Runoff Curve Numbers for Reclaimed Surface Mines in Pennsylvania

The Surface Mining Control and Reclamation Act of 1977 requires evaluation of pre- and postmining hydrology for design of runoff and sediment control structures on mined land. The runoff-curve-number (CN) method as developed by the Soil Conservation Service has been routinely used to predict rainfall-runoff relationships for ungauged watersheds disturbed by surface mining and reclamation. However, lack of sufficient rainfall-runoff data from reclaimed surface mines in the Eastern United States has prevented accurate calibration of CN values for those areas. Median CN values calculated from rainfall-runoff data from reclaimed surface mines in central Pennsylvania indicate that the currently accepted methods for estimating CN values underestimate total runoff from the standard design storm by as much as 50%. Median CN values range between 83 and 88 with CN values as high as 99 for soils with antecedent moisture condition classification III. It is recommended that hydrologic soil group D be adopted for sandy-loam and silty-loam postmine soils and used with the appropriate postmine land use to calculate CN values for reclaimed-surface-mine watersheds.