

Szilagyi, J, Jozsa, J (2009). Estimating spatially distributed monthly evapotranspiration rates by linear transformations of MODIS daytime land surface temperature data. HYDROLOGY AND EARTH SYSTEM SCIENCES, 13(5), 629637. Abstract Under simplifying conditions catchmentscale vapor pressure at the drying land surface can be calculated as a function of its watershedrepresentative temperature (< Ts >) by the wetsurface equation (WSE, similar to the wetbulb equation in meteorology for calculating the drybulb thermometer vapor pressure) of the Complementary Relationship of evaporation. The corresponding watershed ET rate, < ET >, is obtained from the Bowen ratio with the help of air temperature, humidity and percent possible sunshine data. The resulting (< Ts >,< ET >>) pair together with the wetenvironment surface temperature (< Tws >) and ET rate (ETw), obtained by the PriestleyTaylor equation, define a linear transformation on a monthly basis by which spatially distributed ET rates can be estimated as a sole function of MODIS daytime land surface temperature, Ts, values within the watershed. The linear transformation preserves the mean which is highly desirable. < Tws >, in the lack of significant open water surfaces within the study watershed (Elkhorn, Nebraska), was obtained as the mean of the smallest MODIS Ts values each month. The resulting periodaveraged (20002007) catchmentscale ET rate of 624 mm/yr is very close to the waterbalance derived ET rate of about 617 mm/yr. The latter is a somewhat uncertain value due to the effects of (a) observed groundwater depletion of about 1m over the study period caused by extensive irrigation, and; (b) the uncertain rate of net regional groundwater supply toward the watershed. The spatially distributed ET rates correspond well with soil/aquifer properties and the resulting land use type (i.e. rangeland versus centerpivot irrigated crops). DOI:
ISSN: 10275606 