Influence of climate variables on FAO Penman–Monteith reference evapotranspiration in the Upper Chenab Canal command area of Pakistan

Abstract

Examining the changing trends of climatic variables, along with their respective influences identified by conventional sensitivity analysis, could play a decisive role in distinguishing the prominent variables controlling evapotranspiration variations over time. The trends of climatic variables, their influences, and subsequent contributions to FAO Penman–Monteith reference evapotranspiration (ETo) were systematically analysed for the command area of the Upper Chenab Canal, Pakistan, from 1980 to 2012. The nonparametric Mann–Kendall test was employed for trend detection, and ETo sensitivity to climatic variables was quantified via sensitivity coefficients. A significant overall increase in maximum temperature (Tmax) and minimum temperature (Tmin), a significant decrease in sunshine hours (n) and net solar radiation (Rn), and a non-significant decrease in relative humidity (RH) and wind speed (u2) resulted in a slight decrease in ETo. ETo was highly sensitive to Rn, followed by Tmax, n, Tmin, RH, and u2. The highly influential Rn and the comparatively less influential n, Tmin, and u2 appeared to be the active factors controlling the ETo fluctuations. Rn and n contributed to downward trends in ETo in the study area, whereas u2 and Tmin contributed to upward trends. The contributions of Tmax, despite being the second-most influential variable, to ETo variations were not prominent. RH had negligible impact on ETo, as it was least influential and showed non-significant variations.