Defining ‘himalayan catchment’ for better understanding of climate change impact on Himalayan glaciers and river flows

River flow response to the changing climate is a major concern in the Himalayan region. Present understanding regarding the impact of glacier shrinkage on the river flow variations is summarized in the IPCC 2007, which stated that “as these glaciers retreat due to global warming, river flows are increased in the short term, but the contribution of glacier melt will gradually decrease over the next few decades” and “the enhanced melting of glaciers leads at first to increased river runoff and discharge peaks and an increased melt season” (IPCC, 2007b). This paper argues that this conclusion is overwhelmingly biased towards the Alpine glacier systems, where glacier melt occurs during otherwise low flow regime of the annual stream hydrograph. Major parts of the Himalayan glaciers are in a precipitation dominant system, where western disturbances in winter and monsoon in summer dominate the regional hydrology. Monsoon also suggested to be playing an important role in the glacial processes by influencing the growth and decay of these glaciers. Here the ‘Himalayan catchment’ is defined as the glacier catchments experiencing snowfall in winter and monsoon precipitation in summer, where peak discharge from the glacier contributes to the crest of the annual streamflow hydrograph produced by the monsoon rains. It is suggested that the Himalayan glacier catchment could experience higher glacier discharge during the positive mass balance regime of the glacier and lesser discharge during the negative mass balance regime, which is contrary to the Alpine system response. Hence, this paper suggest that the enhanced melting of glaciers under a climate change regime need not necessarily produce higher discharge in a glacier fed stream within the ‘Himalayan catchment’ as suggested by the IPCC 2001 and IPCC 2007. Moreover, glacier melt component in the mountain streams would be highest during the years of low summer runoff. This paper also classify glacier catchments in the Himalayas in to three distinct glacio-hydrological regimes such as Alpine, Himalayan and cold-arid regimes based on the temporal distribution characteristics of precipitation and runoff.