The Koshi Basin, spread across China, Nepal and India, is perceived as having high potential for hydropower and irrigation development, both seen as ways to promote economic development in the region. As climate change (CC) is likely to impact future water resources, it is very important to consider CC in future planning. This paper quantifies and assesses the past and projected future spatial and temporal water balances in the Koshi Basin. A Soil and Water Assessment Tool (SWAT) model was set up, calibrated and validated using measured daily flow data from the basin from 1999 to 2006. The CC analysis is based on the most recent Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) “Representative Concentration Pathways” (RCPs 4.5 and 8.5). The delta change approach was used to generate daily future time series climate data for 2021-2050, considering historical data from 1998-2008 as reference.
Results show that the annual average precipitation, actual evapotranspiration (ET) and net water yield for the reference period are 1,720 mm, 520 mm and 1,124 mm, respectively, with over 75% of precipitation and flow occurring during the monsoon season. The precipitation and net water yield are lowest in the transmountain region and the Tibetan plateau. The values are highest in the mountain region, followed by the hills and Indo-Gangetic Plains. Approximately 65% of average annual precipitation is converted to flows, indicating high water availability. Actual ET, which indicates water use by plants, is highest in the Indo-Gangetic Plains region due to the presence of irrigated agriculture and a few forested mountain watersheds. There is, therefore, a clear mismatch between the area where water availability is highest (mountains) and where water use is highest (Indo-Gangetic Plains). As most of the water from the mountain and hill regions eventually flows down to the plains, the mountain and hill regions in Nepal are important for maintaining agriculture in the plains in both Nepal and India.
Flow analyses show that high-flow pulses (exceeding 75% of daily flows) occur two to five times annually and last for 2 to 20 days during the monsoon season. Extreme low flows occur two to nine times annually and last up to 25 days during the dry season. These results indicate the high temporal variability of flows in the basin. The information on these extreme weather events could be very useful in water-induced disaster management in this region, and also for planning longterm hydraulic structures. The frequent occurrences of both high- and low-flow events demonstrate the existing vulnerability of the region to both floods and droughts, leading to a very risk-prone livelihood system.
The CC projections show an increasing trend in precipitation and net water yield for most of the basin, except the transmountain region. Actual ET also shows an increasing trend for both CC pathways (RCPs 4.5 and 8.5) throughout the basin. Comparison of flow ranges between the past and projected data indicates that future changes during the dry season are within the past data ranges. However, the future monsoon flows will be higher than the past as demonstrated by higher averages as well as maximum flows. An increasing trend is seen in the high-flow pulse frequency, and occurrences of high flows are shifting towards the latter part of the monsoon. Therefore, flooding occurrences in the monsoon are expected to increase with CC.
Results also show that the seasonal variation in contribution to annual flow volume at the outlet of the Koshi Basin for the reference and future periods remained very similar, with over 75% of the total annual flow occurring within the monsoon season. Therefore, the monsoon will remain the main hydrological driver, variability will continue to be high, future dry seasons will be similar to the past dry seasons, and future monsoons will likely be stronger and wetter with increased flood risk.