Preliminary computation of evapotranspiration by land cover type using Landsat TM data and SEBAL

The Boise Valley of Idaho, USA, has a semi-arid climate with approximately 20 to 30 cm of precipitation per year. The predominant land use type has been agriculture, with approximately 370,000 irrigated hectares. Over the last 20 years, the valley has grown in population from 257,000 in 1980 to 431,000 in 2000. An important issue facing water planners is water availability in a valley that is changing from mostly agricultural land-use to more urban types of land use. Department planners have gained insight into the problem by combining LULC polygons from aerial photograph interpretation, and evapotranspiration from Landsat TM data as processed through the Surface Energy Balance Algorithm for Land, both for the year 2000. Seasonal evapotranspiration was generated for the period March 15, 2000 to October 15, 2000. The product was an "image" of seasonal evapotranspiration. LULC polygons were overlaid on the evapotranspiration image and the mean evapotranspiration was computed for all polygons of each LULC class. The preliminary results show that "Irrigated Agriculture" used 812 mm of water, "Urban Residential" used 684 mm, and "New Subdivision" used 606 mm. The water use for irrigated agriculture should be regarded more reliable than for the other two classes, although the error rate is probably not great.