GRL - Future Changes In Snowmelt-driven Runoff Timing Over The Western US

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Sara A. Rauscher

Earth System Physics Section, Abdus Salam International Centre for Theoretical Physics,
Trieste, Italy

Jeremy S. Pal

Department of Civil Engineering and Environmental Science, Seaver College of Science and Engineering, Loyola Marymount University,
Los Angeles, California, USA

Noah S. Diffenbaugh

Purdue Climate Change Research Center and Department of Earth and Atmospheric Sciences, Purdue University,
West Lafayette, Indiana, USA

Michael M. Benedetti

Department of Geography and Geology, University of North Carolina at Wilmington,
Wilmington, North Carolina, USA

Abstract

<1> We use a high-resolution nested climate model to investigate future changes in snowmelt-driven runoff (SDR) over the western US. Comparison of modeled and observed daily runoff data reveals that the regional model captures the present-day timing and trends of SDR. Results from an A2 scenario simulation indicate that increases in seasonal temperature of approximately 3° to 5°C resulting from increasing greenhouse gas concentrations could cause SDR to occur as much as two months earlier than present. These large changes result from an amplified snow-albedo feedback driven by the topographic complexity of the region, which is more accurately resolved in a high-resolution nested climate model. Earlier SDR could affect water storage in reservoirs and hydroelectric generation, with serious consequences for land use, agriculture, and water management in the American West.

EDIT

<13> Precipitation changes do occur in our A2 simulation; precipitation increases over the Northwest and decreases over northern California and the Southwest (Figure 2a), a common feature of climate change simulations that is usually attributed to a northward shift of the mid-latitude winter storm track . In the A2 simulation there is anomalous cyclonic flow over the Southwest and increased upslope flow over western mountain ranges (Figure 2e). Combined with higher atmospheric moisture content, these changes lead to increased precipitation and a weakening of the rainshadow effect over Colorado and Wyoming while contributing to drying over California . However, runoff increases more than precipitation (Figures 2a and 2b), again indicating the effect of higher temperatures and earlier snowmelt.

<14> Further, these circulation changes and higher atmospheric moisture content do not increase accumulated snow since late winter and spring temperatures are higher and there are fewer annual days below freezing (Figures 2g and 2h). Thus, temperature seems to be the dominant factor in determining changes in runoff, consistent with observations . Also, despite the increase in precipitation over the Northwest, accumulated snow decreases in the A2 simulation even at the highest elevations of the Cascades, in agreement with GCM simulations . Moreover, our projected changes in SDR timing are consistent with the observed spatial pattern; larger changes occur over the Northwest and smaller changes are found over interior mountain ranges such as the Rockies .

EDIT

<16> We have used a nested high-resolution climate model to investigate future changes in SDR over the western US. A comparison of modeled SDR with HCDN data reveals that RegCM3 captures the present-day timing of SDR as well as observed trends. Results from a late-21st century simulation (A2 scenario) indicate that increases in temperature, forced by increasing GHGs, could cause early-season SDR to occur as much as two months earlier than present, particularly in the Northwest. Earlier SDR timing of at least 15 days in early-, middle-, and late-season flow is projected for almost all mountainous areas where runoff is snowmelt-driven. These large changes result from an amplified snow-albedo feedback associated with the topographic complexity of the region.

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