How much of the streamflow in the U.S. originates as snow?
Abstract:Snow is a vital component of water supply and ecological systems in the Western United States. Quantifying the overall fraction of streamflow that originates as snowmelt is a critical step for assessing the winter-season snowpack drought and for determining the vulnerabilities of water resources in late summer and fall. However, estimates of this fraction differ in existing literatures; to our knowledge, a systematic study has not been performed to evaluate this fundamental quantity.
In this study, a snowmelt tracker was developed to track the snowmelt-derived runoff from the output of a macroscale hydrological model; the spatiotemporal ratio of the snow-derived runoff to the total runoff (fQ,snow) was calculated over the contiguous U.S. at 0.5° resolution. The tracker calculated fQ,snow based on the Variable Infiltration Capacity model outputs, hydrologic fluxes, and water balance equations. Generally, snowmelt contributes to the total runoff in two ways: surface runoff and baseflow. Since rainfall also contributes to both quantities, the snowmelt tracker operates at every time-step to disentangle the snow-induced runoff from both surface runoff and baseflow and to calculate fQ,snow. In this way, the tracker provides the spatiotemporal distribution of fQ,snow.
In this study, the average fQ,snow from 1979 to 1998 was calculated. A significant fraction of runoff originates as snow across the western US, especially in mountainous areas such as the Rockies, the Cascades, and the Sierra Nevada; fQ,snow ranges from 60% to 80% in these mountains. Other places with relatively high fQ,snow include the northern Great Plains and the northeast Appalachians. Storage reservoirs typically store snowmelt in spring and release water during summer and fall; these reservoirs are critical for ecology, irrigated agriculture, and local water supply. Three out of the five largest reservoirs (in term of capacity) in the U.S. are located in the west; we calculated the runoff-weighted average fQ,snow over the drainage basins of these reservoirs, and found about 50% of the water in each originates from snow. Future studies will be focused on running the tracker with forecasting model results to explore the spatiotemporal patterns of fQ,snow in the future, with emphasis on late-season snow water resource availability.