Variable-source-area controls on phosphorus transport: Bridging the gap between research and design

ABSTRACT: Transport factors incorporated within most versions of the phosphorus index (Pl) are expected to represent the potential for phosphorus (P) to be mobilized on the field and subsequently move from the field source to a stream or other surface-water body. Consequently, these factors must be designed to capture the nature of the hydrologic properties of both the field and the watershed. The field-related transport factors typically included in Pls, e.g., the NRCS soil runoff class and erosion loss via RUSLE are generally accepted as sufficient to represent edge-of-field P loss. However, there has not been a unified approach to represent connectivity of field to stream within the Pl. Here, we develop a generalized connectivity factor for inclusion in Pls based on design rainfall, the variable-source-area (VSA) hydrologic concept, and readily available watershed geomorphic data. The NRCS curve number method is used to determine runoff volume from the watershed for design rainfalls representing a range of return periods. A simplification of VSA hydrology is used to derive the contributing area from which this runoff is expected to occur. Finally, the width of land adjacent to the stream contributing surface runoff is estimated using drainage density. For illustration, the methodology is applied to a small upland watershed in east-central Pennsylvania, and the results are examined in light of hydrologic investigations being conducted on the watershed.