The effects of climate change on hydrological regimes over the coming century have become a priority area, both for process research and for water and catchment management strategies. The development of general circulation models (GCMs) with dynamic modelling of ocean circulation, atmosphere-ocean interactions and sulphate aerosol effects has vastly increased the value of these models for hydrologists. Operating in 'transient' mode, these GCMs now offer physical models of the climatic effects of projected trends in greenhouse gases and aerosols on a year-by-year basis. The results are series of probable scenarios rather than predictions. One of the greatest advances has been the release of scenario sequences of daily weather, which is now enabling research into the frequencies of extreme events.
There remain, however, two major limitations in current GCM output. One is the coarseness of spatial resolution, which is generally orders of magnitude coarser than required for most river basin studies. The other is that the modelling of the hydrological components, precipitation, evapotranspiration, soil moisture and runoff is considerably less reliable than temperature and pressure.
This special issue is devoted to research that is trying to bridge the interface between meteorology and hydrology in this field. Many papers are concerned with developing and testing techniques for 'downscaling' the GCM output. Two papers explore the relationships between hydrometeorology and larger-scale atmospheric circulation patterns, like the North Atlantic Oscillation and El Niรฑo-Southern Oscillation, which might be used to fine-tune or condition stochastic rainfall generators. Others look at the magnitude and return periods of extreme events. These focus on shifts and uncertainties associated with climate change and climate variability, or on the development and testing of limited-area models to simulate localized rainfall events better, analysing, for example, the effects of convective activity, orographic enhancement and local sea surface temperatures. The ultimate aim is to nest these high-resolution models within the framework of the GCM output.
The papers have been selected from work presented to recent meetings of the International Geographical Union (IGU) Study Group on Water Sustainability and its precursor (until 2000) devoted to Environmental Change and Extreme Hydrological Events. Readers interested in the work of the IGU Group are referred to our website at http://water-sustainability.ph.unito.it, which presents our aims and objectives and contains reports on past meetings and details of plans for future meetings.