Seasonal, Variably Saturated Flows in a Vernal Pool Wetland Ecosystem

Vernal pool complexes are an important seasonal wetland ecosystem in California. The pools form in shallow landscape depressions during the rainy winter season. Only under extremely wet conditions, pools become part of a surface drainage network. The surface drainage network is typically much shorter-lived than the pools. Pools may have standing water from as little as two weeks to as long as six months during the rainy season (late fall to late spring). While ecologically important, little is known about the subsurface hydrology associated with vernal pools. We have implemented an extensive hydrologic monitoring program to begin understanding the role of variably saturated flow within these vernal pools catchments. At our field sites on older tertiary alluvial terraces, we find that winter precipitation (PPT) is the principal contribution to a variably saturated aquifer on a shallow, duripan aquitard. During the onset of the winter season, infiltration throughout the local catchment results in a gradual wetting up of the relatively dry soil profile. Soil moisture content increases due to macropore flow in a clay layer overlaying the duripan, thus creating a bottom up increase in saturated soil. Overlying landscape depressions begin to expose surface water not as a result of surface runoff, but as a result of increasing perched water table elevation, thus creating the vernal pools. Subsequent fluctuations in pool basin water levels are primarily controlled by evapotranspiration (ET) within the pool and by subsurface gravitational flows, often into seasonal drainages. Multiple vernal pool basins within a catchment are shown to exhibit a high level of groundwater connectivity. Intense periods of PPT continue to supply the perched aquifer with water. Rapid fluctuations in soil moisture content can occur over periods of 10 to 30 minutes increasing surface soil moisture to near saturation levels. Macroporous clay loam soils in the A horizon quickly discharge water downward to the existing saturated zone. Recharge of the perched aquifer can similarly cause a rapid increase in the height of the saturated soil zone above the duripan. This is often reflected in a rapid increase in the depth of the vernal pool basin surface water, by as much as one foot within one hour. The upper zone of soil saturation has a highly variable peak elevation that is directly related to the frequency and amount of PPT. A significant amount of variation is observed for infiltration rates and moisture content during and following PPT events. These variations are a function of soil hydraulic properties within different stratigraphic layers in the soil profile. Microtopographic variation in the soil texture can cause seasonal variation in soil moisture retention and loss through ET. Vernal pool plant communities increase the pool basin ET. Particularly towards the end of the rain season, this accelerates the drying out of surrounding uplands. Once the season vernal pool plants have completed their life cycle and no longer actively transport water, evaporation and moisture diffusion into the surrounding uplands are the major fluxes out of the vernal pools.