PARAMETERIZATION OF SURFACE MOISTURE AVAILABILITY FOR EVAPOTRANSPIRATION USING COMBINED REMOTELY SENSED SPECTRAL REFLECTANCE AND THERMAL OBSERVATIONS (CANOPY RESISTANCE, ENERGY BALANCE, LATENT HEAT)

Reflected short-wave radiation and thermal emissions from a surface have generally been considered independently in remote sensing based techniques for estimating evapotranspiration. A fundamental postulate developed in this study is that the use of combined spectral reflectance and thermal emission data provide new information that may improve the quality of evapotranspiration estimates. Using data collected from satellite, aircraft and ground-based platforms, it is demonstrated that the relationship between canopy temperature and a spectral vegetation index is inverse and linear. The relationship between these variables is considered to reflect the differences in surface moisture availability for evapotranspiration as parameterized by canopy resistance. This is demonstrated theoretically as well as by using data collected over wheat crops grown near Phoenix, Arizona.;By varying canopy characteristics, soil moisture conditions and meteorologic variables, a range of canopy resistance and canopy temperature values is generated. It is shown that the minimum canopy resistance may be obtained from the spectral vegetation index. A procedure is developed to predict actual canopy resistance from the inferred minimum canopy resistance and observed canopy temperature. A test of the procedure using data collected over a wheat canopy is conducted and the accuracy of the procedure demonstrated. Possible applications of the technique and required future research are outlined.;In order to examine fully the relationships between the spectral vegetation index, thermal emissions and canopy resistance under a variety of environmental conditions, a suite of models is developed to simulate the spectral reflectance and thermal response of specified canopies. An existing model for simulating canopy reflectance is adapted to calculate a spectral vegetation index, canopy albedo and the penetration of photosynthetically active radiation into the canopy. The radiation gradient is used in the determination of minimum canopy resistance. The simulated albedo and canopy resistance values are incorporated into a model which simulates the flows of energy and water in the soil-plant-atmosphere continuum and yields estimated canopy temperatures.