Evidence of runoff production mechanisms in low gradient coastal forested watersheds

Abstract

The mechanisms by which forested watersheds produce rapid increase in flow following rainfall have been an area of inquiry for over 50 years. Overland flow due to rainfall rate exceeding infiltration capacity (Hortonian runoff) seldom occurs on forested watersheds. The idea of a limited source area that expands with rainfall has been widely accepted, although the dynamics of the process and the exact mechanisms have seldom been quantified. Several theories have been developed and tested primarily on watersheds with fairly steep slopes. On low gradient watersheds common to the southeastern US coastal plain little work has been done and the flow producing mechanism has been assumed to be saturation excess overland flow. Data from two watersheds in eastern Georgetown County, SC, reveal complexity in this explanation. Hydrograph analysis shows that the recession can be well predicted assuming two linear reservoirs. One showed a steep decline, approaching zero in about four days. The second has a slower decline, approaching zero in about one month. Ion ratios indicated a groundwater signature during the long decline with high concentrations of Ca and Si. DON and DOC concentrations suggest the peak and fast decline are from near surface sources consistent with the saturation excess overland flow hypothesis. However, piezometry near the stream and NH4 and NO3 concentrations suggest a rapid influx of water from the stream bed contributes to flow prior to the peak. These data are also consistent with observed rapid increase in water table elevation throughout the watershed at the initiation of rainfall. Although data support the saturation excess overland flow process, they also indicate that it is not the sole mechanism involved in stream flow generation on these flat sandy coastal watersheds.