Ecohydrology of Wetlands Occurring on Perched Seasonally Saturated Water Tables in the Central Valley of California

The Central Valley, California has extensive areas of shallow perched groundwater systems associated with geomorphic terraces. Early season water supply is derived from precipitation (PPT) that has infiltrated into soils underlain by a near surface aquitard, typically at less than 2 m depth. Early season water input is regulated by the hydraulic conductivity of the (clay-) loamy soils and by surface and aquitard slope of the local catchments associated with these old alluvial landforms. Research on these landforms and shallow aquifers has identified a complex PPT and evapotranspiration (ET) sensitive system that includes shallow depressions that seasonally produce water table derived wetlands (“vernal pools”). These wetlands have been recognized for a very high level of plant and invertebrate species diversity including endangered species. Our work on these seasonal perched systems shows that as much as 80 percent of the soil column above the aquitard is saturated, during average to high rainfall years, for up to 90 to 120 days. Soil surface topographic depressions reduce the soil depth to the aquitard. Where the water table of this perched system intercepts the land surface, vernal pools develop. The perched groundwater drains into seasonal surface drainages that ultimately supply the Sacramento and San Joaquin rivers. At the end of the rainy season, both the vernal pools and the perched aquifer rapidly and synchronously disappear. Once the soil is unsaturated, water flow is vertically upward due to ET. Aquatic and wetland adapted plant species develop within the basins along a depth gradient. Variably saturated modeling of this system was conducted using HYDRUS 2D/3D. Climate inputs were from local and regional weather stations that measure and calculate daily PPT and ET, respectively. Initial conditions and calibration of the domain were based on field measurements using pressure transducers and soil moisture sensors. Soil pressure flux was measured using a matric potential soil sensor. Field measurements were taken throughout the local catchment and discharge points. The HYDRUS modeling has revealed a high level of sensitivity of the perched system to PPT and ET, with the first major seasonal PPT event generally establishing initial moisture saturation immediately above the aquitard. Plant species adapted to vernal pools were found to occur within narrow (5 to 10 cm) elevation zones in the pool basins and are correlated with specific hydroperiods of surface inundation. Annual variation in the amount and distribution of rainfall can cause a change the plant community composition. Longer term climate changes could result in regional shifts in plant community structure.