ON THE METHODS OF ESTIMATING GROUNDWATER RECHARGE FROM STREAM DISCHARGE

Two common groundwater recharge estimation methods: Seasonal Recession Method (SRM) (or Meyboom Method) and the Recession Curve Displacement Method (RCDM) (or Rorabaugh Method) are found not always giving satisfying results in my New Jersey data study. The average ratios of annual groundwater recharge/river discharge of eight stream discharge series which have at least twenty-year long daily record from New Jersey are calculated. The SRM gives an average ratio of 0.14 and the RCDM by RORA program (Rutledge, USGS Water-Resource Investigations Report 98-4148, Reston, Virginia, 1998) gives an average ratio of 0.82. The averaged ratio of 0.14 is lower and the ratio of 0.82 is higher than the normal ratio of groundwater recharge/river discharge (around 0.45 to 0.5). The explanation of the underestimation by SRM is that SRM uses only the two lowest log average daily discharges on a annual recession limp to define its recession trend line, therefore, ignores the recharge events above the two lowest points.  The alternative of using a daily average trend line, or a monthly average trend line, instead of the two lowest daily average data of a season to defend a recession line seems to improve this situation of underestimation.  The high recharge/river discharge ratios of RCDM by RORA may come from the multiplier 2 in the recharge equation Q_recharge = 2(Q_B-Q_A)t₁/2.3026,where discharges Q_B and Q_A are discharges at a critical time t and t₁ is the time for 1 log cycle of recession. A simplified recharge equation of Q_recharge = (Q_B-Q_A)t₁/2.3026, where Q_B and Q_A are discharges at time t_D=A^0.2 (where A is the catchment area in square miles) gives a more practical recharge/discharge ratio.