Validation and improvement of Effective Monthly Recharge (W em ) model

1. Validation and improvement of Effective Monthly Recharge (Wem) model for simulating wetland hydrology G. Richard Whittecar, Tracy Thornton, John Smith, John McLoed, and Brian Jolemore Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, Norfolk, Virginia Oceana Ridge From Mixon and others, 1989 Oceana VDOT/Navy Mitigation Site Effective Monthly Recharge: Wem A time-weighted recharge value Ws D n S Wmox da-1 Wem = a=1 Response-decay factor (<1.0) Each month’s recharge (Ppt - ET) n S da-1 Normalizing factor Groundwater Flow a=1 n= # preceding months ABSTRACT The Effective Monthly Recharge (Wem) model generates a synthetic hydrograph of water table elevations for toe-slope wetlands dominated by groundwater discharge. Use of post-construction water-level data gathered for 110 months (2000-2006) at a 2.6-acre mitigation wetland in Virginia Beach permits validation of model parameters calibrated using pre-construction data. Model improvements based on evaluation of error sources and sensitivity analyses greatly improve the repeatability and reliability of model parameters developed by future model users. Other changes to the model decrease its complexity making it more flexible and adaptable for inclusion in a package of water-budget models under development for use by wetland design professionals. RESEARCH GOALS One goal of this research is to evaluate the pre-construction model calibration by using post-construction monitoring data. A second goal is to test and develop the uses of the Wem model to make its application easier to use and adaptable in more situations. MODEL DEVELOPMENT In order to improve the model, we addressed issues related to both data collection errors and calculation procedures. After examination of data collection records and the history of daily rainfall, we established the following criterion: eliminate any water level data if more than 0.75 cm rain fell in the month prior to data collection. In order to make more precise and repeatable comparisons of model results with field data, we started using correlation coefficients (R2) to test model runs, replacing the original curve-matching procedure. Users can now compare R2 values generated by multiple model calculations using both n (number of months) and d (decay factor) values to identify more accurate model parameters. PREVIOUS WORK Study Area: VDOT designed a mitigation wetland for a site on a relict barrier island where only precipitation and groundwater flow through the sandy soils can provide water. Fourteen months of water level data collected in 1999-2000 were used to design an excavation pattern for maximizing seepage into the wetland . Compare: Matrix of model results by number of months (n) and decay factor (d) used in present model. Also, pattern of Wem and the water table fluctuations over time, such as used in the previous curve-matching procedure. Correlation coefficients (R2) by n and d Original Wem Model : The Effective Monthly Recharge model estimates past water-table fluctuations. Monthly groundwater recharge values (precipitation minus ET) are combined in time-weighted averages. Model parameters were calibrated by comparing shapes of curves generated for different input values. Wetland designers using this model will ask “When is the best time to measure water levels?” and “Are any of these data I’ve just collected better than others?” To address these issues we plotted data according to standard NRCS wet/normal/dry criteria (3-month window), and by seasonality. Effective Monthly Recharge Values and East Pond Levels Wem-2mo curve matches best R2 = 0.72 R2 = 0.87 CONCLUSIONS Initial Results: Model calculations that use all available weather data and calibrated model parameters produce a synthetic hydrograph – an estimate of past water-table fluctuations. The elevation selected for the new wetland by this method received groundwater discharge consistently during even the driest months. • Analyses of post-construction monitoring data from the site verified the original calibration for the study area. Originally done using pond elevations, model runs with well data generate stronger correlations. • Use of linear regression correlation coefficients makes the model more robust and flexible, mostly by allowing for more precise comparisons of model results that use different input values and model parameters. • Our analyses to date suggest that the best field data come from Spring and Summer months of any wetness, plus other months that are “dry.” Deep (central) Pond looking East after wetland construction Wem-2 mo Values, 1952-95, Oceana, Va 15 10 Effective Monthly Recharge (cm) 5 ACKNOWLEDGEMENTS 0 -5 We acknowledge the assistance of M. Richardson (ODU) and the guidance of T. Wynn and W.L. Daniels (Virginia Tech) and M. Rolband and L. Giese (Wetland Studies and Solutions, Inc.). Funding provided by a grant from the Peterson Family Foundation. Consistent groundwater seeps even during dry months Wem during driest months of calibration in 1999-2000 -10 1952-1995