COMPARISON OF MEASURED AND SIMULATED PHOSPHORUS LOSSES WITH INDEX RECOMMENDATIONS

Authors: Veith, Tameria - Tamie; Sharpley, Andrew; Weld, Jennifer; Gburek, William

Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/4/2005
Publication Date: 4/18/2005
Citation: Veith, T.L., Sharpley, A.N., Weld, J.L., Gburek, W.J. 2005. Comparison of measured and simulated phosphorus losses with index recommendations. Transactions of the ASAE. 48(2):557-565.

Interpretive Summary: Direct measurement of phosphorus (P) loss often is limited by site-specific watershed hydrology and land management. Although simulation models often can bypass field study limitations and use detailed equations to mimic sediment and P movement accurately, they typically require a large amount of data and expertise. Phosphorus indices provide a less complex, less data-intensive means of calculating risk of P loss. Many studies have evaluated the use of simulation models and of P indices separately, but few have compared results of field studies with both complex model simulation and the simpler index. This study uses a daily time-step, process-based, simulation model (Soil and Water Assessment Tool) to predict watershed- and field-level losses from a well-documented watershed with years of land management and water quality data. The model predictions then are compared to ratings from the Pennsylvania P Index for fields within the watershed. This research showed that while the simulation model and the P Index both identified areas of high risk to P loss, the P Index is much simpler to use and requires only a few, easily obtained inputs. The simplicity and accessibility of the P Index promotes its use when more complex models are not feasible. Use of the P Index will aid in selecting appropriate conservation practices to control undesirable sediment and P loss from field to stream and ultimately improve water quality.

Technical Abstract: Measurement of nonpoint source phosphorus (P) loss from a watershed can be accomplished through stream sampling. However, accurate estimation of P loss from individual fields and their relative contributions to watershed export of P is more complex. Simulation models can estimate losses at both field and watershed scales, but typically become data and expertise intensive as scale and predictive accuracy increase. Risk assessment indexes, such as the P Index, are less complex to use, and are designed to indicate the P loss vulnerability of a field based on source and transport to stream characteristics. A 39.5 ha mixed land use watershed in Pennsylvania was studied over three years (1998 ' 2000). Measured watershed exports of dissolved (0.10 kg ha-1 yr-1) and total (0.32 kg ha-1 yr-1) P were similar in magnitude to predicted losses (0.05 and 0.73 kg ha-1 yr-1, respectively) using the Soil and Water Assessment Tool (SWAT) simulation model. Field-level predictions from SWAT were then compared with P Index ratings for 22 fields. The P Index and SWAT categorized 73% of the fields similarly in terms of risk to P loss with Pearson correlation significant at p = 0.07; all except one of the remaining six fields were over or under predicted by only one category. The results indicate that actual P loss from FD-36 was small, and only three fields contributed a major proportion of this loss. Accurate prediction of watershed P export enabled a reliable estimation of P loss from individual fields within the watershed. These factors agreed closely with P loss vulnerability assigned by the P Index, indicating the Index can provide land managers with a reliable assessment of where P loss is actually occurring within a watershed to target more effective conservation practices.