Variability of climate and riverflow in partially glacierised basins in the Karakoram and western Himalaya

High specific discharges from Himalayan headwater basins arises from seasonal winter snow cover, summer monsoon precipitation and melting glacier ice in varying proportions and differing absolute quantities along west –east axes of the Karakoram and western Himalaya. Discharge records for stations in the UIB, Jhelum, Chenab and Sutlej have been examined for periods between 1920 and 2009, together with precipitation and air temperature data for stations with long records (within the period 1876 to 2013) at elevations between 234 and 3015 m a.s.l. Ice-cover age in the basins was between 222.8 and 15,061.7 km2. Climate in the Karakoram had a distinct opposing regime to climate in the western Himalaya. Totals of summer and winter precipitation at climate stations in the UIB reached their maxima in the 1990s. Winter precipitation in the western Himalaya generally declined from the 1980s, while summer precipitation declined since the 1950s but recovered from the 2000s. Mean summer air temperature had an inverse relationship with seasonal totals of precipitation. Air temperatures at UIB climate stations reached their maxima in the 1970s before declining to the 1990s, failing to recover in the 2000s. Air temperatures from mountain stations in the Himalaya displayed maxima in the 2000s, as did climate stations in Tibet. Riverflows of partially glacierised basins varied regionally, being influenced by regional climate and amount of ice coverage within a basin. Discharge in the glacierised Hunza declined from maxima in the 1970s following a reduction of mean summer air temperature and increasing precipitation since the 1990s. Riverflow in the eastern Karakoram, in the glacial Shyok basin inclined from the 1990s in response to increasing temperatures in Tibet. Annual riverflow along the Sutlej declined from the 1970s-2000s, reduction of flow in the range of 5-31%. Flows at large dams were influenced by riverflows upstream of smaller sub-basins. Annual discharge was least variable from year-to-year at Tarbela (Indus), flow being influenced not only by precipitation fluctuations but also by changes in air temperature affecting glacier melt in headwater basins. Riverflow at Bhakra (Sutlej) also had low variability, flow being influenced by winter precipitation in the upper Sutlej and by summer monsoon in the lower Sutlej. Mangla had high variability of annual runoff, as did other basins with small proportions of ice cover. Basins which had intermediate proportions of ice cover moderated intra-annual variations in runoff as in warm dry years icemelt enhanced riverflows, compensating for reduced precipitation