USGS Groundwater Information
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Recession-Curve Displacement Method for Estimating Groundwater Recharge In Humid Regions
Determination of Recession Index (K)
The recession index (K) must be determined for each watershed before recharge can be estimated by the recession-curve displacement method with the RORA program. (Rutledge 1998) K is the time in days required for groundwater discharge to decline by one log cycle after the recession curve becomes nearly linear on a semilog hydrograph. K is determined from construction of a master recession curve representing streamflow during periods when most or all flow is contributed by groundwater discharge. The recession curve can be determined manually or with an automated procedure. A good discussion of the theory and methodology for determining K is provided by Rutledge (1998, p. 9).