Role of Snow in Runoff Processes in a Subalpine Hillslope: Field Study in the Ward Creek Watershed, Lake Tahoe, California, during 2000 and 2001 Water Years

Abstract

Field study is an essential component of hydrologic science because all hydrological studies must be conducted by such observation-based knowledge of real watersheds. Hillslope runoff processes have been intensively investigated, but the flow process at the boundary between the snowpack and ground surface has not been well documented. A field site at the northwest sector of the Ward Creek watershed, Lake Tahoe Basin, was built for observations of overland flow, subsurface stormflow, and channel flow, simultaneously with atmospheric measurements to examine the hydrology at a snow-covered hillslope. Also, the groundwater table under the snowpack was monitored by shallow wells at the hillslope. All field-measured atmospheric data were synthesized with an energy-balance snow model, and the snowmelt rate and energy balance were computed. The results of the analyses indicate that most of the snowmelt water infiltrated into the topsoil layers and that saturated subsurface flow was the largest contributor to the stream channel flow throughout the observation period. However, overland flow or longitudinal flow within the snowpack may still happen even over an unfrozen and unsaturated topsoil on a relatively mild hillslope (16%). The quantification of observations of overland/within snowpack flow showed that the snowpack enhanced the overland runoff, which caused higher peaks in flow discharge and sediment production. Analysis of the major rain-on-snow event of May 7, 2000, revealed that snowmelt water, caused by the energy flux from raindrops, might not be the major contributor to the runoff peak discharge since very little snowmelt was observed during the rain-on-snow event. Consequently, spring storm hydrographs in the Sierra Nevada seem to be affected by the high-soil water in the topsoil because of the daily water supply by snowmelt and by overland/within snowpack flow caused by the capillary suction forces in the snowpack.