Understanding Evapotranspiration Variability between the Eastern and Western Himalayas

Hydrological budgeting of mountainous watersheds like the Indian Himalayan Region (IHR) is critical as they supply water for most of the Indian Sub-continent. Estimating evapotranspiration (ET), a major eco-hydrological process that returns a significant amount of terrestrial precipitation to the atmosphere, however, is a challenge in data-scare regions like IHR. This is further complicated by the geographic extent of the Indian Himalayas, which cover 13 states within the Himalayan range that possess a significant climatic and biotic gradient from east to west. This study aims to compare the applicability of various potential ET (PET) methods for the eastern and western Himalayan regions under different hydro-climatic conditions and suggest the best-fit method using easily accessible hydrometeorological data. The data for this study was obtained from 17 stations covering the IHR extensively during two study periods (1972-1982 and 2003-2009) from the Indian Meteorological Department (IMD). Four temperature-based methods (Thornthwaite, Blaney-Criddle, Kharrufa, and Hargreaves method) were tested against the modified Penman-Monteith (PM) method using reanalysis data (PM-PET). The PET estimates for the stations showed similar variations across three different elevation ranges. Further, it was observed that the Western Himalayas experienced lesser months of drier conditions (i.e., higher PET values during 2-3 months) compared to the Eastern Himalayas, which typically experienced 4-5 months of higher ET demand. It was observed that the PET values from the Hargreaves method were close to PM-PET with NSE (Nash-Sutcliffe Efficiency) values ranging from 0.75-0.92 and r² values ranging from 0.72-0.92 (except for Jammu; NSE = 0.5, r² = 0.41) and was found to serve as the best temperature-based method among the four methods for PET estimation in the Western and Central Himalayan region. However, no temperature-based method provided reasonable PET estimates in Eastern Himalayas, as the NSE and r² values were less than 0.3 for all the methods.  Thus, there is a need to explore why temperature-based PET methods may not be applicable to the Eastern Himalayan region and evaluate other PET methods that are more reliable in this region.