Spatiotemporal changes in precipitation extremes in the arid province of Pakistan with removal of the influence of natural climate variability

摘要

Natural variability of climate considerably affects hydro-climatic trend significance, and therefore, removal of such influence is essential to understand the unidirectional trends due to global warming. The objective of this study was to evaluate the trends in precipitation extremes in the arid province of Pakistan by removing the natural variability of climate to understand the effect of global warming on precipitation extremes during two major cropping seasons, Rabi and Kharif. Daily precipitation data of APHRODITE (Asian Precipitation-Highly Resolved Observational Data Integration Towards Evaluation) for the period 1951-2015 was used for this purpose. An improved form of classical Mann-Kendall (MK) test known as modified Mann-Kendall (MMK) was used which can estimate trends by discarding the influence of natural cycles present in time series. The results were compared with the classical MK test to show the novelty in the findings of this investigation. The results revealed a large influence of climate fluctuations on the trends in all the extreme precipitation indices for both seasons. The reduction in trend significance was noticed between 25 and 100% for different precipitation indices when MMK instead of MK test was used. The reduction was observed more for the positive trends in the indices compared with negative trends. The results revealed that global warming caused an increase in total annual precipitation at a rate of 2.8-34.8 mm/decade during 1951-2015. Besides, the annual number of extreme precipitation days was found to increase in the north by 0.1-0.84 days/decade and the number of annual precipitation days to decrease in the west for all seasons up to - 8.6 days/decade. An increase in continuous precipitation days was detected by 0.6-1.0 day/decade in the northeast while a decrease by - 0.5 to - 1.0 days/decade in the southwest and northwest. The continuous dry days decreased in the north and the central regions by up to - 6.3 days/decade while a rise in 1-day maximum precipitation by 6.6-35 mm/decade in the central north. Analysis of results revealed that the overestimation of trends by classical MK test is more in the arid region of Pakistan compared with other regions.