Hydrologic evolution of drainage basins disturbed by surface mining, central Pennsylvania
The hydrologic evolution of drainage basins disturbed by surface mining in central Pennsylvania is regulated by infiltration capacity of minesoils and its control on hillslope runoff processes. Steady-state infiltration capacity of newly reclaimed minesoils is uniformly low, less than 1 cm/hr, and it generally increases up to 6 cm/hr with time, the magnitude of increase being dependent on soil characteristics and vegetation. In drainage basins where steady-state infiltration capacity recovers to less than 3 cm/hr, the dominant runoff process is infiltration-excess overland flow. Increased peak runoff rates associated with infiltration-excess overland flow are sufficient to initiate drainage network evolution in these basins, with phases of expansion and abstraction. Storm hydrographs are characterized by (1) increasing peak and total runoff and decreasing time to peak runoff during periods of network extension and (2) decreasing peak and total runoff and increasing time to peak runoff during network abstraction. Trends in hydrograph evolution are due to the relative efficiency of channeled-versus overland-flow processes for removing excess rainfall from the basin. In contrast, in drainage basins where steady-state infiltration capacity recovers to greater than 3 cm/hr, runoff is initially dominated by infiltration-excess overland flow, but saturation overland flow is increasingly dominant with time. Drainage network evolution is limited to skeletal network initiation and elongation, which occur while the basin is dominated by infiltration-excess overland flow. Changes in storm hydrographs include decreasing peak runoff and increasing time to peak runoff, the result of the increasing dominance and lagged response of saturation overland flow. Although steady-state infiltration capacity increases in these basins, total runoff remains essentially constant due to the extended and less steep recession limb of storm hydrographs, which is a characteristic of hydrographs from basins dominated by saturation overland flow. The hydrologic response of drainage basins to changes in the dominant runoff process has important implications for models of long-term drainage-basin evolution, particularly those used to evaluate climate change and associated changes in vegetation and soil chemical and physical properties, each of which may affect infiltration capacity.