Hydrologic response to climate change in selected basins across the United States throughout the 21st century

In a global climate-change study, air temperature and precipitation data for the entire 21st century simulated from five General Circulation Models were used as input to pre-calibrated watershed models for 14 selected basins across the United States. Simulated daily streamflow and energy output from the watershed models were used to compute a range of statistics. With a side-by-side comparison of the statistical analyses for the 14 basins, regional climatic and hydrologic trends over the 21st century could be identified. Low-flow statistics (95-percent exceedance, 7-day mean annual minimum, and summer mean monthly streamflow) decreased for almost all basins. Annual maximum daily streamflow also decreased in all the basins except for all four basins in California and the Pacific Northwest. For all 14 basins the Julian date of the annual maximum daily streamflow moved to an earlier date in the year. An analysis of the supply of available energy and water for the basins indicated that ratios of evaporation to precipitation and potential evapotranspiration to precipitation for most of the basins will increase. Probability density functions (PDFs) were developed to assess the uncertainty and multimodality in the impact of climate change on mean annual streamflow variability. With exception of four basins, all located in the Western U.S., a Kolmogorov-Smirnov test showed a significant difference between the beginning and ending 21st century PDFs for the basins. Two basins in the upper Midwest were more dispersed and skewed than for all the other basins. Approximately half of the 14 basins PDFs showed some bio-modal shape.