Divergence of potential evapotranspiration trends over Pakistan during 1967-2016

Abstract

Knowledge of the variability and the changes in potential evapotranspiration (PET) is imperative for agriculture and water resource planning and management. There is a growing concern on alteration of PET because of global climate change. The spatial patterns of the changes in PET for annual and two key cropping seasons of Pakistan namely, Kharif and Rabi, are investigated in the present study for the period 1967–2016. The gauge-based gridded PET data of Climatic Research Unit (CRU) is used for this purpose. The rate of changes in PET over various CRU grid cells of Pakistan is assessed with Sen’s Slope estimator, and the significance of the changes is evaluated using a modified version of Mann-Kendall trend test that has the ability to separate the natural variability of PET from unidirectional trend due to global climate change. Besides, trends in annual and seasonal PET for different 30 years with an interval of 10 years are estimated to assess the variations of the trends with time. The results show higher PET in the southern coastal region and lower PET in the northern mountainous regions. The spatial characteristics of annual PET trend showed a significant decrease (0.74 to 1.65 mm/year) in the south and an increase (1.04 to 1.59 mm/year) in the east during 1967–2016. Among the two crop growing seasons, the PET is found to increase (0.51 to 0.66 mm/year) in a small area in the southeast and decrease over a large area in the center (0.87 to 1.29 mm/year) during Kharif, and only increase (0.38 to 0.85 mm/year) in the southeast during Rabi. The time-varying trends in annual and two crop growing seasons reveal that PET has increased significantly at more grid cells in recent years which indicates higher impact of climate change on PET in recent years in Pakistan.