Role of Runoff Generation Mechanisms on Phosphorus Runoff from an Agricultural Hillslope in Central Pennsylvania.

The variable source area (VSA) concept provides the underlying paradigm for managing nutrient losses in runoff in the northeastern U.S.  This study sought to explain factors controlling runoff generation and losses of phosphorus along a single hillslope with contrasting soils and field management.  The experimental hillslope is located within a 27.4 ha agricultural watershed in the Ridge and Valley Province of central Pennsylvania.  A total of 94 storm events were monitored from 2002 to 2004 using runoff monitoring plots (2 m x 1 m) established in three distinct landscape positions.  The seepage-slope and transportational mid-slope plots were located in well-drained residual soils, whereas the foot-slope plots were located nearest the stream channel in somewhat poorly drained colluvial soils with fragipans.  During the study period, manure and fertilizer sources of phosphorus only were applied to fields upslope and including the seepage-slope landscape position.  Results of the study showed that storms occurring in residual soils generated small volumes of infiltration-excess surface runoff that resulted in high phosphorus concentrations but relatively small phosphorus losses.  In contrast, the foot-slope position produced much larger volumes of primarily saturation-excess surface runoff that significantly diluted phosphorus concentrations, but led to much more substantial phosphorus losses.  The overall trends in this study demonstrate the importance of hydrology as an important controlling variable on phosphorus losses in agricultural watersheds.