RECALIBRATED RECHARGE: SOLVING COMPLEX PROBLEMS WITH MODELS

Models are often used to estimate recharge by varying recharge values until results approximate or “match” historical data. The resulting recharge value is assumed to be a reasonable estimate of the actual value. This model calibration technique to determine recharge is a fairly easy and simple approach to a rather difficult and complex problem. However, it is popular because recharge values are often the missing part of the input data needed for the model. When more than one model is used to determine recharge values or when multiple calibrations based on differing assumptions are used to determine recharge values, sometimes the model can be recalibrated with the “improved” recharge values, and a “better” recharge number may result.

The use of the finite difference numerical model, MODFLOW, the analytical Soil Water Assessment Tool, SWAT, and the analytical evapotranspiration model 3-PG were used in a recalibration interchange to determine recharge in a seemingly simple sand outlier setting. The study area is a topographically high recharge area with little relief and no runoff. However, the vegetation/landuse and meteorology changed in both space and time. Data collected to be used in the models included rainfall, soil moisture, spring flow, well hydrographs, transmissivity and storage coefficients from local aquifer tests, groundwater usage and leaf area index (LAI) values. MODFLOW was calibrated to spring flow and water levels independently, SWAT was calibrated to soil moisture and 3-PG was calibrated to LAI values. The recharge results were compared among the appropriate model methods and to literature values. Although not defensible statistically at this time, the final analysis yielded a narrower range of values and perhaps a better estimate of average annual recharge than existed previously.

Results could be improved further with better input data but a similar technique may prove useful in a variety of settings and applications.