Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/20/2001
Publication Date: 9/20/2001
Citation: Mcdowell, R.W., Sharpley, A.N., Folmar, G.J. 2001. Phosphorus export from an agricultural watershed: linking source and transport mechanisms. Journal Of Environmental Quality. 30(5):1587-1595. Interpretive Summary: Many source and transport factors control P loss from agricultural landscapes at a watershed scale. We found that the release of P from soil and stream sediments was related to the concentration of P at several points in a stream draining an agricultural watershed in central Pennsylvania. Phosphorus concentrations in base flow were greater at the watershed outlet than the upper-most point, while the opposite occurred during storms. These changes in P concentration in the stream were related to flow and chemical processes occurring over the watershed and within the stream. During base flow, P concentrations were controlled by the release of P from stream sediments. However, erosion and runoff of P from the upper parts of the watershed, coupled with some binding of P by stream sediments, controlled the concentration of P in the stream during storm flows. These results show the importance of the different processes that control P concentrations in a stream and how the processes vary spatially and temporally. This understanding will enable us to better target and manage P losses from an agricultural watershed.
Technical Abstract: Many source and transport factors control phosphorus (P) loss from agricultural landscapes; however, little information is available on how these factors are linked at a watershed scale. Thus, we investigated mechanisms controlling P release from soil and stream sediments in relation to storm and baseflow P concentrations at four flumes in the channel of an agricultural watershed. Baseflow DRP concentrations were greater at the watershed outflow (flume 1; 0.042 mg/L) than upper-most flume (flume 4; 0.028 mg/L). Conversely, DRP concentrations were greater at flume 4 (0.304 mg/L) than flume 1 (0.128 mg/L) during stormflow. Similar trends in total phosphorus (TP) concentration were also observed. During stormflow, stream P concentrations are controlled by overland flow generated erosion from areas of the watershed coincident with high soil P. In-channel decreases in P concentration during stormflow were attributed to sediment deposition, ,resorption of P, and dilution. The increase in baseflow P concentrations downstream were controlled by channel sediments. Phosphorus sorption maximum of flume 4 sediment (532 mg/kg) was greater than at the outlet flume 1 (227 mg/kg). Indeed, the greater desorption of P from flume 1 than 4 sediment (0.046 to 0.025 mg/L) was similar to the increase in baseflow dissolved reactive phosphorus (DRP) from flume 4 to 1 (0.028 to 0.042 mg/L). This study shows erosion, soil P concentration, and channel sediment P sorption properties influence streamflow DRP and TP. A better understanding of the spatial and temporary distribution of these processes and their connectivity over the landscape will aid targeting remedial practices.