Simulating vernal pool hydrologic regimes for two locations in California, USA

Ecological processes and community composition in vernal pools are dominated by a suite of interrelated hydrologic factors collectively described as a hydrologic regime. This study reports on the development of a hydrologic regime simulation model called PHYDO and its application to two locations in California. PHYDO calculates daily water balance using meteorological data and parameters describing basin morphology, soil characteristics, and vegetation cover. The model provides daily estimates of water depth, volume, and temperature, as well as statistics on seasonal and inter-annual variability. Model predictions are compared to field observations for coastal vernal pools in southern Santa Barbara County, CA, USA (2000–2001), and interior vernal pools near Oroville in the Central Valley of CA, USA (1994–1998). The model was also run for the period 1990–2000 to investigate patterns of inter-annual variation in hydrologic conditions at both sites. Results indicate that vernal pool hydrologic regimes can be modeled primarily based on the direct interception of precipitation and the loss of water to evapotranspiration. Results also suggest differences in hydrologic limiting factors between the sites. Storm frequency appears to be the primary control of hydrologic conditions for Santa Barbara pools, while basin morphology dominates conditions in Oroville. These findings suggest ecologically important differences in hydrologic regimes between the two vernal pool localities, and PHYDO provides a new set of tools for quantifying these differences for a range of hydrologic variables.