SIMULATION OF HISTORICAL GROUNDWATER FLUCTUATION IN SUFFOLK, VIRGINIA USING EFFECTIVE MONTHLY RECHARGE (WEM) MODEL

The Effective Monthly Recharge (W_em) model generates a synthetic hydrograph of water table elevations for precipitation-driven groundwater systems. A time-weighted averaging technique, the W_em water-budget model simulates recharge fluctuations over time using historical weather data. Recharge equals precipitation, adjusted for interception (I), minus FAO Penman-Monteith evapotranspiration estimates (ET). Model users calibrate calculated monthly Wem values against 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 preceding years; the estimates are generated with historic weather data. Forty years of local weather data and daily data from a USGS well in Suffolk allow us to test the model using the head on first day of each month. Data from 2003-2005 suggest n=18 months and d=0.9 provide the best fit (R²=0.87) for this site; this correlation made using filtered data (remove all data affected by rain during, or one day before, the chosen day) greatly exceeds the correlation generated by using unfiltered data (R²=0.56). Comparison of all monthly USGS well data (unfiltered) and the model-generated head values for 1981-1999 produces a significant correlation (R²=0.68). Sensitivity analyses of interception estimates suggest an I=0.25 maximizes the correlation coefficient for these data. These estimates of historical long-term water-level fluctuations provide guidance to wildlife habitat managers, wetland design professionals, and agency regulators about the probable availability of groundwater and will be used in a comprehensive package of water-budget models now under development.