Preferential flow is an important subsurface hydrological process, resulting in rapid fluxes of water and contaminants from hillslopes laterally to the stream channel and vertically to the water table. While preferential flow has been studied extensively, research has traditionally focused on individual irrigation experiments, post-event excavations, and limited storm monitoring. In this study conducted at the Shale Hills Critical Zone Observatory, we analyzed over 3 years of real-time monitoring of soil moisture and precipitation, totaling 175 distinct storm events. For each storm we have identified real-time soil moisture response at 10 sites and categorized the response as preferential flow (non sequential response to precipitation with soil depth), sequential flow (a sequential response with soil depth indicative of a vertical wetting front from the surface to a certain soil depth), or no flow (either no response to precipitation with soil depth). From the analysis of 45 indices of precipitation and soil moisture dynamics prior to and during each storm, we developed a comprehensive understanding on the controls and spatial-temporal patterns of preferential flow in this small forested catchment. We found that preferential flow occurred throughout this catchment with various soils and landforms, and the frequency of preferential flow occurrence was insensitive to topographic position. The controls on preferential flow initiation, however, varied with soil depth, distance upslope and upslope contributing area. Ridge and hillslope sites with shallow soils were controlled by initial soil moisture as preferential flow occurred more often when the soils at these sites were dry during the late summer. Preferential flow at the upper swale sites occurred more likely when the soil was wet, but depended on the internal storm intensity dynamics. When storms were long, with an extended prewetting, followed by a late spike in precipitation intensity, preferential flow occurred more frequently in mid slope sites. Preferential flow at the valley bottom was most likely when the antecedent precipitation was high, and soils were wet. Widespread preferential flow occurred throughout the catchment when a mixture of these conditions was met (long storms with a late peak in precipitation intensity falling on initially dry soils saturated by the storm event). These findings will help determine the locations and timing of preferential flow occurrence in forested environments and have implications for catchment hydrologic modeling.
Graham, C.B., Lin, H. (2010): Controls on preferential flow in the vadose zone. AGU Annual Fall Conference Proceedings.
This Paper/Book acknowledges NSF CZO grant support.