Managing the nations water in a changing climate

Among the many concerns associated with global climate change, the potentia l effects on water resources are frequently cited as the most worrisome. In contrast, those who manage water resources do not rate climatic change amo ng their top planning and operational concerns. The difference in these vie ws can be associated with how water managers operate their systems and the types of stresses, and the operative time horizons, that affect the Nation' s water resources infrastructure. Climate, or more precisely weather, is an important variable in the management of water resources at daily to monthl y time scabs because water resources systems generally are operated on a da ily basis. At decadal to centennial time scales, though, climate is much le ss important because (1) forecasts, particularly of regional precipitation, are extremely uncertain over such time periods, and (2) the magnitude of e ffects due to changes in climate on water resources is small relative to ch anges in other variables such as population, technology, economics, and env ironmental regulation. Thus, water management agencies find it difficult to justify changing design features or operating rules on the basis of simula ted climatic change at the present time, especially given that reservoir-de sign criteria incorporate considerable buffering capacity for extreme meteo rological and hydrological events.