SUMMARY

Water planning for Oklahoma necessitates a precise understanding of the components of the water budget. Of these components evapotranspiration is recognized to be of major importance. Yet, the very nature of evapotranspira-tion renders measurement difficult and expensive. Confounding matters is the fact that Oklahoma possesses amazing spatial and temporal variability in evapotranspiration. Evapotranspiration can not be accurately portrayed through averaging daily pan evaporation measurements for a location. Nor can existing pan evaporation sites be considered adequate to characterize the place-to-place differences in evapotranspiration. The purpose of this. work was to develop a model for estimating place to place differences in evapo-transpiration over large areas of Oklahoma. A geographic information system was used-to concatenate precipitation, elope angle, slope aspect, soils, and satellite-derived landcover.data to numerically produce a monthly evapotran-spiration estimate for each 200 x 200 m grid cell in the upper Little Washita River drainage basin in south central Oklahoma. The total of per-grid-cel