SOIL MOISTURE MEASUREMENTS AND ESTIMATION OF RECHARGE UNDER A MELTING SNOWPACK

Recharge into bedrock under a melting snowpack is being investigated as part of a study designed to understand hydrologic processes involving snow at Yosemite National Park, in the Sierra Nevada of California. The use of a basin-scale water-balance model that accounts for melting snow along with the physical characteristics of a location dominated by granite bedrock and shallow soils, generally results in calculations indicating a higher potential for runoff than in-place recharge into the bedrock. Measurements of the snowpack, and water content and matric potential measurements of the soil under the snowpack, allowed for the estimation of infiltration into the soil and percolation into the bedrock during snowmelt. Infiltration into the 30-cm-deep loamy sand was approximately 35 mm per day, which exceeded the permeability of the bedrock and caused ponded conditions to be sustained at the soil-bedrock interface during the snow melt period. During a 7-day period with no measured snowmelt, drainage into the underlying fractured granitic bedrock was estimated to be 16 mm/day. These measurements have shown that simple, inexpensive instrumentation can provide insight into hydrologic processes involving snowmelt and soil moisture. Available data indicate that such processes are actively occurring in the soil under a melting snowpack. These observations support the notion that although most snowmelt on shallow soils overlying relatively impermeable upland bedrock tends to run off and contribute directly to streamflow, at least some of the snowmelt can infiltrate and potentially provide recharge to regional aquifers.