Spatial-Temporal Variability and Hydrologic Connectivity of Runoff Generation Areas in Sand Mountain Region of Alabama

Abstract

This study delineated spatially- and temporally-variable runoff generation areas in the Sand Mountain region of North Alabama under natural rainfall conditions and demonstrated that hydrologic connectivity is important for generating hillslope response when infiltration-excess runoff mechanism dominates. Data from four rainfall events (rainfall amounts 1.4 – 3.2 cm) on an intensively-instrumented pasture hillslope (0.12 ha) were analyzed. Two of these rainfall events occurred in the summer months, while the two others occurred in the winter months. Analysis of data from surface runoff sensors, tipping bucket rain gauge, and HS-flume demonstrated spatial and temporal variability in runoff generation areas. Results showed that the maximum runoff generation area, which contributed to runoff at the outlet of the hillslope varied between 87 to 100%. Furthermore, because infiltration-excess was the main runoff generation mechanism on the hillslope, the data showed that, as the rainfall intensity changed during a rainfall event, the runoff generation areas expanded or contracted. During rainfall events with 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. In situ soil hydraulic conductivity (k) measured and interpolated across the hillslope confirmed hydrologic connectivity of the runoff generation areas. Combined surface runoff sensor and ln(k) interpolated data clearly showed that during a rainfall event, lower k areas start generating runoff first, and then, depending on rainfall intensity during the event, runoff at the outlet is generated by hydrologically connected areas. It was concluded that in infiltration-excess runoff dominated areas, rainfall intensity and soil hydraulic conductivity can explain hydrologic response. The study demonstrated that only hydrologically connected areas of low hydraulic conductivity generate surface runoff during high intensity rainfall events. Quantification of these areas would serve as an important foundation for controlling nonpoint source pollution.