Hydrologic sensitivities of Colorado River runoff to changes in temperature and precipitation

The Colorado River is the primary water source for much of the rapidly growing southwestern United States. Several recent studies have projected reductions in Colorado River flows by mid-century ranging from less than 10% to almost 50%, a range so large as to greatly complicate the potential for management responses. These differences in projections are attributable to wide variations in climate model projections, but also reflect variations in hydrologic model sensitivities to changes in precipitation and temperature. We evaluate the second contribution to the overall uncertainty, specifically, variations in land surface model runoff change sensitivities to changes in precipitation and temperature. We compare multi-decadal simulations from five commonly used semi-distributed hydrological models (Variable Infiltration Capacity model, Noah land surface scheme, Sacramento Soil Moisture Accounting Model, Catchment model, and Community Land Model) all applied over the entire Colorado River Basin at 1/8-degree latitude by longitude spatial resolution. We find that the annual elasticity of model runoff to precipitation (percent change in annual runoff divided by fractional change in annual precipitation) ranges from a little over one to almost eight depending on the hydrological model and reference climate. Elasticities generally are higher in lower precipitation and/or runoff regimes, hence some of the highest values are for models that are biased low in terms of runoff production. As temperatures increase, streamflow timing shifts to earlier springtime peaks and lower overall annual flow values. However the magnitude of annual temperature sensitivity (percent change in annual runoff for an imposed increase in annual temperature) differs depending on the construction of the particular hydrological model, ranging from basinwide declines in flow of about 3% to 10% per °C increase in annual average temperature minimum and maximum, but on rare occasion, small areas of mid-elevation show increasing flows with increasing temperature. Generally, spatial resolution in complex terrain plays an important role in the range of results and subsequent uncertainty in Colorado River flows because the basin's runoff is strongly controlled by the relatively small (less than 25%) headwaters area.