Spatial distribution of the trends in precipitation and precipitation extremes in the sub-Himalayan region of Pakistan

The northern sub-Himalayan region is the primary source of water for a large part of Pakistan. Changes in precipitation and precipitation extremes in the area may have severe impacts on water security and hydrology of Pakistan. The objective of the study is to evaluate the spatial characteristics of the trends in annual and seasonal precipitation and precipitation extremes in Gilgit Baltistan, the northern administrative territory of Pakistan surrounded by Hindu Kush, Karakoram, and the Himalayan regions. The daily gridded rainfall data (1951-2007) of Asian Precipitation-Highly Resolved Observational Data Integration Towards Evaluation (APHRODITE) at 0.25 degrees spatial resolution was used for evaluating the trends. The novelty of the present study is the application of the modified Mann-Kendall (MMK) trend for the evaluation of the significance of precipitation trends in order to differentiate the secular trends from climate natural variability. Besides, cumulative distribution function (CDF) plots of daily rainfall for the early (1951-1980) and later (1981-2007) periods were used to show the changes in extremes. The results revealed no significant change in annual precipitation but increase in summer rainfall in the range of 0.25 to 1.25mm/year in the upper part and decrease in winter precipitation from 0 to -0.25mm/year in the west part of the region. Annual number of rainy days was also found to decrease in winter up to -1.33days/decade where the region receives a major portion of total precipitation. The decrease in winter rainfall and rainy days caused an increase in continuous dry days (around 0.27days/year) and decrease in continuous wet days (up to -0.26days/year). Trend analysis and CDF plot revealed that though the numbers of rainy days are decreasing, the numbers of extreme rainfall days are increasing, which indicates rainfall become more erratic and intense in the region. The increases in both continuous dry days and extreme rainfall days indicate more droughts and floods may have adverse impacts on the hydrology of Pakistan.