High-resolution models better simulate historical snowpack declines in the Upper Colorado River Basin

Abstract The Colorado River supplies water to 40 million people, 4.5 million acres of irrigated land, seven US states, over two dozen federally recognized and sovereign tribes and Mexico. River discharge is strongly controlled by snowpack accumulation and melt in the headwater regions, making upper-basin snow dynamics a primary driver of water availability throughout the basin. Therefore, changes in snow water equivalent (SWE) are critical for water resources planning in the Upper Colorado River Basin (UCRB), yet there is considerable uncertainty even in historical SWE trends. We compare historical UCRB SWE trends across various gridded snow products, including new Weather Research and Forecasting simulations at 45, 9, and 3 km resolution, against snow telemetry station data. We find that SWE trends are systematically amplified with higher model resolution and that high resolution (<10 km) is needed to accurately capture not only SWE amounts but also historical SWE declines across high-elevation mountains. Trend amplification primarily results from higher resolution simulations having higher snowfall due to stronger orographic forcing, an effect further modulated by synoptics. This is confirmed in a set of alternative historical realizations from a downscaled climate model large ensemble, which, despite a tendency for SWE declines, highlight the important role of internal variability in historical UCRB SWE trends. These results demonstrate that model resolution plays an important role in shaping assessments of snow-related climate change, with implications for a wide range of impact studies.