Potential impacts of climatic warming on glacier-fed river flows in the Himalaya

The Himalayan region is one of the most highly glacierised areas on Earth. Regarded as the “water towers” of Asia, the Himalayas are the source of several of the world’s major rivers. The region is inhabited by some 140 million people and ten times as many (~1.4 billion) live in its downstream river basins. Freshwater from the mountains is vital for the region’s economy and for sustaining the livelihoods of a fast-growing population. Climatic warming and the rapid retreat of Himalayan glaciers over recent decades have raised concerns about the future reliability of mountain melt-water resources, leading to warnings of catastrophic water shortages. Several previous studies have assessed climate change impacts on specific glacier-fed rivers, usually applying meso-scale catchment models for short simulation periods during which glacier dimensions remain unchanged. Few studies have attempted to estimate the effects on a regional scale, partly because of the paucity of good quality data across the Himalaya. The aim of this study was to develop a parsimonious grid-based macro-scale hydrological model for the Indus, Ganges and Brahmaputra basins that, in order to represent transient melt-water contributions from retreating glaciers, innovatively allowed glacier dimensions to change over time. The model initially was validated over the 1961-90 standard period and then applied in each basin with a range of climate-change scenarios (sensitivity analysis- and climate-model-based) over a 100-year period, to gain insight on potential changes in mean annual and winter flows (water availability proxies) at decadal time-steps. Plausible results were obtained, showing impacts vary considerably across the region (catchments in the east appear much less susceptible to glacier retreat effects than those in the west, due to the influence of the summer monsoon), and, in central and eastern Himalayan catchments, from upstream to downstream (effects diminish rapidly downstream due to higher runoff from non-glaciated parts).