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United States Department of Agriculture

Agricultural Research Service

Title: Combining the Penman-Monteith Equation with Measurements of Surface Temperature and Reflectance to Estimate Evaporation Rates of Semiarid Grassland

Authors
item Moran, Mary
item Rahman, M - UNIVERSITY OF ARIZONA
item Washburne, J - UNIVERSITY OF ARIZONA
item Goodrich, David
item Weltz, Mark
item Kustas, William

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 4, 1995
Publication Date: N/A

Interpretive Summary: Current interest in the prediction of drought, forest fires and regional changes in climate has resulted in a related interest in making maps of vegetation health. For example, unhealthy vegetation is indicative of lack of water, insect infestation, disease or extreme weather conditions. Though there are proven methods to monitor the health of the vegetation at a local scale, these methods can't be applied at the regional scale with a heterogeneous landscape. Images of the earth from orbiting satellites provide a viable means of monitoring the health of vegetation at a regional scale. In this experiment, satellite images of an area in southeast Arizona were used in conjunction with information about air temperature and wind speed to map the health of grassland. On each of four dates during the growing season, an index (termed the Water Deficit Index: WDI) was computed to rate the plant health from good (WDI=0) to poor (WDI=1). The results corresponded well with conventional measurements of plant health and our knowledge of general plant conditions. The technique could provide accurate vegetation health information for use in predicting such phenomenon as large-scale drought or forest fire potential.

Technical Abstract: The Penman-Monteith equation is useful for computing evaporation rates of uniform surfaces, such as dense vegetation or bare soil. This equation becomes less useful for evaluation of evaporation rates at the regional scale, where surfaces are generally characterized by a patchy combination of vegetation and soil. This is particularly true in the arid and semi-arid regions of the world. The approach proposed here is an attempt to use remotely-sensed measurements of surface reflectance and temperature to allow application of the Penman-Monteith theory to partially-vegetated fields without a-priori knowledge of the percent vegetation cover and canopy resistance. The approach was first tested using ground-based measurements of surface reflectance and temperature at a rangeland site; the results compared well with on-site measurements of surface evaporation rate (RMSE=29 W m-2). Then, the approach was tested based on a set of four Landsat Thematic Mapper (TM) images acquired in southeast Arizona during 1992, along with ground-based measurements of evaporation rates and near-surface meteorological conditions. Maps of surface air temperature and wind speed were combined with maps of surface temperature and spectral vegetation index to produce regional estimates of vegetation water status for the grassland biome. Results showed that there was potential for application at a regional scale.

Last Modified: 9/2/2014
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