Development and comparative evaluation of radiation-based reference evapotranspiration equations for sub-humid Hazaribagh region of Jharkhand

Abstract

In this study, two site-specific solar radiation (R_s) and net radiation (R_n) based equations for estimating reference evapotranspiration (ET_­0) were developed and their performance were statistically analysed in comparison to widely accepted FAO Penman-Monteith method (PMF-56) and four other standard radiation methods for sub-humid Hazaribagh region of Jharkhand. These two equations were developed with daily values of R_s and R_n in conjunction with maximum and minimum air temperature by taking daily PMF-56 ET₀ values as index with weather dataset of 15 years (1990-2004). The performance of these equations validated with carefully screened daily weather dataset of eight years (2005-2012) with other considered standard methods revealed that they estimated ET₀ better. The eight year average ratio of ET₀ values calculated with developed R_s- and R_n- based equations and PMF-56 on daily basis were obtained as 1.07 and 1.10 respectively. These two derived equations resulted in better average values of SEE on daily (0.57 and 0.61) and monthly (1.24 and 1.21) basis. The higher value of Agreement index (D) on monthly ET₀ values on daily and monthly basis during validation period confirms efficacy of derived equations. Considering the limitations associated with reliability and availability of weather data especially in developing countries, derived equations presented in this study are recommended to estimate ET₀ in sub-humid Hazaribagh region if standard PMF-56 equation cannot be used due to non-availability of required weather parameters.