EVALUATION OF DIFFERENT METHODS FOR POTENTIAL EVAPOTRANSPIRATION ESTIMATION

Abstract

Accurate and reliable potential evapotranspiration estimation depends on the method through which it is estimated. The aim of this research study is to compare and evaluate different potential evapotrasnpiration estimation methods against the standard FAO Penman -Monteith equation (FPM). The methods evaluated included simple Penman -Monteith equation (SPM), Hergreaves’s method (HM), Priestly Taylor method (PTM) and Makkink method (MM). Mean monthly data of all the climatic variables including maximum and minimum air temperature, relative humidity, wind speed, solar radiation and rainfall was recorded during 2010 from weather station installed inside the Agriculture Research Institute (ARI) Tarnab, Peshawar, Pakistan. Results revealed that all methods underestimated potential evapotranspiration value except the HM. A t and paired t-test was applied on overall means of the all methods and individually monthly means against FPM. There was no overall significant difference for all methods when compared against FPM annually. Significant differences were observed for all methods when subjected to paired t-test for individual monthly mean subjected against FPM. The SPM is considered best after FPM (R2 =0.99), but it also need high numbers of climatic parameters. While the HM which worked on only temperature variable and PT on solar radiation showed high correlation (R2 =0.98) with FPM. HM and PT are simpler and rely only on temperature and radiation data, can be used as an alternative to FPM if some of climatic data are missing or unreliable.