Semi-empirical approach for estimating actual evapotranspiration in Greece

Reliable estimation of actual evapotranspiration rates (ET) may be achieved if extensive information on the soil-plant-atmosphere system is available. Agricultural and irrigation engineers facing the problem of rational irrig ation planning, rarely have at their disposal such information. Therefore, there is a demand for simpler approaches to estimate actual evapotranspirat ion. In this paper, a semi-empirical approach is proposed for estimating ac tual water losses from crops. It is assumed that the ratio of actual to max imum evapotranspiration (ET/ETm) is an exponential function of the water co ntent w in the root-zone, of the form: ET/ETm = exp[c(w - w(fc))/ (w - w(wp ))], where c is a constant introduced to adjust the decrease of the ratio E T/ETm according to what is observed for the climatic conditions, soils and crops of Greece, wfc the water content at field capacity and w(wp) denotes the water content at wilting point. Verification of the above approach for estimating actual evapotranspiration was achieved by comparing ET-values ob tained by the soil moisture profile changes and the ET-values obtained by t he above equation. Meteorological, crop and soil data required were collect ed from experimental fields of the Agricultural University of Athens (38 de grees 23 'N, 23 degrees6 'E). The agreement of actual versus computed ET-va lues for three widely grown crops in Greece (cotton, wheat and maize) may b e considered as satisfactory. (C) 2001 Elsevier Science B.V. All rights res erved.