Topographic distribution of snow water equivalent in the Sierra Nevada

Abstract

Water supply forecasts in the Sierra Nevada using ground-based measurements of snow water equivalent (SWE) are uncertain because neither point measurements nor transects adequately explain spatial or temporal variability in mountainous terrain. To address this problem, we combine satellite-based retrievals of fractional snow cover in 2006 with energy balance calculations to reconstruct the SWE values throughout the melt season. Model estimates when compared to snow pillows at maximum accumulation are unbiased and have an RMS error of 297 to 417 mm. We compare this retrospective calculation of distributed SWE with two real-time models: (i) interpolation from pillows, courses, and satellite snow cover, and (ii) the Snow Data Assimilation System (SNODAS). The interpolation and SNODAS models show complete melt out more than a month earlier than reconstruction, and their total SWE volumes are 68% and 87% of the reconstructed volume. At elevations below 1500 m, the reconstruction model has less total SWE because of early season melt. Above 3000 m, the reconstruction shows more SWE than the real time models, which depend on surface measurements that do not sample the higher elevations. The results indicate that spatial patterns from the reconstruction could improve estimates of snow accumulation and duration.