Water resources implications of global warming: A US regional perspective

The implications of global warming for the performance of six U.S. water re source systems are evaluated. The six case study sites represent a range of geographic and hydrologic, as well as institutional and social settings. L arge, multi-reservoir systems (Columbia River, Missouri River, Apalachicola -Chatahoochee-Flint (ACF) Rivers), small, one or two reservoir systems (Tac oma and Boston) and medium size systems (Savannah River) are represented. T he river basins range from mountainous to low relief and semi-humid to semi -arid, and the system operational purposes range from predominantly municip al to broadly multi-purpose. The studies inferred, using a chain of climate downscaling, hydrologic and water resources systems models, the sensitivit y of six water resources systems to changes in precipitation, temperature a nd solar radiation. The climate change scenarios used in this study are bas ed on results from transient climate change experiments performed with coup led ocean-atmosphere General Circulation Models (GCMs) for the 1995 Intergo vernmental Panel on Climate Change (IPCC) assessment. An earlier doubled-CO 2 scenario from one of the GCMs was also used in the evaluation. The GCM sc enarios were transferred to the local level using a simple downscaling appr oach that scales local weather variables by fixed monthly ratios (for preci pitation) and fixed monthly shifts (for temperature). For those river basins where snow plays an important role in the current cl imate hydrology (Tacoma, Columbia, Missouri and, to a lesser extent, Boston ) changes in temperature result in important changes in seasonal streamflow hydrographs. In these systems, spring snowmelt peaks are reduced and winte r flows increase, on average. Changes in precipitation are generally reflec ted in the annual total runoff volumes more than in the seasonal shape of t he hydrographs. In the Savannah and ACF systems, where snow plays a minor h ydrological role, changes in hydrological response are linked more directly to temperature and precipitation changes. Effects on system performance va ried from system to system, from GCM to GCM, and for each system operating objective (such as hydropower production, municipal and industrial supply, flood control, recreation, navigation and instream flow protection). Effects were generally smaller for the transient scenarios than for the dou bled CO2 scenario. In terms of streamflow, one of the transient scenarios t ended to have increases at most sites, while another tended to have decreas es at most sites. The third showed no general consistency over the six site s. Generally, the water resource system performance effects were determined by the hydrologic changes and the amount of buffering provided by the syst em's storage capacity. The effects of demand growth and other plausible fut ure operational considerations were evaluated as well. For most sites, the effects of these non-climatic effects on future system performance would ab out equal or exceed the effects of climate change over system planning hori zons.