Water Management Decisions Using Multiple Hydrologic Models within the San Juan River Basin under Changing Climate Conditions

A modified version of the U. S. Bureau of Reclamation (Reclamation) long-term planning model, Colorado River Simulation System (CRSS), is used to evaluate whether hydrologic model choice has an impact on critical decision variables within the San Juan River Basin when evaluating potential effects of climate change through 2099. The distributed variable infiltration capacity (VIC) model and the lumped National Weather Service (NWS) River Forecast System (RFS) were each used to project future streamflow; these projections of streamflow were then used to force Reclamation's CRSS model over the San Juan River Basin. Both hydrologic models were compared to evaluate whether or not uncertainty in climatic input generated from general circulation models outweighed differences between the hydrologic models. Differences in methodologies employed by each hydrologic model had a significant effect on projected streamflow within the basin. Both models project decreased water availability under changing climate conditions within the San Juan River Basin, but disagree on the magnitude of the decrease. On average, total naturalized inflow within the San Juan River Basin into the Navajo Reservoir is approximately 15% higher using inflows derived using the VIC model than those inflows developed using the RFS model; average projected tributary inflow from the San Juan River Basin to the Colorado River is approximately 25% higher using inflows derived by using the VIC model than those inflows developed by using the RFS. Overall, there is a higher risk and magnitude of shortage within the San Juan River Basin using streamflow developed with the RFS model as compared with inflow scenarios developed by using the VIC model. Model choice was found to have a significant effect on the evaluation of climate change impacts over the San Juan River Basin. DOI: 10.1061/(ASCE)WR.1943-5452.0000237. (C) 2012 American Society of Civil Engineers.