INTEGRATED ASSESSMENT AND ANALYSIS OF PHYSICAL LANDSCAPE PROCESSES THAT IMPACT THE QUALITY AND MANAGEMENT OF AGRICULTURAL WATERSHEDS
Location: Watershed Physical Processes Research Unit
Title: Quantifying Relative Contributions from Sediment Sources in Conservation Effects Assessment Project Watersheds
Submitted to: Journal of Soil and Water Conservation Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 11, 2008
Publication Date: November 3, 2008
Citation: Wilson, C.G., Kuhnle, R.A., Bosch, D.D., Steiner, J.L., Starks, P.J., Tomer, M.D., Wilson, G.V. 2008. Quantifying Relative Contributions from Sediment Sources in Conservation Effects Assessment Project Watersheds. Journal of Soil and Water Conservation. 63(6):523-532.
Interpretive Summary: As part of the Conservation Evaluation and Assessment Project (CEAP), the effect of conservation practices on the environmental health of agricultural watersheds is being evaluated on 12 benchmark watersheds. One of the main pollutants of these watersheds is sediment. Knowledge of the sources where sediment originates on these watersheds is being used to compare and contrast the ways in which sediment moves through these watersheds. This report presents results on sediment sources from five CEAP benchmark watersheds. The activities of two naturally produced radioactive elements (7Be and 210Pb) were used together to make the distinction between sediment sources from fields and stream banks. Identifying the main sources of sediment delivered to the stream will assist in focusing Best Management Practices to maximize their effectiveness on these watersheds. The two main sources of sediment, eroded surface soils and collapsed bank sediment, have been found to have distinctly different activities of the two radioactive elements which allows for determinations on their fraction in the suspended sediment of the streams and rivers. The suspended sediment carried downstream in these watersheds is a mixture of these two types of sediment. It will have an activity that reflects this mixture by having a value that is between the activities of the two sources. A simple mixing model was used to determine the amount of sediment that came from the landscape surface and the stream banks following a rain event. Preliminary results have shown that sediment from channel sources is the dominant source in three of the five watersheds. This information is useful for determining which conservation practices will be most effective on a given watershed.
As part of the USDA Conservation Effects Assessment Program (CEAP), 12 watersheds across the country have been designated as benchmarks to address one of the major objectives of CEAP, which is to evaluate the effectiveness of conservation practices under various landscape and agricultural conditions. Identifying the major contributors of fine sediment to the suspended sediment loads of the streams within these watersheds will provide valuable information regarding Best Management Practice (BMP) efficiency and allow for better focus of future conservation practices.
A novel technique, which quantifies the relative proportion of eroded surface soils and entrained channel sediments in the fine suspended sediment load of streams during runoff events, was applied to the benchmark watersheds. This method utilizes naturally occurring radionuclides, namely 7Be and 210Pbxs, as tracers to differentiate eroded surface soils and sediments contributed from channel banks and the streambed. Due to different erosion processes controlling the sediment delivery to streams (sheet erosion, bank collapse, or resuspension), eroded surface soils have higher activities of 7Be and 210Pbxs than collapsed bank sediment or entrained bed sediments. The fine suspended sediment, which is a mixture of eroded surface soils and channel sediment, has an intermediate radionuclide signature that is quantified in terms of the relative contribution from both source materials. A simple two-end member mixing model was used to determine the relative contribution from each source area to the total fine sediment load. Data presented here focus on runoff events at sites in Georgia, Iowa, Mississippi, and Oklahoma. Preliminary analysis suggests that eroded surface soils were more abundant in the suspended sediment early in the runoff event; however, channel contributions dominated the suspended load later in the runoff event.