Estimating actual evapotranspiration using a detailed agro-hydrological model

Actual evapotranspiration (ET) is an important term of the water balance an d actual crop transpiration is considered to he the determining factor in c rop production. However. measurements of actual ET are complicated and espe cially the distinction between soil evaporation, which is considered as a n on-beneficial loss, and crop transpiration requires specific measurement te chniques. An attractive alternative is the use of simulation models to esti mate actual transpiration and evaporation. As part of a comparison of diffe rent methods to estimate the actual ET. thr physically based simulation mod el for the saturated-unsaturated zone. SWAP. was used to estimate actual ET for two locations in the Gediz basin in western Turkey. The locations sele cted are: a field with a single crop (cotton) and a valley with a mixed cro pping pattern (grapes. cotton, olive trees). Actual crop transpiration as w ell as actual soil evaporation were simulated taking into account crop deve lopment stage and reduction as consequences of actual soil moisture status. For the valley, with the mixed cropping pattern, a lumped approach was use d to estimate actual ET. Results presented show that a distinction between actual crop transpiration and soil evaporation call be made and that the lu mped method is an attractive method to estimate areal actual ET. (C) 2000 E lsevier Science B.V. All rights reserved.