Analysis of future precipitation in the Koshi river basin, Nepal

Abstract

We analyzed precipitation projections for the Koshi river basin in Nepal using outputs from 10 General Circulation Models (GCMs) under three emission scenarios (B1, A1B and A2). The low resolution future precipitation data obtained from the GCMs was downscaled using the statistical downscaling model LARS-WG. The data was downscaled for 48 stations located in the six physiographic regions in the Koshi basin. The precipitation projections for three future periods, i.e. 2020s, 2055s and 2090s, are presented using empirical Probability Density Functions (PDFs) for each physiographic region. The differences between the mean values of individual GCM projections and the mean value of the multi-model for the three scenarios allow for the estimation of uncertainty in the projections. We also analyzed the precipitation of the baseline and future periods using six indices that are recommended by the Expert Team on Climate Change Detection, Monitoring and Indices (ETCCDMI). Results indicate that not all GCMs agree on weather changes in precipitation will be positive or negative. A majority of the GCMs and the average values of all the GCMs for each scenario, indicate a positive change in summer, autumn and annual precipitation but a negative change in spring precipitation. Differences in the GCM projections exist for all the three future periods and the differences increase with time. The estimated uncertainty is higher for scenario A1B compared to B1 and A2. Differences among scenarios are small during the 2020s, which become significant during the 2055s and 2090s. The length of the wet spell is expected to increase, whereas the length of the dry spell is expected to decrease in all three future periods. There is a large scatter in the values of the indices: number of days with precipitation above 20 mm, 1-day maximum precipitation, 5-day maximum precipitation, and amount of precipitation on the days with precipitation above 95th percentile, both in direction and magnitude of the change.