GLOBAL WARMING AND THE HYDROLOGIC-CYCLE

Starting with a review of the basic processes that govern greenhouse w arming, we have demonstrated that the hydrologic cycle plays a key rol e in the heat balance of the Earth's surface-atmosphere system. Throug h the water and other climatic feedbacks, the hydrologic cycle is show n to be a key factor in the climate's evolution as greenhouse gases co ntinue to build up in the atmosphere. This paper examines the current predictive capability of general circulation models linked with macros cale and landscape-scale hydrologic models that simulate regional and local hydrologic regimes under global warming scenarios, Issues concer ning hydrologic model calibration and validation in the context of cli mate change are addressed herein, It is shown that the natural uncerta inty in hydrologic regimes in the present climate introduces a signal- to-noise interpretation problem for discerning greenhouse-induced vari ations in regional hydrologic regimes. Simulations of river basins by means of macroscale hydrologic models nested within general circulatio n models have been implemented in a few selected cases. From the persp ective of water resources management, such simulations, carried out in detail under greenhouse-warming scenarios in midlatitudinal basins of the United States, predict shorter winter seasons, larger winter floo ds, drier and more frequent summer weather, and overall enhanced and p rotracted hydrologic variability. All these predictions point to poten tially worsening conditions for flood control, water storage, and wate r supply in areas of semiarid midlatitudinal climates currently depend ent of spring snowmelt. Little information of this type is currently a vailable for other areas of the world. Practice of sound water resourc es engineering principles ought to be adequate to cope with additional hydrologic uncertainty that might arise from global warming.