Phosphorus Transport by Surface and Subsurface Flow Pathways in An Upland Agricultural Watershed.

Improved understanding of phosphorus transport by surface and subsurface flow pathways is critical to protecting water quality in agricultural watersheds. While considerable attention has been devoted to understanding phosphorus losses in overland flow, comparatively limited research has examined phosphorus transport in subsurface flow. We sought to compare phosphorus transport in overland and subsurface flow on two opposing hillslopes (north versus south facing), each with soils having different drainage properties. The experimental hillslopes are located within Mattern, an 11 ha agricultural watershed in the Valley and Ridge physiographic province of central Pennsylvania. Storm events were continuously monitored from September 2010 through December 2011 using small hillslope trenches (1 m wide × 3 m long × 1.5 m deep). The hillslope trenches were constructed to route, measure, and sample subsurface flow from major soil horizons (bottom of A horizon, center of argillic or cambic horizon, top of fragipan or top of parent bedrock). In addition, a slotted PVC pipe (5 cm diam., 3 m long) was installed in front of each trench in order to collect and measure overland flow. On each hillslope, one trench was located at the shoulder position in well-drained residual soils, while the second trench was situated at the footslope position in poorly drained soils possessing a fragipan at approximately 0.5 m depth. Preliminary hydrologic results showed that the residual soil sampling sites generated small volumes of runoff that were dominated by overland flow. Subsurface flows from these sites tended to infiltrate vertically as opposed to moving laterally downslope. In contrast, much larger volumes of runoff (approximately 140-fold greater) were generated from soils with a fragipan, with the majority of water draining laterally on top of the fragipan surface. In larger storms, these subsurface flows could persist for several days following the cessation of an event. Phosphorus concentrations in runoff from sites with fragipan soils were generally dilute in comparison to runoff from the residual soil sampling locations, but larger runoff volumes led to much greater phosphorus losses. The results of this study highlight the importance of lateral subsurface flow from fragipan soils as a critical pathway for phosphorus transport during storms.