Hydrology of Vernal Pools on Non-Volcanic Soils in the Sacramento Valley

The hydrology of vernal pools near Mather Air Force Base in Sacramento County was investigated to assess the relative importance of direct precipitation, overland flow, and subsurface flow on the water level regime of vernal pools located on a Redding-Red Bluff soil complex, and to determine if constructed pools affected the hydrology of naturallyoccurring pools. Studies included direct continuous measurements of vernal pool water levels and volumes, precipitation, evaporation, and groundwater levels. Lateral subsurface flow rates were studied using a bromide tracer. An upland watershed model was developed to estimate overland and subsurface flow under different combinations of soil depths and slopes over a range of rainfall regimes. The site’s moderate to deep upland soils and the light to modest rainfall intensities, during the study period resulted in little overland flow, but the model predicts it can occur during periods of intense rainfall. In most years, overland flow contributions are probably limited to periods when pools are already full, so that overland flow is excess to the pool. In 1990, direct precipitation was the primary source of water in the pools, especially during the early winter. Using a direct precipitation-evaporation model, direct precipitation was found to be capable of filling the pools beyond capacity during most years. Although watershed contributions might be considered minor from a volumetric perspective, water exchange between the pool and surrounding upland plays a major role in controlling water level relationships, and subsurface inflows tend to dampen water level fluctuations during the late winter and early spring. This dampening may affect pool vegetative composition and play a role in the life cycle of biota in the pool and adjacent uplands. CITATION. Pages 38-49 in: C.W. Witham, E.T. Bauder, D. Belk, W.R. Ferren Jr., and R. Ornduff (Editors). Ecology, Conservation, and Management of Vernal Pool Ecosystems – Proceedings from a 1996 Conference. California Native Plant Society, Sacramento, CA. 1998.