Improving understanding of mixed-land-use watershed suspended sediment regimes: Mechanistic progress through high-frequency sampling.

Given the importance of suspended sediment to biogeochemical functioning of aquatic ecosystems, and the increasing concern of mixed-land-use effects on pollutant loading, there is an urgent need for research that quantitatively characterizes spatiotemporal variation of suspended sediment dynamics in contemporary watersheds. A study was conducted in a representative watershed of the central United States utilizing a nested-scale experimental watershed design, including five gauging sites (n=5) partitioning the catchment into five sub-watersheds. Hydroclimate stations at gauging sites were used to monitor air temperature, precipitation, and stream stage at 30-min intervals during the study (Oct. 2009-Feb. 2014). Streamwater grab samples were collected four times per week, at each site, for the duration of the study (Oct. 2009-Feb. 2014). Water samples were analyzed for suspended sediment using laser particle diffraction. Results showed significant differences (p<0.05) between monitoring sites for total suspended sediment concentration, mean particle size, and silt volume. Total concentration and silt volume showed a decreasing trend from the primarily agricultural upper watershed to the urban mid-watershed, and a subsequent increasing trend to the more suburban lower watershed. Conversely, mean particle size showed an opposite spatial trend. Results are explained by a combination of land use (e.g. urban stormwater dilution) and surficial geology (e.g. supply-controlled spatial variation of particle size). Correlation analyses indicated weak relationships with both hydroclimate and land use, indicating non-linear sediment dynamics. Suspended sediment parameters displayed consistent seasonality during the study, with total concentration decreasing through the growing season and mean particle size inversely tracking air temperature. Likely explanations include vegetation influences and climate-driven weathering cycles. Results reflect unique observations of spatiotemporal variation of suspended sediment particle size class. Such information is crucial for land and water resource managers working to mitigate aquatic ecosystem degradation and improve water resource sustainability in mixed-land-use watersheds globally.