Predictions for snow cover, glaciers and runoff in a changing climate

2 CH-7270 Davos-Platz, Switzerland Abstract The problem of evaluating the hydrological effects of climate change has opened a new field of applications for snowmelt runoff models. The Snowmelt Runoff Model (SRM) has been used to evaluate climate change effects on basins in North America, the Swiss Alps, and the Himalayas. Snow covered area depletes about one month earlier in response to warmer temperatures (+4 o C) with runoff peaks shifted accordingly. Runoff will be higher in winter at the expense of summer runoff. In glacerized basins, runoff is not only redistributed, but increased due to glacier melting. This improved knowledge facilitates long-term decisions concerning hydropower, flooding, water allocations, and water management in general. NEW TASKS FOR SNOWMELT RUNOFF MODELS The growing awareness of the problem of climate change opened a new field of applications for snowmelt runoff models. Originally designed to simulate and forecast the seasonal runoff, they should now be capable to predict snow cover and runoff in a distant future. Thanks to its deterministic approach, the SRM model has been easily adapted to this new task. In the present climate, it is run with the real seasonal snow cover monitored by satellites as one of the input variables. In a future climate, this snow cover is transformed by changed temperatures and precipitation according to any given climate scenario, so that the future runoff can be computed. The necessary amendments to the computer program were facilitated by the transparent structure of the model which had been made possible by taking into account the role of the subsurface runoff (Martinec & Rango, 1999). As has been pointed out by other investigators (e.g. Klemes, 1985) hydrological models which depend on calibration of their parameters are not suitable for evaluations of the climate change effect. In the meantime, such studies have been carried out by the non-calibrated SRM model in different climate zones of North America, the Swiss Alps, and the Himalayas. HYDROLOGICAL EFFECTS OF CLIMATE CHANGE The effects of global warming, combined in some cases with changed precipitation, were evaluated in a wide range of basin size and altitude, as illustrated in Table 1. Various climate types are represented so that the runoff coefficient (runoff/ precipitation ratio in a hydrological year) varies from 0.25 in Rio Grande, 0.57 in Kings River and to 0.78 in Illecillewaet. With the exception of the Himalayan basins, runoff is dominated by snowmelt so that it occurs mostly in the summer half of the hydrological year (April-September).