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Evapotranspiration (ET; evaporation + transpiration) is central to a wide range of biological, chemical, and physical processes in the Earth system. Accurate remote sensing of ET is challenging due to the interrelated and generally scale dependent nature of the physical factors which contribute to the process. The evaporation of water from porous media like sands and soils is an important subset of the complete ET problem. Chapter 1 presents a laboratory investigation into this question, examining the effects of grain size and composition on the evolution of drying sands. The effects of composition are found to be 2-5x greater than the effects of grain size, indicating that differences in heating caused by differences in reflectance may dominate hydrologic differences caused by grain size variation. In order to relate the results of Chapter 1 to the satellite image archive, however, the question of information loss between hyperspectral (measurements at 100s of wavelength intervals) laboratory measurements and multispectral (≤ 12 wavelength intervals) satellite images must be addressed. Chapter 2 focuses on this question as applied to substrate materials such as sediment, soil, rock, and non-photosynthetic vegetation. The results indicate that the continuum that is resolved by multispectral sensors is sufficient to resolve the gradient between sand-rich and clay-rich soils, and that this gradient is also a dominant feature in hyperspectral mixing spaces where the actual absorptions can be resolved. Multispectral measurements can be converted to biogeophysically relevant quantities using spectral mixture analysis (SMA). However, retrospective multitemporal analysis first requires cross-sensor calibration of the mixture model. Chapter 3 presents this calibration, allowing multispectral image data to be used interchangeably throughout the Landsat 4-8 archive. In addition, a theoretical explanation is advanced for the observed superior scaling properties of SMA-derived fraction images over spectral indices. The physical quantities estimated by the spectral mixture model are then compared to simultaneously imaged surface temperature, as well as to the derived parameters of ET Fraction and Moisture Availability. SMA-derived vegetation abundance is found to produce substantially more informative ET maps, and SMA-derived substrate fraction is found to yield a surprisingly strong linear relationship with surface temperature. These results provide context for agricultural applications. Chapter 5 investigates the question of mapping and monitoring rice agricultural using optical and thermal satellite image time series. Thermal image time series are found to produce more accurate maps of rice presence/absence, but optical image time series are found to produce more accurate maps of rice crop timing. Chapter 6 takes a more global approach, investigating the spatial structure of agricultural networks for a diverse set of landscapes. Surprisingly consistent scaling relations are found. These relations are assessed in the context of a network-based approach to land cover analysis, with potential implications for the scale dependence of ET estimates. In sum, this thesis present a novel approach to improving ET estimation based on a synthesis of complementary laboratory measurements, satellite image analysis, and field observations. Alone, each of these independent sources of information provides novel insights. Viewed together, these insights form the basis of a more accurate and complete geophysical understanding of the ET phenomenon.

Graduation date: 1994

12 pp. / This publication describes the procedure that is now recommended as a US standard for computing reference evapotranspiration. Included in the publication are: the rationale for developing the new procedure, the equations utilized in the new procedure, a discussion of how the new procedure differs from the established AZMET procedure, and tables to facilitate conversion between procedures (new and AZMET).

In the arid southwestern United States, where water is a limiting factor in agricultural and industrial development, a sizeable portion of the annual precipitation may be lost through evapotranspiration. In Arizona such losses account for approximately 95 per cent of the annual precipitation. Tamarisk (Tamarix pentandra Pall.) is estimated to occupy over one million acres of the flood plains and streambanks in the southwest. Although reported to use a large quantity of water, accurate estimates of evapotranspiration are unknown. Evapotranspiration processes are complex and depend on many interrelationships of the soil-plantatmosphere system. Although, water use by tamarisk has been intensively studied, evapotranspiration measurements under different climatic and hydrologic conditions are not available. The evapotranspiration tent was selected to measure evapotranspiration rates of tamarisk under varying climatic and hydrologic conditions. Intensive investigations of the enclosure effect of the tent were performed. Modifications of the tent reduced serious enclosure effects of the original tent. Evapotranspiration rates measured by the tent agreed favorably with rates computed by Penman's equation. Evapotranspiration rates for an area where the water table depth was approximately 20-feet was greater than an area where the Water table depth was 14-feet. This deviation, which may be attributed to salinity, led to a laboratory investigation of the effects of salinity on transpiration rates of tainarisk. An intensive laboratory study was conducted to determine the effect of salinity on transpiration rates of tamarisk at different vapor pressure deficits. Results indicated that the effect of salinity is dependent on vapor pressure deficit. Transpiration rates were linearily related to vapor pressure deficits at low salinity levels, but a curvilinear relationship was obtained at high salinity levels. An estimate of saturation deficit of the mesophyll cells was determined by extrapolation of transpiration and vapor pressure deficit relationships. These data indicate minimial increases in salt concentrations in the stomatal cavities as indicated by small increases in the mesophyll saturation deficits as the salinity of the root substrate was increased. Root permeability tests were conducted on plants subjects to varying salinity and vapor pressure deficit levels. Results indicated a significant reduction only at the highest salinity and vapor pressure deficit levels.

