1. How much water will be available in the upper Colorado River Basin under projected climatic changes? Abstract The upper Colorado River Basin (UCRB), is a principal source of water for urban and agricultural demands in the arid and semi-arid Southwestern United States. Projected climatic changes in the basin suggest some precipitation increases in the very highest regions, and no changes or declines for the middle and low elevations, while air surface temperatures are expected to increase by about 5 ºC by the end of the century. We model the effects of these climatic changes on all hydrologic components (snowmelt, ET, surface runoff, subsurface runoff, groundwater, and streamflow) on the subbasin scale to project future changes in water resources using SWAT. We find that spring and summer season flows are likely decrease substantially by the end of the century, %. Many subbasins will turn from semi-arid to arid conditions by the 2080’s. The role of the individual hydrologic components and the impact on the availability of water resources for urban and agricultural use in the region are quantified. Iris T. Stewart 1, Darren L. Ficklin 1, Edwin P. Maurer 2 1 Department of Environmental Studies and Sciences, Santa Clara University, Santa Clara, CA 95053 2 Civil Engineering Department, Santa Clara University, Santa Clara, CA 95053 Objective and Methods: The goal is to quantify the effects of climate change in the UCRB on all hydrologic flow components and water availability on the subbasin scale A calibrated and validated SWAT model of the UCRB using 46 unimpaired sites and driven by downscaled output from 16 GCMs was used to investigate the effects of climate change under the A2 emission scenario. All flow components (snowmelt, streamflow, ET, surface runoff, subsurface runoff, groundwater flow) were extracted and analyzed. Conclusions Although there is a large range of projected future precipitation patterns, the model ensemble in general suggests less precipitation for large areas of the UCRB. Projected decreases in streamflow from changes in hydrologic components (surface runoff, subsurface flow, soil water storage, and evapotranspiration) at the subbasin-level due to climate change are likely to have serious impacts on water resources in the UCRB, with the greatest changes expected at lower elevations. The expected changes in the volume and seasonality of streamflow and other hydrologic components have potential consequences for land cover and land use. A decreased availability of soil water in arid landscapes is likely to lead to a decrease in vegetative cover, and increased soil exposure, soil erosion, and soil loss. Due to increases in aridity in many low-elevation regions within the CRB, many areas that are currently under rain-fed agricultural production will likely have to transition to irrigated agriculture, drought-tolerant crops, or other uses. Acknowledgements We gratefully acknowledge financial support for this work from the U.S. Environmental Protection Agency through EPA STAR Grant No. RD-83419101-0. Fig. 1: Watersheds, elevation, and calibration sites in the upper Colorado River Basin. Fig. 2: Average (over 16 GCMs) projected temperature and precipitation changes under the A2 scenario (20 th, 50 th, and 80 Th percentiles. Fig. 3: The propagation of streamflow timing changes from the headwaters to the Lee Ferry outflow. Fig. 4: Projected changes in spring streamflow under the A2 scenario by the 2050’s and 2080’s. The 25 th, 50 th, and 75 th percentile over the 16 GCMs are shown. Fig. 5: Projected changes in spring nowmelt under the A2 scenario by the 2050’s and 2080’s. Fig. 6 Projected changes in the aridity index under the A2 scenario by the 2050’s and 2080’s. Results For the UCRB (Fig. 1), the ensemble of GCM models projects increases in temperature by 4-6 °C, precipitation projections vary in direction, magnitude, and spatial pattern, but drier conditions are more likely, especially in the San Juan and Colorado River watersheds (Fig. 2). The SWAT model yielded satisfactory calibration and validation results (NS > 0.7, R^2 > 0.75). Our modeling results suggest statistically significant changes in streamflow towards earlier runoff (Fig. 3) and a general decrease in annual average streamflow (23% at Lee’s Ferry for the 2080’s). Projections range from a 44% decrease (25% percentile) to a 15% increase (75% percentile). Streamflow changes propagate from headwaters to Lee’s Ferry outflow (Fig 3). Spring streamflow, an important contribution to annual flow, is likely to decrease by 20 – 90% (Fig. 4). Snowmelt is likely to decrease throughout the mid and low elevations of the upper Colorado River Basin. In the San Juan and Colorado Basins, snowmelt is likely to be reduced by more than 80% by the 2080’s (Fig. 5). The lower elevation areas where the largest decreases in spring streamflow and snowmelt are expected, are the most likely to stay or become arid landscapes, while the highest elevations are likely to remain mostly humid (Fig. 6). With expected climatic changes, the snowmelt, streamflow, soil water, and surface flows are expected to decrease and arrive earlier while ET is expected to increase. Fig. 7 Hydrologic flow components.