SIMULATE HISTORICAL GROUNDWATER FLUCTUATIONS AT POTENTIAL MITIGATION WETLAND SITES

The Effective Monthly Recharge (W_em) model generates a synthetic hydrograph of water table elevations for toe-slope wetlands dominated by groundwater discharge. A time-weighted averaging technique, the W_em water-budget model simulates recharge fluctuations over time using historical weather data. Model results are calibrated to monthly head data by varying the number of months of weather data (n) used in the calculation, and the weight (d) applied to antecedent conditions. The combination of n and d that generates the best correlation of W_em vs measured heads is used to estimate monthly head values for the preceding thirty years using historic weather data. Use of post-construction water-level data gathered for 80 months (2000-2006) at a 2.6-acre mitigation wetland in Virginia Beach, Virginia permits validation of the model parameters originally calibrated using pre-construction data (1998-2000). Sensitivity analyses using post-construction values suggest FAO Penman-Monteith estimates of evapotranspiration provide better explanation of measured hydraulic head (R²= 0.77) than Thornthwaite ET calculations (R²= 0.65). Also, correlations using data for winter, summer and fall months (R²= 0.83, 0.86, and 0.83, respectively) far exceed those for spring months (R²= 0.46). One model result is a probability estimate for how often a particular head was exceeded at the beginning of a given month during the past thirty years. These estimates provide guidance to wetland design professionals and regulators about the probable availability of groundwater and will be used in a comprehensive package of water-budget models now under development.