Re- EXAMINING tHE VALIDITY OF REFERECE Evapotranspiration estimation in Herat, Afghanistan

Abstract

The aim of this study is to contribute in irrigation scheduling by proposing adaptable models that are widely used for the estimation of reference evapotranspiration ET0 in Herat, Afghanistan. Six well-known models, The Penman-Monteith (ET0PM), Hargreaves (ET0Hrg), Hamon (ET0Ham), Thornthwaite (ET0Trw), solar radiation based (ET0Rs) and Net radiation based (ET0Rn) were compared, and the pan evapotranspiration ETpan model was used as indicator. The pan coefficient (kP) proposed by Pereira was used to convert pan evaporation (Epan) to ETpan. Results obtained showed that, the ET0values estimated by all the methods were shown to be close to those of ETpan in the second period (spring, fall and winter). However, large differences emerged in the first period (the windy summer), with the exception of  ET0PM. This method displayed a small difference only in June and July. Pearson’s correlation (R) showed that the estimates produced by all the simpler methods were significant correlated with those of ETpan in the second period, but weakly correlated in the first period. The ET0PM method produced the lowest value of 1.3 mm day-1, based on the standard error estimation (SEE). The seasonally-based average difference between ETpan and ET0PM was smaller than that of the other methods in the first period, at 1.9 mm day-1. The ET0PM estimation rate was therefore closest to ETpan. It is concluded that the methods that used wind factor are more adaptable than those not used wind factor especially in Herat, Afghanistan. The wind might be the reason of the differences between ETpan and ET0PM in the windy summer.