USING CHLORIDE IN STREAMS TO ESTIMATE BASE FLOW AND RECHARGE IN WATERSHEDS

The chloride mass-balance method is often used in semi-arid to arid regions to estimate recharge. In contrast, recharge in humid regions is often estimated using analyses of stream hydrographs. The validity of the hydrograph techniques is often questionable for very small or large watersheds. A new method has been developed to estimate recharge that expands the chloride mass-balance method to the watershed scale. Based on chloride values measured in a stream over a range of flow rates, and combined with a long-term stream flow record, the long-term, hydrologic budget of the watershed can be estimated. Water balance and chloride balance equations for the watershed are rearranged to yield several equations, including a time-averaged chemical hydrograph equation, that can be solved sequentially for all of the water budget components. The method does not require measurements of chloride in precipitation, nor is it compromised by the anthropogenic sources at the land surface, such as road salts. A number of chloride concentrations are required over a range of stream flow rates, such that a rating curve can be developed. Riparian ET estimates can been made using either median chloride concentration in ground water, or the percentage of the watershed land surface occupied by floodplain. Many USGS stream-flow sites have sufficient historical chloride and stream-flow data available to make these calculations. Specific conductance can serve as a proxy where chloride data is limited. In cases where there is a deep ground-water source of chloride, other conservative solutes can serve the same purpose. The method gives good agreement with base-flow estimates made using long-term tritium data and the hydrograph separation method on the Potomac River. Examples from several sites in Virginia demonstrate that the chloride-based method can be used in conjunction with the hydrograph-based methods to better constrain estimates of base flow and recharge.