ESTIMATING GROUND-WATER RECHARGE IN THE NORTH CAROLINA PIEDMONT FOR LAND-USE PLANNING

Quantitative information regarding recharge rates to aquifers and ground-water storage is needed to manage the development of ground-water resources in the Piedmont. In order to determine the maximum population that can be supplied by ground water, planners and managers of suburban development must know the amount of ground water that can be withdrawn without exceeding recharge and(or) overdrafting water in long-term storage.

Seasonal and long-term recharge rates were estimated for 27 selected drainage basins and subbasins in the north-central Piedmont of North Carolina by using streamflow data from 33 gaging stations and an analytical technique known as hydrograph separation. Mean annual recharge in the 27 basins and subbasins ranges from 4.03 to 9.69 inches per year, with a mean value of 5.67 inches per year for all basins. The distribution of recharge rates is approximately the reverse of the distribution of precipitation across the study area, suggesting that recharge rates are highly dependent on hydrogeologic conditions.

There are many methods of hydrograph separation and some produce more reliable estimates of usable (recoverable by wells) ground-water recharge than others. In a comparison of methods based on data from 16 gaging stations, recharge estimates were found to differ by as much as 21 percent. Methods that analyze base flow recession appear to include a large proportion of subsurface stormflow (interflow), much of which will not reach the water table. Geochemical and experimental field data suggest subsurface stormflow may be as much as 65 percent of recharge.

Development that uses on-site wastewater treatment may increase evapotranspiration and reduce net recharge in a watershed. Housing density, changes in land use, and off-site wastewater treatment also can affect the depth to the water table, the amount of ground water in storage, and the carry-over capacity of the ground-water system during droughts. Therefore, conservative methods of hydrograph separation that result in lower planned housing densities will more accurately reflect long-term sustainable yields to wells.