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 irrigation planning, rarely have at their disposal such information. Therefore, there is a demand for simpler approaches to estimate actual evapotranspiration. In this paper, a semi-empirical approach is proposed for estimating actual water losses from crops. It is assumed that the ratio of actual to maximum evapotranspiration (ET/ETm) is an exponential function of the water content 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 ET/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 obtained by the soil moisture profile changes and the ET-values obtained by the above equation. Meteorological, crop and soil data required were collected from experimental fields of the Agricultural University of Athens (38 degrees 23 'N, 23 degrees6 'E). The agreement of actual versus computed ET-values for three widely grown crops in Greece (cotton, wheat and maize) may be considered as satisfactory. (C) 2001 Elsevier Science B.V. All rights reserved.