Assessment of water availability and river health in the Marshyangdi watershed, Nepal

Abstract

Understanding water availability is essential for water resource development and management, as well as devising river health interventions. Climate change/variability impacts the hydrology of a river system which subsequently affects human and ecological health by altering the structure and function of the aquatic ecosystem. This study, therefore, aims to assess water availability and river health under current and future climate scenarios in the Marshyangdi Watershed, central Nepal, which has a huge potential for water infrastructure development. The specific objectives are i) to assess historical trends in the climatic variables, ii) to project future climate, iii) to evaluate the impacts of projected changes (climatic) on streamflow, and iv) to assess river health under current and future climatic conditions. Historical (1983-2013) and future (near-future: 2014-2033; mid-future: 2034-2053) trends in the climatic extremes were computed using RClimDex and hydrologic extremes using the Indicators of Hydrologic Alteration (IHA) tool. Bias-corrected projected future climate for the near and mid future under moderate (RCP 4.5) and pessimistic (RCP 8.5) scenarios was developed based on multiple regional climate models. Further, trends in extreme indices were estimated using the Mann-Kendall test and Sens’s slope estimator. A hydrological model was set up in the Soil and Water Assessment Tool (SWAT). It was calibrated and validated at multiple hydrological stations. Simulated hydrological time series was used to assess water availability under current and future conditions. Similarly, river health conditions under current, as well as future scenarios, were evaluated based on a customized indicator-based framework. The annual maximum temperature was observed with a significant increasing trend over the historical period at all the stations whereas temperature-related extremes showed both increasing and decreasing trends (e.g., warm spell duration index, warm days, and summer days are increasing whereas cold spell duration index, cool days, and warm nights are decreasing). Further, trends in precipitation extremes such as the number of heavy and very heavy precipitation days and maximum 1-day precipitation were decreasing along with the average annual precipitation amount in the entire watershed, indicating drier and hotter conditions over the historical period in the watershed. vi The climate in the Marshyangdi Watershed was projected to be hotter and wetter in the future. Among the stations, maximum climatic variation was observed at the Chame Station (Index: 816), with average annual precipitation projected to increase by 10% under RCP8.5 for mid-future, and maximum temperature increase at the rate of 0.06 o C/year. Maximum temperature and temperature-related extreme indices (hot nights and warm days) have been projected to have an increasing trend for both scenarios. Similarly, average annual precipitation has been also projected to increase at all the stations in the future for both RCPs but further decreases in consecutive dry days at most stations indicate wetter conditions in the future. Climate change is anticipated to increase hydrological alterations from low (in the current) to high (in the future) as revealed by the IHA tool. Annual average water availability increased, varying across seasons, and seasonal trends followed the annual trends. The average annual volume of water in the Marshyangdi Watershed was estimated to be 9,335 Million Cubic Meters (MCM), which will increase by 15% in the near future and 11% in the mid-future under RCP8.5 scenarios. Statistically, at a 5% level of significance, current river health showed moderate condition (67% of the sites) and it is projected to remain the same condition in the future, for both near and midfuture under both RCPs (4.5 and 8.5) scenarios but with varying degrees.

Keywords: Climate Change, Indicators of Hydrologic Alteration, Marshyangdi Watershed, River Health, SWAT