Snowpack

Abstract: [1] Output atmospheric fields from seven global climate models (GCMs) were extracted over a domain covering the Adour-Garonne basin in southwestern France in order to calculate precipitation and temperature anomalies for the decade 2050–2060 relative to the present climate. These anomalies showed a general trend of increasing precipitation in wintertime and decreasing precipitation in summertime, together with an increase in the annual average temperature of approximately 2°C. The anomalies were used to create seven modified climate-forcing data sets, which were then used to drive the SAFRAN-ISBA-MODCOU (SIM) hydrometeorological modeling system. The river discharge simulated by the SIM model under each modified climate for the 2050–2060 decade was compared to the discharge simulated for the 1985–1995 reference decade. The results show a slight decrease in the low river flow, on the order of 11% ± 8% on average for all of the climate-forcing data sets and the hydrometric stations. However, there was a significant impact on the snowpack in terms of reduced snow cover depth and duration. These changes provoked a discharge decrease in the spring and a large increase in winter due to the additional liquid precipitation. Considering the large range in climate conditions of the period studied, it appears that the hydrological sensitivity of the river basin is greater when applying the same climate modification to a wet year as opposed to a dry year. Finally, a transient climate forcing covering the 1985–2095 period provokes a general tendency to decrease the river discharge for all seasons.

Abstract: [1] Diurnal cycles in snow-fed streams provide a useful technique for measuring the time it takes water to travel from the top of the snowpack, where snowmelt typically peaks in the afternoon, to the river gauge, where the daily maximum flows may arrive many hours later. Hourly stage measurements in nested subbasins (6-775 km2) of the Tuolumne River in Yosemite National Park illustrate travel time delays at different basin scales during the spring 2002 and 2003 melt seasons. Travel times increase with longer percolation times through deeper snowpacks, increase with longer travel times over land and along longer stream channels, and increase with slower in-stream flow velocities. In basins smaller than 30 km2, travel times through the snowpack dominate streamflow timing. In particular, daily peak flows shift to earlier in the day as snowpacks thin and mean discharges increase. In basins larger than 200 km2, snowpack heterogeneity causes the hour of peak flow to be highly consistent, with little or no variation as the snowpack thins. Basins with areas in between 30 and 200 km2 exhibit different sequences of diurnal streamflow timing in different years, sometimes acting like small basins and other times like large basins. From the start of the melt season until the day of peak snowmelt discharge, increasing travel distances in channels as the snow line retreats to higher elevations do not cause long enough travel delays to offset the observed decrease in mean travel times through the snowpack. A model that couples porous medium flow through thinning snowpacks with free surface flow in stream channels can reproduce the observed patterns, provided that the model incorporates snowpack heterogeneity.

Abstract: Snow accumulation and melt patterns play a significant role in the water, energy, carbon, and nutrient cycles in the montane environments of the Western United States. Recent studies have illustrated that changes in the snow/rainfall apportionments and snow accumulation and melt patterns may occur as a consequence of changes in climate in the region. In order to understand how these changes may affect the snow regimes of the region, the current characteristics of the snow accumulation and melt patterns must be identified. Here we characterize the snow water equivalent (SWE) curve formed by the daily SWE values at 766 snow pillow stations in the Western United States, focusing on several metrics of the yearly SWE curves and the relationships between the different metrics. The metrics are the initial snow accumulation and snow disappearance dates, the peak snow accumulation and date of peak, the length of the snow accumulation season, the length of the snowmelt season, and the snow accumulation and snowmelt slopes. Three snow regimes emerge from these results: a maritime, an intermountain, and a continental regime. The maritime regime is characterized by higher maximum snow accumulations reaching 300 cm and shorter accumulation periods of less than 220 days. Conversely, the continental regime is characterized by lower maximum accumulations below 200 cm and longer accumulation periods reaching over 260 days. The intermountain regime lies in between. The regions that show the characteristics of the maritime regime include the Cascade Mountains, the Klamath Mountains, and the Sierra Nevada Mountains. The intermountain regime includes the Eastern Cascades slopes and foothills, the Blue Mountains, Northern and Central basins and ranges, the Columbia Mountains/Northern Rockies, the Idaho Batholith, and the Canadian Rockies. Lastly, the continental regime includes the Middle and Southern Rockies, and the Wasatch and Uinta Mountains. The implications of snow regime classification are discussed in the context of possible changes in accumulation and melt patterns associated with regional warming.