Phosphorus loading in agricultural and forested headwater streams in the upper Etowah River basin, Georgia
Romeis, James Joshua
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Few watershed-scale studies have evaluated how phosphorus (P) source and hydrologic transport factors in watersheds managed by poultry operations translate into in-stream P loading. In this study, a combination of continuous (5-minute) streamflow and mixed-frequency water quality data sets were used to estimate total P (TP) loads in three forested (FORS) and nine agricultural (poultry-pasture) (AG) headwater streams (2.4 – 44 ha) in the upper Etowah River basin of Georgia. Specific P source (soil P) and transport (watershed land cover and physical characteristics) factors were also investigated. The data collection duration at study sites ranged between 18 and 22 months. A total of 1,603 water quality samples were collected from the study sites. Significant (P < 0.1) inverse relationships were detected between extreme flow response variables (i.e. Q0.1) and drainage area and percentage of forest cover. Order-of-magnitude differences in TP and dissolved reactive P (DRP) concentration were observed between AG and FORS sites and among AG sites specifically. At AG sites, stormflow TP concentrations depended on streamflow, but the concentration-flow relationships were strongest on a storm-event basis. At most AG sites, stormflow TP concentrations appeared to reach supply-limited concentrations that were independent of streamflow rate. A documented exception was at an AG site where a storm event was sampled soon after poultry litter application. Three load estimators were examined in the study—planning level, flow-duration rating curves (FDRC), and log-transformed regression models with and without bias-correction techniques. Based on FDRCs, FORS site TP yields ranged between 0.01 and 0.1 kg-P ha-1. At AG sites, the yields ranged 0.031 to 3.17 kg-P ha-1. With confidence intervals factored in, AG site yields ranged 0.025 to 13.1 kg-P ha-1. Total P yields were significantly related (P < 0.005) to area-weighted Mehlich-1 soil test P (AWSTP) concentrations. Dissolved reactive P concentrations observed during non-storm flow conditions were significantly related (P < 0.05) to AWSTP and P yield. Results suggest that water quality sampling during non-stormflow conditions may be a useful screening tool for watershed scale P risk-based management.