Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/11/2008
Publication Date: 3/1/2008
Citation: Sharpley, A.N., Kleinman, P.J., Heathwaite, A.L., Gburek, W., Folmar, G.J., Schmidt, J.P. 2008. Phosphorus loss from an agricultural watershed as a function of storm size. Journal of Environmental Quality. 37:362-368. Interpretive Summary: Due to the importance of storm flow to watershed phosphorus loss, storm characteristics exert a strong control on the potential of phosphorus loss to occur. However, little information is available documenting the effect of storm size on phosphorus loss, even though this information is fundamental to the concept of risk assessment and indexing phosphorus loss and to targeting watershed remediation efforts in general. We found that for a mixed-land use watershed in Pennsylvania that most of the phosphorus exported over 10 years occurred during storm flows with less than a 1-year return period. However, the two largest storms contributed more than 20% of phosphorus loss. The fact that runoff contributing area likely increases with storm size or return period has important implications to watershed management of phosphorus. For example, phosphorus-based management would need to be applied to an increasingly large area of the watershed to minimize the risk of phosphorus loss. Thus, at some stage implementation of remedial strategies in general will need to address what level of risk management is acceptable. This will need to be weighed against an increased area of land that may require more restrictive phosphorus-based management to affect a phosphorus loss reduction.
Technical Abstract: Phosphorus (P) loss from agricultural watersheds is generally greater in storm rather than base flow. Although a fundamental concept of P-based risk assessment tools, few studies have quantified the effect of storm size on P loss. Thus, the loss of P as a function of flow type (base and storm flow) and size was quantified for a mixed-land use watershed (FD-36; 39.5 ha) from 1997 to 2006. Storm size was ranked by return period (less than 1, 1 – 3, 3 – 5, 5 – 10, and greater than 10 yr), where increasing return period represents storms with greater peak and total flow. From 1997 to 2006, storm flow accounted for 32% of watershed discharge, yet contributed 65% of dissolved reactive P (DP, 120 g**ha**yr) and 76% of total P (TP) exported (555 g**ha**yr). Of 248 storm flows during this period, 93% had a return period of less than 1 yr, contributing most of the 10-yr flow (6507 m3**ha; 63%) and DP (774 g**ha; 64%) and TP (2981 g**ha; 54%). Two 10-yr storms contributed 21% of P exported between 1997 and 2006. A significant increase in storm flow DP concentration with storm size (0.10 to 0.29 mg**L) suggests that P release from soil and or area of the watershed producing runoff increase with storm size. Thus, implementation of P-based BMPs will need to consider what level of risk management is acceptable.