USE OF CHEMICAL HYDROGRAPH SEPARATION AND MULTIPLE REGRESSION TO ASSESS WATER AVAILABILITY ACROSS THE COMMONWEALTH OF VIRGINIA, USA

Long-term groundwater availability is affected strongly by long-term average rates of recharge. Many techniques have been developed to measure recharge locally but for regional scales these values always need to be scaled up to account for substantial spatial and temporal heterogeneity. In order to assess long-term recharge and other hydrologic conditions across the entire Commonwealth of Virginia, a hydrologic budget approach was coupled with a chemical-hydrograph separation technique that was used to quantify base flow and surface runoff. A regression equation was developed for runoff/precipitation as a function of the landscape parameters of physiographic province and bedrock type. Mean precipitation was estimated from the PRISM national climate dataset. Evapotranspiration (ET) was estimated using a regression equation developed using the difference between precipitation estimated from the PRISM dataset and stream flows from 60 watersheds. Recharge could be calculated from the remainder of the water balance once ET, precipitation, and surface runoff were calculated.

Chemical hydrograph separation was used at 51 watersheds that were instrumented for specific conductance for 18 months from 2007 to 2008. Results of the separations yielded base-flow percentages between 50 and 93 with a mean of 72. Graphical hydrograph separation for these same watersheds and time periods yielded base-flow percentages between 44 and 88 with a mean of 61.

The regression and hydrologic balance equations were used to estimate ET, surface runoff, base flow, and recharge for all 95 counties and 39 independent cities in Virginia by calculating mean values in a GIS framework. ET values were lowest (25-26 in/yr) in the Valley and Ridge Province, and highest (30-32 in/yr) in southeastern Virginia. Surface runoff values were lowest (3-4 in/yr) in the Blue Ridge Province, and Northern Neck and Eastern Shore peninsulas, and highest (9-10+ in/yr) in the Appalachian Plateau and cities. Base-flow percentages were lowest (50-55 percent) in the Appalachian Plateau and siliciclastic regions of the Valley and Ridge provinces, and highest (70-80 percent) in the Blue Ridge, Northern Neck, and Eastern Shore. Recharge values were lowest (9-11 in/yr) in some of the western counties and highest (16-18 in/yr) along the Blue Ridge and the Eastern Shore.