IMPACTS OF CLIMATE CHANGE ON THE SURFACE WATER BALANCE OF THE CENTRAL UNITED STATES, 1984-2007

The climate system and the hydrologic cycle are strongly connected with each other. Understanding the interactions between these two systems is important, since variations in climate can trigger extensive changes in the hydrologic cycle, with significant impacts on agriculture, ecosystems, and society. Observations over the central U.S. in recent decades show numerous changes in climatic variables. This includes decreases in cloud cover and wind speed, increases in air temperature, and seasonal shifts in precipitation rate and rain/snow fraction. To assess the impacts of these variations in climate on the regional water cycle, a terrestrial ecosystem/land surface hydrologic model (Agro-IBIS) is employed in this study, forced by observed climatic inputs for the period 1984-2007. The results generally show an acceleration of the water cycle in the Upper Mississippi, Missouri, Ohio, and Great Lakes basins, but with significant seasonal and spatial complexity. Over the past 24 years, evapotranspiration has increased in most regions and most seasons, particularly during the fall, which is also a time of pronounced solar brightening. Trends in runoff are characterized by distinct spatial and seasonal variations. Since recent warming has led to a greater fraction of winter precipitation falling as rain rather than snow, spring runoff in some snow-dominated regions (such as the northern Great Lakes) has declined significantly since 1984. Other regions, however, such as the northern Missouri basin, show large increases in runoff throughout all seasons, primarily as a result of increased precipitation. Sensitivity experiments show that the water balance is most linearly sensitive to solar radiation and relative humidity, followed by precipitation, air temperature and wind speed. Because of the interdependencies among the climate factors, the hydrological responses of climate change are highly non-linear. Seasonal hydrological responses are notably dependent on regional water and energy availability, and are affected by seasonal conditions of soil moisture and snow cover. Furthermore, precipitation is characterized as the predominant factor that affects the decadal scale hydroclimatic changes in the central U.S.. Adviser: John D. Lenters