Snowpack response to warmer temperatures: a southern Sierra Nevada case study

Abstract:

The State of California is reliant upon spring meltwater runoff from accumulated winter snowfall in the Sierra Nevada to meet agricultural and municipal demands. The response of snow water resources to anticipated warmer climate conditions, and particularly how changes may be manifested in time (e.g., spring vs. winter; wet vs. dry years) and across an elevation profile is examined. We present model simulations of snowpack response to warmer climate over a 3600 m elevation gradient in the southern Sierra Nevada. The Alpine3D snow model was used to simulate the snow mass and energy balance for three reference years: a moderately dry snow season, near-average and moderately wet snow season. Compared to measurements, the uncalibrated model well represented the date of snow disappearance, depth and SWE. A pseudo-climate-warming experiment was conducted by modifying the measured air temperature by +1°C to +6°C along with the corresponding increase in downwelling longwave radiation. In general, the drier snow season and the forested elevations exhibited the most sensitivity to warmer temperatures. At middle elevations (2000 m to 2700 m), 70-80% of the present-day snowpack volume was lost in a +2°C scenario and nearly 90% in a +4°C scenario. At lower to middle elevations, >50% of the predicted change in SWE was caused by a conversion of precipitation from snow to rain while at higher elevations, a majority of the SWE change was caused by enhanced melt. It is found that warmer temperature scenarios caused seasonal average melt rate reductions of ~1 mm day^-1 °C^-1. The simulated melt rate reduction is explained by a shift in the seasonality of snowmelt occurrence; more frequent and earlier melt occurred under lower net available energy conditions compared to the present-day snowmelt regime, which is characterized by high melt production in the relatively warm and typically cloud-free spring and early summer. The results raise questions about future rates of meltwater mobilization and the hydrological partitioning between ET and runoff. For example, will a shift toward earlier snowmelt correspond with a decrease or increase in ET demand? And how will snowmelt and ET changes combine with more frequent rain-on-snow events to shape future changes in seasonal runoff?