Hydrologic Sensitivities of Colorado River Runoff to Changes in Precipitation and Temperature*

Abstract

The Colorado River is the primary water source for much of the rapidly growing southwestern United States. Recent studies have projected reductions in Colorado River flows from less than 10% to almost 50% by midcentury because of climate change-a range that has clouded potential management responses. These differences in projections are attributable to variations in climate model projections but also to differing land surface model (LSM) sensitivities. This second contribution to uncertainty-specifically, variations in LSM runoff change with respect to precipitation (elasticities) and temperature (sensitivities)-are evaluated here through comparisons of multidecadal simulations from five commonly used LSMs (Catchment, Community Land Model, Noah, Sacramento Soil Moisture Accounting model, and Variable Infiltration Capacity model) all applied over the Colorado River basin at 1 /88 latitude by longitude spatial resolution. The annual elasticity of modeled runoff (fractional change in annual runoff divided by fractional change in annual precipitation) at Lees Ferry ranges from two to six for the different LSMs. Elasticities generally are higher in lower precipitation and/or runoff regimes; hence, the highest values are for models biased low in runoff production, and the range of elasticities is reduced to two to three when adjusted to current runoff climatology. Annual temperature sensitivities (percent change in annual runoff per degree change in annual temperature) range from declines of 2% to as much as 9% per degree Celsius increase at Lees Ferry. For some LSMs, small areas, primarily at midelevation, have increasing runoff with increasing temperature; however, on a spatial basis, most sensitivities are negative.FIG. 10. Combined precipitation and temperature changes. Maps show the percent difference in Q DTDPest from Q DTDP ; note that the scale is considerably small relative to previous figures (e.g., total range of 2.5% vs the temperature sensitivity range of 40%). Histograms show total difference in flow with the majority of differences in all LSMs being within 60.0005 mm day 21 of zero.

Enhanced groundwater recharge rates and altered recharge sensitivity to climate variability through subsurface heterogeneity

“…Beyond a correlation analysis that simply evaluates the strength of relations between variables, elasticity quantifies "how responsive one variable is to change in another variable" (18) or "the percentage change in a first variable to the percentage change in second variable, when the second variable has a causal influence on the first variable" (19). Other than several applications of elasticity on stream flow (20)(21)(22), we apply elasticity to groundwater recharge in hydrology. Here we define recharge sensitivity as the median ratio of interannual changes of recharge rates to the interannual changes of three climatic variables that drive recharge and evapotranspiration using a 20-y period: (i) Annual precipitation expresses general water availability, (ii) mean annual temperature is used as proxy for potential evapotranspiration, and (iii) the mean intensity of high-intensity events is used to account for the nonlinear impact of strong rainfall events (23).…”

“…Similar to ref. 22, we preferred temperature over net radiation as a proxy for potential evapotranspiration because net radiation is temperature dependent and temperature is the best-understood and most common input variable to large-scale hydrological models. Recharge sensitivity with large positive or negative values indicates that recharge is highly sensitive to variations of these input variables.…”

Combined impacts of current and future dust deposition and regional warming on Colorado River Basin snow dynamics and hydrology

“…Anthropogenic warming and enhanced evapotranspiration over the coming decades put this supply at risk. Coupled climate-hydrology model simulations show a 5-20 % decrease in flow depending on the study, indicative of a greater risk of a dry future (Christensen et al, 2004;Christensen and Lettenmaier, 2007;McCabe and Wolock, 2007;Reclamation, 2011a, b;Colorado Water Conservation Board, 2012;Vano et al, 2012;Seager et al, 2012).…”

Projections of declining surface-water availability for the southwestern United States

“…California obtains most of its water from the Sierra Nevada Mountain Range, on which it largely falls as snow in winter 21 . The second region is the Colorado River headwaters (37 • -42 • N, 112 • -106 • W), which receive substantial winter snowfall as well as summer rainfall, and feed the Colorado River, which provides water to 7 states and Mexico 8,12,22 . The third region is Texas (26 • -36 • N, 103 • -93 • W, land areas alone), which uses water from rivers and groundwater within its own borders 23 .…”