Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/26/1996
Publication Date: N/A
Citation: N/A Interpretive Summary: The patterns and concentrations of phosphorus and nitrate in streamflow were analyzed in terms of environmental impact. The analysis was structured so to identify the primary source areas and source times controlling the export of these nutrients from a typical hill land agricultural watershed located in the northeastern U.S. The results showed dmost losses to occur from relatively small areas within the watershed or during short time periods. Most of the nitrate was exported in nonstorm flow (mostly through late fall-spring), whereas most of the phosphorus (sediment-associated as well as dissolved) was exported in storm flow. Thus, there are critical areas or time periods that are most important to sample or treat for control purposes. This paper develops a watershed scale perspective for perceiving and assessing the impact of critical areas using stream export data.
Technical Abstract: Nutrient patterns and export in streamflow were determined for a 7.4-km2 agricultural hill land watershed (MCR) located in Pennsylvania. Data included mostly nutrient export in the water phase (soluble P, orthophosphorus, NO3) routinely collected for 9 years and in the suspended sediment phase (labile P, algal available P, total P) composited for 37 large storms. A smaller (9.9 ha) watershed contained within MCR and monitored for less time, provided similar results. The data set, representing the water phase, was subdivided into storm flow, baseflow and elevated baseflow categories. P export was dominated by storm flow whether associated with sediment or water phase. Even though storm flow periods occupied 10% of the time, 65-90% of the soluble P or ortho phosphorus exported occurred during storm flow. The proportion of P exported with sediment during storms ranged from 30-90%, depending on whether labile, algal available or total P were of concern. Depending on the watershed, soluble P accounted for 30% to 50% of the algal available P exported. Dissolved P concentrations were very low for non storm periods, substantially higher for storm periods, and overall exceeded EPA standards for orthophosphorus (10 ppb), less than 25% of the time. The EPA soluble P standard for lakes (50 ppb) was exceeded infrequently and then only during storm flow. The streams standard (100 ppb) was not exceeded. NO3 export was mostly in nonstorm flow (60%) with the higher concentrations in elevated baseflows. The significantly higher NO3 concentration in elevated baseflow compared to baseflow appears to result from increased drainage from a shallow fracture layer that underlays cropland. Only about 2% of flows exceeded the 10 ppm NO3-N standard. Because storm flow dominates P