The control of climate sensitivity on variability and change of summer runoff from two glacierised Himalayan catchments

Abstract. The response of catchment runoff to climate forcing is determined by its climate sensitivity. We investigate the sensitivity of summer runoff to precipitation and temperature changes in winter-snow dominated Chandra (western Himalaya), and summer-rain dominated upper Dudhkoshi (eastern Himalaya) catchments in order to understand the nature of climate-change impact on the mean summer runoff and its variability. The runoff over the period 1980–2018 is simulated with a semi-distribute hydrologic model, which is calibrated using available discharge and glacier mass loss data. An analysis of the interannual variability of the simulated summer runoff reveals that the runoff from the glacierised parts of the catchments is sensitive to temperature changes, but is insensitive to precipitation changes. The behaviour of the summer runoff from the non-glacierised parts is exactly opposite. Such precipitation-independent runoff from the glacierised parts stabilises the catchment runoff against precipitation variability to some degree. With shrinking glacier cover over the coming decades, the summer runoff from the two catchments is expected become more sensitive to the precipitation forcing and less sensitive to the temperature forcing. Because of these competing effects, the impact of the glacier loss on the interannual variability of summer runoff may not be significant. However, the characteristic ‘peak water’ in the long-term mean summer runoff, which is caused by the excess meltwater released by the shrinking ice reserve, may lead to a detectable signal over the background interannual variability of runoff in these two catchments.