Seismic refraction and electrical resistivity tomography to investigate subsurface controls on vegetation distribution in a mountain watershed

Abstract:

The objective of this work is to investigate subsurface controls on the distribution of vegetation at two sites located within the Reynolds Creek Critical Zone Observatory (CZO). Located in southwestern Idaho, the Reynolds Creek CZO extends over a steep elevation-climatic gradient (mean annual precipitation 250 - 1100 mm/yr, mean annual temperature 5.5 °C to 11°C). The existing, publically available hydroclimatic data are long-term and spatially extensive, including precipitation (>50 yr), snow course SWE (>50 yr), temperature (30-50 yr), soil moisture and temperature (>10 yr), and some soil depth data. Both sites we investigated were at elevation greater than 2000m, and both sites showed abrupt changes in vegetation with no surface expression of changes in the underlying geology. The first site, termed Dry Meadow (DM), consists of a grassy meadow that transitions from being saturated to the surface during the spring runoff to dry with a water table at a depth of 4-6m in the late summer. The grassy meadow transitions abruptly to sage brush dominated terrain with no significant change in elevation. The second site, termed the Aspen Grove (AG), shows an abrupt transition from dry grassy terrain to an Aspen grove along a constant, and low gradient hill slope. At both sites we acquired high density seismic refraction data (1m receiver spacing) along transects that ranged from 95 to 160 m. Additionally we acquired 107 m long electrical resistivity profiles in both dipole-dipole and Wenner arrays with 2 m electrode spacing. At both sites, both seismic and ERT data indicate a distinct and abrupt drop in depth to the top of the weathered rock surface of 10-15 m. These topographic lows in the bedrock may be either erosional or structurally controlled, but in either case create accommodation space for the accumulation of sediment and an altered groundwater distribution that can accommodate a shift in the dominant vegetation type.