Increased Temperatures Overwhelm Precipitation Changes Leading to Streamflow Declines in the Colorado River Basin

The Colorado River Basin (CRB) has experienced widespread and prolonged drought in the 21^st century with recent precipitation (P) up to 25% below historical means and air temperature (T) up to 0.8 ^oC warmer. The extent that continued warming will lead to streamflow (Q) decline is unclear given the high interannual variability of P. Here we explore physically plausible ways that climate change could impact Q using the Variable Infiltration Capacity (VIC) model. We integrated advances in VIC using Landsat- and MODIS-based products to produce more realistic land surface conditions and used this setup to simulate long-range Q projections. Meteorological datasets were sourced from gridded daily observations (1950-2013) and downscaled historical (1950-2005) and future projections (2006-2099) derived from multiple CMIP5 models under a low and a high emission scenario to explore forcing uncertainties and cases where P increase could offset warming. We compared the impacts of anticipated climate change on hydrologic responses in subbasins key for water management to gauge their importance for basin-wide water budgets and how these relationships could evolve in time, as this has been a largely unexplored aspect in the CRB. Results showed that spatial gradients in seasonal P changes led to contrasting seasonal responses in runoff (R) across the CRB. Whereas most of the Upper Basin had a shift to greater R during the winter, summer R declined over most of the CRB due to heightened evapotranspiration in the northwest (Green, Upper Colorado, Glen Canyon, and Grand Canyon subbasins) and large P decline in the southeast (San Juan, Little Colorado, and Gila subbasins). The strength of seasonal runoff signals across different climate models and their impacts to annual Q were dependent on subbasin area and emission scenario. Annual Q at the CRB outlet declined in most cases, however, reflecting the pervasive drying effect of warming.