Hargreaves versus Penman-Monteith under Humid Conditions

Abstract

The Food and Agriculture Organization of the United Nations (FAO) hasproposed using the Penman-Monteith equation as the standard forestimating reference evapotranspiration (ET0), and for evaluating otherequations. The basic obstacle to using the FAO-56 Penman-Monteithequation (FAO-56 PM) widely is the required weather data which are notavailable in most of the stations. In such circumstances, a simpleempirical Hargreaves equation (HARG) is often used. However, thisequation generally overestimates ET0 at humid locations. Therefore, HARGrequires local calibration. The main objective of this study is toinvestigate the possibility for calibrating the equation in the WesternBalkans region, South East Europe through the adjustment of Hargreavesexponent (HE). Data from Palic, Sarajevo, and Nis have been used forestimating the adjusted HE. A value of 0.424 is proposed instead of theoriginal 0.5 as one which should be used in the adjusted Hargreavesequation (AHARG) for the Western Balkan locations. The ET0 valuesestimated by AHARG were compared with FAO-56 PM estimates for eighthumid locations (Varazdin, Zagreb, Bihac, Novi Sad, Negotin, Kragujevac,Nis, and Vranje). Estimates by AHARG were in close agreement with FAO-56PM estimates at most of the locations. The SEE ranged from 0.17 mmday(-1) for Varazdin to 0.24 mm day(-1) for Vranje, averaging 0.21 mmday(-1). The average overestimation was about 1%. These resultsstrongly support the use of the adjusted Hargreaves equation at humidWestern Balkan locations in the case when only the temperature data areavailable. Further research should be undertaken for evaluating thevalidity of the approach presented in this paper in other regions.