Sensitivity Analysis of FAO-56 Penman-Monteith Method for Different Agro-ecological Regions of India

Abstract

This paper analyzes the sensitivity of reference evapotranspiration (ETo) to climatic variables for different agro-ecological regions of India: semi-arid (Kovilpatti and Parbhani), humid (Mohanpur), and sub-humid (Ludhiana and Ranichauri). The FAO-56 Penman-Monteith (FAO-56 PM) method is used to estimate ETo, and sensitivity of ETo has been studied in terms of change in maximum air temperature (T max ), minimum air temperature (T min ), solar radiation (R s ), average relative humidity (RH avg ), and wind speed (W s ). Sensitivity analysis is performed by increasing and decreasing the climate variables such as T max , T min , R s , and RH avg by one unit of increment and decrement, respectively, up to five units (except for W s ) while keeping the other variables and parameters constant. However, wind speed W s (km h−1) is only increased with an increment of one km h−1 up to five km h−1. The results showed that the change in ETo is linearly related to change in all climate variables (r 2 = 0.97 in most cases) at all sites. Further, ETo is most sensitive to R s at Kovilpatti, Mohanpur and Ranichauri, and to W s at Ludhiana and Parbhani. However, the sensitivity of ETo to the same variable shows considerable variation from site to site and at the same site within the year. ETo is less sensitive to RH avg followed by T min at all sites.