Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: July 7, 2011
Publication Date: November 1, 2011
Citation: Hatfield, J.L., Prueger, J.H. 2011. Spatial and temporal variation in evapotranspiration. In: Gerosa, G.A., editor. Evapotranspiration from Measurements to Agricultural and Environmental Applications. Rijeka, Croatia: InTech. p. 3-16.
Spatial and temporal variation in evapotranspiration occurs at multiple scales as the result of several different spatial and temporal patterns in precipitation, soil water holding capacity, cloudiness (available energy), types of crops, and residue and tillage management practices. We have often assumed there is minor variation in evapotranspiration at the field scale where the same crop is grown; however, at this scale there are differences induced by a combination of soil water holding capacity, crop growth patterns, and responses to nitrogen management. Since we often use evapotranspiration as a method for assessing how well a specific crop or management practice performs, then it even becomes more critical to understand how evapotranspiration varies in time and space. The results we have gathered from a range of experiments reveal that spatial variation is induced primarily by the capacity of the soil to hold water and release it to the crop to meet the evaporative demand, and by the amount of rainfall that has occurred in a given area. We assume rainfall is nearly uniform across a field; however, at the watershed scale where distances exceed 5-10 km between measurement points, then rainfall variation becomes a dominant part of the evapotranspiration variation. We have also found that within a day, variation in evapotranspiration can be induced by differences in cumulus clouds affecting the amount of incoming energy. Spatial variation can also be induced by changing the albedo of the surface, which in turn changes the amount of energy available for evapotranspiration.