Impact of climate change on flow regime in Himalayan basins, Nepal

Abstract

General Circulation Models (GCMs) successfully simulate future climate variability and climate change on a global scale; however, poor spatial resolution constrains their application for impact studies at a regional or a local level. The dynamically downscaled precipitation and temperature data from CGCM3 model were used for the climate projection, under A2 and A1B SRES scenarios. In addition, the observed historical temperature, precipitation and discharge data were collected from 14 different hydro-metrological locations for the implementation of this studies, which include watershed and hydro-meteorological characters, trends analysis and water balance computation. Estimation of the impact of climate change on flow regime at three river basins of Nepal (Annapurna, Langtang and Khumbu regions) was computed by using different Hydro-meteorological models. The simulated precipitation and temperature were corrected for bias before implementing in the Hydraologiska Byrans Vattenbalansavde (HBV), HVB-light 3.0 conceptual rainfall-runoff model to predict the flow regime, in which Groups Algorithms Programming (GAP) optimization approach and then calibration were used to obtain several parameter sets which were finally reproduced as observed stream flow. Except in summer, the analysis shows that the increasing trends in annual as well as seasonal precipitations during the period 2001 - 2060 for both A2 and A1B scenarios over three basins under investigation. In these basins, the model projects warmer days in every seasons of entire period from 2001 to 2060 for both A1B and A2 scenarios. These warming trends are higher in maximum than in minimum temperatures throughout the year, indicating increasing trend of daily temperature range due to recent global warming phenomenon. Furthermore, there are decreasing trends in summer discharge at Langtang Khola which is increasing in Modi Khola as well as Dudh Koshi river basin. The flow regime is more pronounced during later parts of the future decades than during earlier parts in all basins. The annual water surplus of 1419 mm, 177mm and 49mm are observed at Annapurna, Langtang and Khumbu region respectively.