Climate change and river flow in partially-glacierised catchments

Climatic and hydrological data for stations in the Himalayan headwaters of the Ganges and Indus rivers were obtained where available from national meteorological institutes and bodies responsible for river flow gauging. The aim was to develop a database of climatic variables with long homogenous series at monthly resolution for periods of up to 150 years, and of river flow data for as long as available for riverswith natural flow, i.e. upstream of dams which regulate discharge at seasonal timescale. Glaciologically-relevant climatic variables which influence glacier massbalance were taken from records for stations at high elevations generally in the range 1000 to 2000 m a.s.l. and up to 4531 m a.s.l. along the Himalayan arc. A northwestto south-east transect was combined with two stations on the Tibetan plateau.Hydrological observations with long term records cover rivers tributary to the Indus and Ganges, which drain partially-glacierised basins. The stations are located are at elevations between 191 and 3688 m a.s.l. and have records of between 8 and 45 years in length. Mean annual and summer (May through September) temperaturesacross Himalayan arc increased with initial warming between 1900 and 1950,followed by dips until around 1980, before recovering to levels just above 1950 maxima. Total winter precipitation between November and May in north-westregions of the Himalaya was stable at many stations but declined 18% and 41% at Srinagar and Shimla respectively. Total monsoon (summer) precipitation (June through September) declined considerably, by around 50% in the western and central Himalaya but 35-40% in far eastern Himalaya.River flows declined also in this period. In the upper Indus tributaries, flows reduced by up to 24 % between the 1960s and 1990s, whilst the upper Sutlej River declined by about 30% in the same period by comparison with about 10 % in central Himalaya of Nepal. Summer temperatures have been insufficient to enhance snow and icemelt to offset falling flows resulting from reducing precipitation. Precipitation is the main driver of river flow in the Himalaya as the monsoon covers wide swaths of discharge fail to offset declining monsoon precipitation