Estimating actual evapotranspiration using a detailed agro-hydrological model

Actual evapotranspiration (ET) is an important term of the water balance and actual crop transpiration is considered to he the determining factor in crop production. However. measurements of actual ET are complicated and especially the distinction between soil evaporation, which is considered as a non-beneficial loss, and crop transpiration requires specific measurement techniques. An attractive alternative is the use of simulation models to estimate actual transpiration and evaporation. As part of a comparison of different methods to estimate the actual ET. thr physically based simulation model for the saturated-unsaturated zone. SWAP. was used to estimate actual ET for two locations in the Gediz basin in western Turkey. The locations selected are: a field with a single crop (cotton) and a valley with a mixed cropping pattern (grapes. cotton, olive trees). Actual crop transpiration as well as actual soil evaporation were simulated taking into account crop development stage and reduction as consequences of actual soil moisture status. For the valley, with the mixed cropping pattern, a lumped approach was used to estimate actual ET. Results presented show that a distinction between actual crop transpiration and soil evaporation call be made and that the lumped method is an attractive method to estimate areal actual ET. (C) 2000 Elsevier Science B.V. All rights reserved.