Simulating vernal pool hydrology in central Minnesota, USA

A dominant determinant of successful amphibian dispersal from vernal pools and colonization of other pools in a metapopulation is pool hydrology and hydroperiod. We created a hydrologic model for vernal pools inhabited by wood frogs (Rana sylvatica). The hydrologic model used a classic water-budget approach, which had been applied to larger water bodies but not vernal pools. Depth profiles for eight pools in Cloquet, Minnesota, USA were measured for the open water seasons from June 2000 to September 2002. In the model, we adjusted infiltration rates and runoff coefficients to match depth profiles measured in 2000 and 2001 most closely. The simulated and observed depth profiles for 2000 and 2001 agreed well. In an assessment using novel depth profiles from 2002, modeled depths were somewhat deeper than observed depths for three pools, and four pools included sources of inflow that could not be explained by the model. We believe that the difference was from ground-water inflow rather than systematic changes in parameters, such as runoff curve numbers, because fit was good in 2000 and 2001, and some pools actually filled in 2002 during conditions when they typically would lose volume. Modeling hydroperiods using our methods for all pools in a region is problematic, as depth information is required for each pool studied, but the model will perform reasonably well for even small pools with adequate depth data.