Problems associated with estimating ground water discharge and recharge from stream-discharge records

Ground water discharge and recharge frequently have been estimated with hyd rograph-separation techniques, but the critical assumptions of the techniqu es have not been investigated. The critical assumptions are that the hydrau lic characteristics of the contributing aquifer (recession index) can be es timated from stream-discharge records; that periods of exclusively ground w ater discharge can be reliably identified; and that stream-discharge peaks approximate the magnitude and timing of recharge events. The first assumpti on was tested by estimating the recession index from stream-discharge hydro graphs, ground water hydrographs, and hydraulic diffusivity estimates from aquifer tests in basins throughout the eastern United States and Montana. T he recession index frequently could not be estimated reliably from stream-d ischarge records alone because many of the estimates of the recession index were greater than 1000 days. The ratio of stream discharge during baseflow periods was two to 36 times greater than the maximum expected range of gro und water discharge at 12 of the 13 field sites, The identification of the ground water component of stream-discharge records was ambiguous because dr ainage from bank-storage, wetlands, surface water bodies, soils, and snowpa cks frequently exceeded ground mater discharge and also decreased exponenti ally during recession periods. The timing and magnitude of recharge events could not be ascertained from stream-discharge records at any of the sites investigated because recharge events were not directly correlated with stre am peaks. When used alone, the recession-curve-displacement method and othe r hydrograph-separation techniques are poor tools for estimating ground wat er discharge or recharge because the major assumptions of the methods are c ommonly and grossly violated. Multiple, alternative methods of estimating g round water discharge and recharge should be used because of the uncertaint y associated with any one technique.