The potential effects of climate change on the hydrology and water resources of the Colorado River basin are assessed by comparing simulated hydrologic and water resources scenarios derived from downscaled climate simulations of the U. S. Department of Energy/ National Center for Atmospheric Research Parallel Climate Model (PCM) to scenarios driven by observed historical (1950 - 1999) climate. PCM climate scenarios include an ensemble of three 105-year future climate simulations based on projected 'business-as-usual' (BAU) greenhouse gas emissions and a control climate simulation based on static 1995 greenhouse gas concentrations. Downscaled transient temperature and precipitation sequences were extracted from PCM simulations, and were used to drive the Variable Infiltration Capacity ( VIC) macroscale hydrology model to produce corresponding streamflow sequences. Results for the BAU scenarios were summarized into Periods 1, 2, and 3 ( 2010 - 2039, 2040 - 2069, 2070 - 2098). Average annual temperature changes for the Colorado River basin were 0.5 degreesC warmer for control climate, and 1.0, 1.7, and 2.4 degreesC warmer for Periods 1 - 3, respectively, relative to the historical climate. Basin-average annual precipitation for the control climate was slightly (1%) less than for observed historical climate, and 3, 6, and 3% less for future Periods 1 - 3, respectively. Annual runoff in the control run was about 10% lower than for simulated historical conditions, and 14, 18, and 17% less for Periods 1 - 3, respectively. Analysis of water management operations using a water management model driven by simulated streamflows showed that streamflows associated with control and future BAU climates would significantly degrade the performance of the water resources system relative to historical conditions, with average total basin storage reduced by 7% for the control climate and 36, 32 and 40% for Periods 1 - 3, respectively. Releases from Glen Canyon Dam to the Lower Basin ( mandated by the Colorado River Compact) were met in 80% of years for the control climate simulation ( versus 92% in the historical climate simulation), and only in 59 - 75% of years for the future climate runs. Annual hydropower output was also significantly reduced for the control and future climate simulations. The high sensitivity of reservoir system performance for future climate is a reflection of the fragile equilibrium that now exists in operation of the system, with system demands only slightly less than long-term mean annual inflow.
