Assessment of impacts to hydroclimatology and river operations due to climate change over the Colorado River Basin

This dissertation investigated the impacts of climate change to the hydroclimatology and river and reservoir management operations within the Colorado River Basin. Preliminary research indicated observed warming trends throughout the Colorado River Basin and corresponding seasonal trends to the magnitude and timing of runoff in the Colorado River Basin. Subsequent research investigated the changing character of precipitation and corresponding impacts to streamflow over the Colorado River Basin. Analysis of snowpack telemetry (SNOTEL) stations over the American West and Colorado River Basin indicated decreasing trends in annual snowpack, often at least at the 95% confidence interval. A shorter snowpack season was observed within the gage record at most SNOTEL locations throughout the western United States; the length of the snowpack season decreased approximately 1 day per year throughout much of the Colorado River Basin. Decreasing snowpack trends correspond with decreased runoff over the Colorado River Basin. Research then focused on the derivation of streamflow projections under changing climate conditions. Using temporally disaggregated, bias corrected and spatially downscaled climate projections of temperature and precipitation to force the National Weather Service River Forecasting System developed over the Colorado River Basin by the Colorado Basin River Forecast Center, projections of unregulated streamflow under climate change conditions were derived over three Colorado River headwater basins. Projections of unregulated streamflow over the Gunnison and San Juan River Basin decreased approximately 15% to 20% over the 90 year projection period. Over the Green River Basin, an increase of approximately 3% was projected over the same 90 year period. Information from these streamflow projections were then used to force a river management planning model utilized by the United States Bureau of Reclamation (Reclamation) over the San Juan River Basin. This research contributed to the understanding of hydroclimatology within the Colorado River Basin and impacts to river hydrology and management under changing climate conditions. This was done primarily in three sections. First, trends in snowpack characteristics were compared to annual and seasonal trends in streamflow to improve understanding of how hydroclimatic indices impact streamflow within the Colorado River Basin. Secondly, temporally disaggregated bias-corrected spatially downscaled projections of climate were used to derive streamflow projections over the Green, Gunnison, and San Juan River Basin. Changes to evapotranspiration with temperature were taken into consideration, and projections were subjected to analysis for evidence of nonstationary behavior. Finally, this dissertation represents Reclamation\u27s first effort in the Colorado River Basin to incorporate climate change information into a planning model. This research improves the understanding of the relationship between climatic variables and hydrology within the Colorado River Basin, and successfully derives projections of streamflow using projections of temperature and precipitation over Colorado River headwater basins. These streamflow projections may be used by water resource managers to evaluate potential ranges of resource management as impacts from climate change are realized. Information from these streamflow projections are incorporated into a Reclamation planning model. This research provides a proof of concept that may be followed to incorporate climate change information into environmental water resource planning and operations. With changing climate conditions, Reclamation must maintain proactive conservation efforts and efficient water management practices to meet water delivery requirements and flow recommendations