Abstract
Despite considerable research performed on forested catchments in the Ouachita Mountains of Oklahoma and Arkansas, little information on hydrological processes in operation is available. Based on catchment physical characteristics, subsurface flow was thought to be an important hydrological process in the region. Therefore, this study was undertaken to determine the occurrence, rates, timing and volumes of subsurface flow, and to estimate the importance of subsurface flow as a streamflow generating process. Subsurface flow was collected from three hillslope sites on a 7.7 ha forested catchment. Hillslope sites drained through natural seepage faces located near stream channels. Subsurface flow was collected from three depths at each hillslope site, below the litter layer, below the a horizon, and within the B horizon (Bt21). Subsurface flow occurred and was measured during 11 of 31 rainfall events. Subsurface flow responded rapidly to the initiation of and to changes in intensity of rainfall at all depths. the rapid response was indicative of flow through soil macropores. B horizon subsurface flow commenced within 10 to 180 min of the initiation of rainfall. Multiple linear regression showed that the volume of subsurface flow generated during a given storm was directly related to rainfall depth and a 7βday antecedent precipitation index used to represent antecedent water content. About 67 per cent of the total subsurface flow collected during the study was produced in one large storm under wet antecedent conditions. the storm was equal to the 2βyear, 24βhour storm for the region. Measured subsurface flow volumes were extended to the watershed scale to provide estimates of catchmentβwide contributions to streamflow. It was estimated that subsurface flow contributed from 1 to 48 per cent of total quickflow measured at the catchment outlet. Based on the timing of subsurface flow, it was estimated that subsurface flow May, contribute up to 70 per cent of quickflow before and soon after peak flow.