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

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 (ET_o) 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 ET_o sensitivity to climatic variables was quantified via sensitivity coefficients. A significant overall increase in maximum temperature (T_max) and minimum temperature (T_min), a significant decrease in sunshine hours (n) and net solar radiation (R_n), and a non-significant decrease in relative humidity (RH) and wind speed (u₂) resulted in a slight decrease in ET_o. ET_o was highly sensitive to R_n, followed by T_max, n, T_min, RH, and u₂. The highly influential R_n and the comparatively less influential n, T_min, and u₂ appeared to be the active factors controlling the ET_o fluctuations. R_n and n contributed to downward trends in ET_o in the study area, whereas u₂ and T_min contributed to upward trends. The contributions of T_max, despite being the second-most influential variable, to ET_o variations were not prominent. RH had negligible impact on ET_o, as it was least influential and showed non-significant variations.