The problem
The hydrological system of Northern India is based on two main phenomena: the monsoon precipitation in summer and the growth and melt of the snow and ice cover in the Himalaya, also called the "Water Tower of Asia". However, climate change is expected to change these phenomena and it will have a profound impact on snow cover, glaciers and its related hydrology, water resources and the agricultural economy on the Indian peninsula (Singh and Kumar, 1996, Divya and Mehrotra, 1995). Especially the perennial rivers in the north, Ganga, Indus and Brahmaputra, are susceptible to climate change as they originate from the Himalayas. Snow and glacier melt run-off form a great part of the rivers’ flow (Singh and Bengtsson, 2004).

Climate change is projected to have a short term and long term impact on the hydrological system. On the short term discharge of rivers in the north will increase due to the melting of snow and glaciers. On the long term the snow and glaciers will have melted for a great part and their contribution to the rivers’ flow will decrease. (Eriksson, 2006).

The presence of the snow cover and the timing of snow fall on the mountains and the Tibetan plateau also influences the monsoon. Cold, wet winters cause a less severe monsoon as the snow cools down the air in summer. Excessive snowfall in winter and spring delays the build up of the monsoonal temperature gradient as solar energy is used to melt the snow or reflected by the snow. The heat low in the northwest of India will be less strong resulting in a weak monsoon and decrease in precipitation.

The latest IPCC report of WG1 states: “There is a tendency for monsoonal circulations to result in increased precipitation due to enhanced moisture convergence, despite a tendency towards weakening of the monsoonal flows themselves. However, many aspects of tropical climatic responses remain uncertain” (Christensen et al., 2007). In the same report, it is concluded that the observed maximum rainfall during the monsoon season is poorly simulated by many models. The most likely cause being the coarse resolution of the models preventing a good representation of the steep georography of the area.

The Challenge
Recent work indicates that time-slice experiments using an AGCM with prescribed SST's, as opposed to a fully coupled system, are not able to accurately capture the South Asian monsoon response (Douville, 2005). The hydrological effects of climate change can best be obtained from hydrological models which have the advantage over GCM's that they can incorporate regional characteristics and climatic variations (Singh and Bengtsson, 2004).

Three-member ensembles of baseline simulations (1961–1990) from a RCM (PRECIS) at 50 km resolution have confirmed that significant improvements in the representation of regional processes over South Asia can be achieved (Rupa Kumar et al., 2006).

It is a great challenge to integrate the spatial and temporal glacier retreat and snow melt and changed monsoon pattern in weather prediction models under different climate scenarios. Furthermore, the output of these models will have an effect on the input of the hydrological models. The retreat of glaciers and a possible change in monsoon precipitation and pattern will have a great impact on the temporal and spatial availability of water resources in Northern India.

Besides climate change, socio-economic development will also have an influence on the use of water resources, the agricultural economy and the adaptive capacity. Socio-economic development determines the level of adaptive capacity. It is a challenge to find appropriate adaptation strategies with stakeholders for each of the sectors agriculture, energy, health and water supply by assessing the impact outputs of the hydrological and socio-economical models.