The perennial rivers that originate from high Himalayas are the major source of water during dry seasons when we need them the most. The flow in such rivers during dry seasons is supplied by melting of snow and glaciers-the natural frozen water reserves in the high altitudes. That’s why they are often referred as the Water Towers. Besides being an important natural resource for socio-economic activities of humans, the snow and glaciers are repositories of information on global climate change scenario. This study makes a simple attempt to estimate the contribution of snow and glacier melt discharge in river flow as well as change in river flow under climate change scenario.

In this study, Gandaki and Karnali basin in Nepal is selected as a study area. This study comprises of use of two models, Energy budget model and HEC-HMS model for the simulation of hydrology of the basin. Energy budget model is used in the upper part of the basin with elevation greater than 3000m where there is glacerised area and HEC-HMS model is used in lower part of the basin below 3000m. For the determination of snow and glacier area to be used in EBM, LANDSAT TM7 image is processed by using ERDAS IMAGINE. Energy budget model is used to determine the melt volume of the debris covered and clear ice glacier. These melt volumes along with the precipitation in higher altitude is given as input to TANK model in order to generate the out flow at the basin outlet. The river discharge thus obtained from TANK is fed to HEC-HMS along with meteorological data at lower elevation in order to simulate river discharge at different locations of the basin. HEC-HMS model is calibrated and validated. Significance of snow and glacier melt contribution at different downstream stations is assessed. Finally, future precipitation and temperature data obtained by using AR5 scenario is used in order to assess the significance of climate change in river flow.

While analysing the aerial distribution of satellite image it is seen that the area of glacier in Gandaki basin is 2.3% while that in Karnali basin is 1.2%. The area of glacier decreases while moving toward western part of the country. For the hydrological simulation of glacerised watershed three glacier stations are used namely Kyangjing, Bagar Khola and Halji.

Result shows that in Gandaki basin snow and glacier contribute about 7.40% of total annual flow at Narayanghat. The contribution is more significant in dry period in which, snow and glacier contributes about 13.68%. Among different sub-basins of Gandaki basin, Seti River has maximum contribution of 14.18% at Phoolbari and Trishuli River has minimum contribution of 9.65% at Betrawati. Similarly, in Karnali basin snow and glacier contribute about 10.13% of total annual flow and about 13.69% of dry period flow. The contribution of snow and glacier in three sub-basins of Karnali basin are similar (i.e. around 11%).

While assessing the effect of climate change, it is seen that in most of the sub-basins of Gandaki basin the flow volume has decreased in 2030 A.D. but has increased in 2050 A.D. But in Kaligandaki and sub-basins of Karnali the flow volume has increased in both 2030 A.D. and 2050 A.D. than in base year (1990 A.D.).

For determination of climate change impact on hydropower potential for storage project, change in total annual volume of river flow is analysed.  While analysing the total annual volume of Gandaki basin it is seen that there will be decreased in flow volume in 2030 A.D. and increase in flow volume in 2050 A.D. for most of the sub-basins. But for Karnali basin, there will be increase in volume for all sub-basins in both the years.

Similar process is used for determination of climate change impact in RoR project. The result shows that the designed discharge for RoR project at Q₄₀ design will decrease in 2030 A.D. for two sub-basins of Gandaki basin while this will increase for remaining    sub-basins. But the flow will increase in all the sub-basins in 2050 A.D in Gandaki Basin. For Karnali basin the flow volume has increased in both the years.