Evapotranspiration in Crop and Hydrologic Models: Variation in Observations.

Evapotranspiration represents the combination of soil water evaporation and plant transpiration and is governed by energy availability, water vapor gradient, wind speed gradient, and soil water availability. The amount of evapotranspiration varies throughout the growing season because of the changes in the leaf area of the plant as it develops. Experimental observations of these parameters across a number of research studies provide detailed insights into the dynamics of the processes governing evapotranspiration.  Preliminary evaluation of the evapotranspiration component in crop models revealed there was a large degree of variation among crop models using the daily total amounts. To evaluate these models more fully for their ability to estimate evapotranspiration, an effort was made to secure data sets across a number of locations and crops. The primary emphasis was on maize and wheat data sets because these crop models had the most extensive intercomparison among the models at this point; however, in anticipation of future intercomparisons, data sets were identified for soybean, cotton, and groundnut.  These data have been assembled into databases using daily totals of all of the available parameters and quality controlled to assure the most reliable data possible. These data are compared with the estimates from different evapotranspiration models to determine the sensitivity to input parameters.