Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/20/2002
Publication Date: 11/20/2002
Citation: Gburek, W.J., Drungil, C.C., Srinivasan, M.S., Needelman, B.A., Woodward, D.E. Variable-source area controls on phosphorus transport: Bridging the gap between research and design. 2002. Journal of Soil and Water Conservation. 57(6):534-543. Interpretive Summary: Phosphorus (P) accumulation on farms throughout many areas of the U.S. has increased farm-scale soil P levels to the extent that they commonly exceed crop needs. Loss of P in surface runoff from these high-P soils can lead to significant off-site environmental and economic impacts related to deterioration in water quality. A major issue facing nutrient management and watershed planners today is how management of P at the field or farm scale relates to reduction in P levels in watershed outflow. To help address this issue, NRCS has proposed the use of a user-oriented tool called the Phosphorus Index (PI). Most versions of the PI being developed by individual states include what is termed a connectivity factor, ostensibly representing the hydrologic connection of a field to the stream. However, no consensus has developed regarding how this factor should be quantified. Building on a site-specific connectivity factor developed on a aresearch watershed in east-central Pennsylvania, an ARS-NRCS work group ha developed a generalized, design-oriented connectivity factor for inclusion in the PI. The method is based on design rainfall, the NRCS curve number runoff generation technique, the variable-source-area hydrologic concept, and readily available watershed geomorphic data. The connectivity factor is also formulated in a risk-based format to address the uncertainty inherent in use of the PI. Here, the rationale upon which the generalized connectivity factor is based is explained in detail, and for illustration, the factor is applied to a small watershed in east-central Pennsylvania. By including this generalized connectivity factor in the many versions of PI being considered today, P management based on the PI will become more uniform across states.
Technical Abstract: Transport factors incorporated within most versions of the Phosphorus Index (PI) are expected to represent the potential for phosphorus (P) to move from a field source to a stream or other surface water body. Consequently, these factors should 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 PIs, 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 PI. Here, we develop a generalized connectivity factor for inclusion in PIs 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 detailed hydrologic investigations being conducted on the watershed.