Abstract

In recent years, regions such as the western United States have been projected to have drastically changing snow and soil moisture conditions in mountainous regions. The importance of subsurface flow during snowmelt for groundwater recharge and streamflow has long been known. This study was conducted to investigate the variability of soil wetting and drying dynamics within a network of soil moisture sensors beneath a seasonally persistent snowpack in California’s southern Sierra Nevada. Data were analyzed to quantify the persistence of soil moisture, the rate of change in soil moisture, and vertical gradients of soil moisture during the 2009, 2010, and 2011 water years at varying elevations (1750–2000 m), aspects (north, south, and flat), and canopy conditions (open, drip edge, and under canopy). Results from this study demonstrated snowmelt water moving through a wetted shallow region of the soil in most locations, while other locations displayed drier conditions in the top layers of soil and higher rates of soil moisture change deeper within the soil. This occurred at various depths within the top 1 m of soil, displaying high variability across the catchment and under different canopy conditions. More than 1400 pulse events above a rate of change in water content of 0.06 cm cm−1 d−1 were observed, with many >0.10 cm cm−1 d−1. Average numbers of pulse events per year ranged from 1 to 18, with a median of 4.3 for all sensors. The lower elevation sensors averaged 6.0 wetting events per year reaching at least 60-cm depth and the upper sensors averaged 4.7, with standard deviations among elevation and aspect clusters ranging from 2.5 to 3.0 for upper elevation sites and 2.9 to 3.2 for lower elevation sites. This demonstrates the high variability of the wetting and drying dynamics of the top 1 m of soil at the sub-hillslope scale. These results have implications for groundwater recharge, plant production, and streamflow response in a changing climate.

Citation

Webb, R.W., Fassnacht, S.R., Gooseff, M.N. (2015): Wetting and Drying Variability of the Shallow Subsurface Beneath a Snowpack in California’s Southern Sierra Nevada. Vadose Zone Journal. 14 (8). DOI: 10.2136/vzj2014.12.0182