Increased pressure on the available limited water resources for agricultural production has a significant impact on sugarcane production. Routine monitoring of evaporation with reliable accuracy is essential for irrigation scheduling, for more efficient use of the available water resources and for management purposes. An indirect method for estimating evaporation involves measuring the sensible heat flux (H) from which latent energy flux and hence total evaporation can be calculated, as a residual using the shortened energy balance from measurements of net irradiance and soil heat flux. Various methods for measuring H may include Bowen ratio energy balance, eddy covariance (EC), flux variance (FV), optical scintillation, surface renewal (SR) and temperature variance (TV). Each method has its own advantages and disadvantages, in terms of method theoretical assumptions, accuracy, complexity, cost, fetch requirements and power consumption. The TV and SR methods are inexpensive and reasonably simple with a reduced power requirement compared to other methods since they require high frequency air temperature data which is obtained by using an unshielded naturally-ventilated type-E fine-wire thermocouple at a single point above the canopy surface. The TV method is based on the Monin-Obukhov similarity theory (MOST) and uses the mean and standard deviation of the air temperature for each averaging period. Currently, there are two TV methods used for estimating sensible heat flux (HTV) at sub-hourly time intervals, one includes adjustment for stability, and a second that includes adjustment for air temperature skewness. Another method used to estimate sensible heat flux from the mean and standard deviation of air temperature is based on MOST and uses spatial second-order air temperature structure function. For the TV method adjusted for stability and the method based on MOST that uses a spatial second-order air temperature structure function, the Monin-Obukhov atmospheric stability parameter () is needed. The parameter  can be estimated from EC measurements or alternatively estimated independently using an iteration process using horizontal wind speed measurements. The TV method including adjustment for air temperature skewness requires the mean and standard deviation of the air temperature and air temperature skewness for each averaging time period as the only input. The SR method is based on the coherent structure concept. Currently, there are various SR models method for estimating sensible heat flux. These include an ideal SR analysis model method based on an air temperature structure function analysis, the SR analysis model with a finite micro-front period, combined SR with K-theory and combined SR model method based on MOST. The ideal SR analysis model based on an air temperature structure function analysis should be calibrated to determine the SR weighting factor (). The other SR approaches require additional measurements such as crop height and horizontal wind speed measurements. In all of the SR approaches, air temperature time lags are used when calculating the air temperature structure functions. In this study, the performance of TV and SR methods were evaluated for estimation of sensible heat and latent energy fluxes at different heights for air temperature time lags of 0.4 and 0.8 s for daytime unstable conditions against EC above a sugarcane canopy at the Baynesfield Estate in KwaZulu-Natal, South Africa. For all methods, latent energy flux (LE) and hence evaporation was estimated as a residual from the shortened energy balance equation using H estimates and net irradiance and soil heat flux density measurements. The ideal SR analysis model method based on an air temperature structure function analysis approach was calibrated and validated against the EC method above the sugarcane canopy using non-overlapping data sets for daytime unstable conditions during 2008. During the calibration period, the SR weighting factor was determined for each height and air temperature time lag. The magnitude of ranged from 0.66 to 0.55 for all measurement heights and an air temperature time lag of 0.8 s. The value increased with a decrease in measurement height and an increase in air temperature time lag. For the validation data set, the SR sensible heat flux (HSR) estimates corresponded well with EC sensible heat flux (HEC) for all heights and both air temperature time lags. The agreement between HSR and HEC improved with a decrease in measurement height for the air temperature time lag of 0.8 s. The best HSR vs HEC comparisons were obtained at a height of 0.20 m above the crop canopy using = 0.66 for an air temperature time lag of 0.8 s. The residual estimates of latent energy flux by SR and EC methods were in good agreement. The LESR at a height of 0.20 m above the canopy yielded the best comparisons with LEEC estimated as a residual. The performance of the TV method, including adjustment for stability, and / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.