Estimating Streambank Phosphorus Loads at the Watershed Scale with Uncertainty Analysis Approach

Streambank nutrient loading rates are a growing concern within many watersheds. Only a few studies exist on streambank soil chemistry and phosphorus (P) concentrations, spatial distributions in watersheds, and P loading rates with a consideration of the potential uncertainty associated with the estimates. More so, limited studies compare streambank P loading for streams within similar watersheds and with similar land use and management. The objectives of this research included (1)quantifying the magnitude and spatial distribution of soil pH, electrical conductivity (EC), total P concentration, dissolved P concentration, and the degree of P saturation of streambanks in a watershed; (2)quantifying whether water-soluble phosphorus (WSP) and total phosphorus (TP) loads entering the stream from streambanks are significant based on a combined mass balance and uncertainty analysis approach; and (3)contrasting streambank P concentrations and loadings between two similar streams: Spavinaw Creek (SC) versus Barren Fork Creek (BFC) in eastern Oklahoma. Both SC and BFC flow through the Ozark ecoregion, possess similar geomorphology, and are characterized by similar land uses. Following procedures conducted for BFC, streambank sampling occurred at five sites along SC, and samples were processed for pH, EC, WSP, and TP. Unlike BFC, there were no clear longitudinal trends in WSP, TP, pH, and EC; similar to BFC, no consistent vertical trends were observed. Using estimated sediment loading (727x106kg) from aerial images and Monte Carlo analysis, it was estimated from 2003 to 2013 there was 1.5x103kg WSP and 1.4x105kg TP loaded into SC from streambanks in Oklahoma. Average in-stream estimates were an order of magnitude larger for WSP and comparable for TP. Streambank P contributions and erosion rates along one stream cannot be used to accurately predict P loading along other streams even in similar watersheds with similar hydrology, geomorphology, and land use because of watershed-specific variability in streambank erodibility and streambank P concentrations. Due to the uncertainty associated with critical input parameters, the uncertainty in streambank P loads at the watershed scale can be large and therefore uncertainty analysis approaches should be used in future research.