A detailed hillslope-scale hydrologic study was conducted in a pasture at the Sand Mountain region of north Alabama, USA. A 0.12 ha hillslope was intensively instrumented using 31 distributed surface and subsurface runoff sensors, a tipping-bucket rain gage, and a 0.31 m HS-flume. Data sets were collected during several rainfall events occurred in 2006 and 2007. This data was used in the hydrologic modeling part of this study.
Results from three rainfall events of differing characteristics, which occurred in 2006, using sensor data at four locations with different soil hydraulic properties along the hillslope showed that the main surface runoff generation mechanism in pastures of this region is infiltration excess. Rainfall intensity and soil hydraulic conductivity were found to play a dominant role in the surface runoff generation process. Furthermore, it was observed that only periods of high intensity rainfall (relative to saturated hydraulic conductivity) produced surface runoff.
Data analysis for six rainfall events in 2007 showed that the maximum runoff generation area that contributed to runoff at the outlet of the hillslope, varied between 67 and 100%. Furthermore, the data showed that as the rainfall intensity changed during a rainfall event, the runoff generation areas expanded or contracted. During rainfall events of high-intensity short- to medium-duration, 4 to 8% of total rainfall was converted to runoff at the outlet. Rainfall events with medium- to low-intensity, medium duration were found less likely to generate runoff at the outlet.
A physically-based, distributed hydrological model, HIRO2, which considers infiltration-excess (Hortonian overland flow) runoff generation as the dominant mechanism, and incorporates most of the hydrologic processes occurring over a hillslope, was found to be applicable at a hillslope-scale. The model showed agreement with the observed spatial and temporal variability of runoff generation areas. The model results helped explain the interaction among hydrologic characteristics such as topography, soil parameters, and rainfall characteristics and their relation to surface runoff mechanisms.
The study demonstrates that only the areas of low hydraulic conductivity zones that are connected generate surface runoff during high intensity rainfall events. Since only high intensity periods of a few rainfall events generated runoff and also because less than about 10% of the rainfall was converted to runoff, this study indicates that subsurface flow is more important in the pastures of this region.