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Problems associated with estimating the connection between ground-water discharge and stream-discharge


, : Problems associated with estimating the connection between ground-water discharge and stream-discharge. Abstracts with Programs - Geological Society of America 32(7): 60

Ground-water discharge to surface water frequently has been estimated with hydrograph-separation techniques, but the critical assumptions of these techniques have not been investigated. The assumptions are that (1) hydraulic characteristics of the contributing aquifer (recession index) can be estimated from stream-discharge records; (2) periods of exclusively ground-water discharge can be reliably identified; and (3) stream-discharge peaks approximate the magnitude and timing of recharge events.The assumptions were tested using stream-discharge hydrographs, ground-water hydrographs, and hydraulic diffusivity estimates from aquifer tests in basins throughout the eastern United States and Montana. The recession index frequently could not be estimated reliably from stream-discharge records alone because many of the estimates of the recession index were greater than 1,000 d. The ratio of stream discharge during baseflow periods was 2 to 36 times greater than the maximum expected range of ground-water discharge at 12 of the 13 field sites. The identification of the ground-water component of stream-discharge records was ambiguous due to drainage from bank-storage, wetlands, surface-water bodies, soils, and snowpacks. These frequently exceeded ground-water discharge and also decreased exponentially 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 stream peaks.When used alone, hydrograph-separation techniques are poor tools for estimating the connection between ground-water discharge and stream discharge because the major assumptions of the methods are commonly violated. Multiple, alternative methods such as aquifer tests and geochemical methods should be used to estimate ground-water discharge because of the uncertainty associated with any one technique.

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