Snow distribution over an elevation gradient and forest snow hydrology of the southern Sierra Nevada, California.

摘要

The Mountain West region of the United States is highly dependent on ecosystem services from the mountain snowpack, one of the most vulnerable components of earth's fresh water cycle. The growing demand for fresh water in a period of climatic non-stationarity requires new approaches to monitoring and prediction. We investigate snow distribution and its effect on subsurface water storage in the southern Sierra Nevada, California using the combination of in-situ measurements, airborne LiDAR-snow-depth altimetry, satellite snow-cover maps, and novel spatial analysis. Using these data and methods we address questions about the mountain snowpack pertaining to: i) broad-scale distribution of snow accumulation governed by elevation and topography, ii) the effects of forest canopy on snow accumulation and ablation, at multiple scales, and iii) the partitioning of water in the vadose zone after snowmelt. Our results show that snow depth as a function of elevation increased at a rate of approximately 15 cm 100 m-1 until reaching an elevation of 3300 m where depth sharply decreased at a rate of 48 cm 100 m-1. Departures from this trend were mostly negative below 2050 m, mostly positive between 2050-3300 m and negative above 3300 m, and attributed to orographic processes, mean freezing level, slope, terrain orientation and wind redistribution. High point-density LiDAR measured 31-44% of under-canopy area, where snow depth was12- 24% lower than in the open, depending on forest vegetation type. The metrics of mean canopy height, canopy-to-ground surface ratio, fractional canopy cover, and canopy- height standard deviation individually explained half 45-58% of the storm accumulation variability. Sky view factor explained up to 87% of the variability in snow ablation rates in the cloudiest snow-melt seasons and direct beam solar irradiance explained up to 58% in the clearest. The timing of soil dry-down is relatively uniform, but due to the heterogeneity of snowmelt it's timing is offset by up to 4 weeks at the same elevation depending on location. Baseflow and evapotranspiration continue after soil dry down has reached a plateau, suggesting that water is drawn from soil saprolite and saprock at depths >1 m below the surface.