Orientador: Fernando Braz Tangerino Hernandez / Resumo: O feijão comum (Phaseolus vulgaris L.) de grande importância sócio-econômica para o Brasil, sendo o segundo maior produtor e principal consumidor mundial. O objetivo neste trabalho foi compreender as metodologias disponíveis para manejo e avaliação do uso da água em culturas irrigadas e estimar a evapotranspiração atual (ETa) e ainda determinar coeficientes de cultura nas áreas cultivadas com feijão de inverno utilizando as técnicas baseadas em sensoriamento remoto (algoritmo SAFER - Simple Algorithm for Retrieving Evapotranspiration), confrontando-os com as estimativas da evapotranspiração da cultura (ETc) preconizados pelo Boletim FAO 56. O estudo foi realizado em condições irrigadas por pivô central com sete épocas de semeadura de feijão de inverno nos municípios de Rubinéia e Pereira Barreto. Cada época de semeadura recebeu um manejo de irrigação, realizado pelo próprio irrigante e uma simulação sugerida de irrigação mantendo os mesmos dados de ETo e chuva. As imagens do satélite LandSat 8 foram utilizas para execução do algoritmo SAFER para a obtenção da relação entre a evapotranspiração atual e a evapotranspiração de referência. Foram feitas a análise da produtividade de água em função da produção obtida para cada época de semeadura. Para as semeaduras de feijão de outono/inverno em Rubinéia, o irrigante possui a opção de utilizar dados das Estações Ilha Solteira e Marinópolis, fato que as ETo são semelhantes. Resultados obtidos das irrigações utilizadas pelo irrigante ... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Common bean (Phaseolus vulgaris L.) of great socioeconomic importance for Brazil, being the second largest producer and main consumer worldwide. The objective of this work was to understand the available methodologies for the management and evaluation of water use in irrigated crops and to estimate current evapotranspiration (ETa) and to determine crop coefficients in areas cultivated with winter bean using techniques based on remote sensing SAFER - Simple Algorithm for Retrieving Evapotranspiration), comparing them with estimates of crop evapotranspiration (ETc) recommended by FAO Bulletin 56. The study was conducted under central pivot irrigation conditions with seven winter bean sowing seasons in the municipalities of Rubinéia and Pereira Barreto. Each sowing season received an irrigation management, performed by the irrigator and a suggested irrigation simulation, maintaining the same ETo and rainfall data. The images of the LandSat 8 satellite were used to execute the SAFER algorithm to obtain the relation between the current evapotranspiration and the reference evapotranspiration. The water productivity was analyzed according to the production obtained for each sowing season. For the autumn / winter bean sowing in Rubinéia, the irrigant has the option to use data from the Ilha Solteira and Marinópolis Stations, a fact that ETo are similar. Results obtained from the irrigations used by the irrigator overestimated the water storage in the soil, so it was necessary to carr... (Complete abstract click electronic access below) / Mestre

The application of the widely used Penrnan-Monteith evapotranspiration equation to hybrid poplar trees is impossible without a valid surface resistance. The increase in applications of drip-irrigated hybrid poplar trees for wood chip stock and veneer production, as well as bioremediation, constitutes a need for estimating the evapotranspiration of these trees. To the author's knowledge, there are no published estimates of surface resistance for poplar trees. Six years of weekly soil moisture content for drip-irrigated, hybrid poplar trees were used in a water balance to compute evapotranspiration. The weekly data were adjusted with reference evapotranspiration data to compute a daily evapotranspiration. Only data that represent fully leaved hybrid poplars are used in this study and the data were screened for the effects of drainage. Additional parameters applied in this study include solar radiation, temperature, wind speed and relative humidity taken at a nearby AGRIMET Weather Station. The results of this study indicate that surface resistance values cannot be described as a function of meteorological data within the constraints of the current experiment design. The graph of poplar evapotranspiration versus surface resistance shows that for a given evapotranspiration there can be multiple rs values. This scatter is the influence of parameters other than rs within the Penman-Monteith model. The use of an instrument to directly measure the surface resistance is recommended in further studies. / Graduation date: 2000

Estimating recharge is a critical aspect of groundwater management, when aquifer resources are constrained by multiple users. The Edwards Aquifer, an artesian aquifer underlying Austin and San Antonio, Texas, sustains municipalities, farmers and fragile habitats at discharge locations. Rising municipal demand for Edwards water supports the need for effective conservation over time to maintain the well-being of all users. Predicting recharge is a valuable tool for determining future available resources. Evapotranspiration (ET) accounts for a majority of water loss following precipitation, significantly affecting recharge. Developing a method for accurate regional estimates of ET is complicated by aquifer characteristics, expensive instrumentation, and a variable climate. This study investigates a specific method for estimating regional potential ET (ETp), by combining the Priestley-Taylor equation with data primarily retrieved from the Moderate-Resolution Imaging Spectroradiometer. Improved resolution and timing of satellite measurements provides greater regional specificity for variables related to ET calculations. ETp is then estimated for 2004 and 2005, utilizing data from MODIS, aboard NASA’s Aqua and Terra satellites. Land surface temperature, leaf area index and albedo retrieved from MODIS replace in situ measurements, which are often nonexistent in a regional context. Incoming radiation, a direct input in the Priestley-Taylor equation, is retrieved from the National Center for Environmental Prediction’s North American Regional Reanalysis Model (NARR). Results show methods overestimate ET between 400% to over 1000% when compared to actual ET (ETa) at two locations in the northeast portion of the aquifer. Correlation is improved when ETp is treated as an instantaneous rate rather than daily. During months of above average precipitation, which are more representative of potential conditions, instantaneous ETp exceeded ETa by an average of 81%, with a root mean squared error of 1.15 mm/30min and an average positive bias of 2.84 mm/30min. Considering the soil moisture limited conditions throughout Central Texas, a positive bias is not surprising. Incorporating a calibrated Priestly-Taylor could improve accuracy, but estimating regional ETp remains restricted by available daily data necessary for calculations and comparison. / text

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