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Precipitation Effects on Sediment and Associated Nutrient Discharges from Rhode River Watersheds

Smithsonian Environmental Research Center, Edgewater, MD 21037.

^* Corresponding author (correll{at}serc.si.edu).

ABSTRACT

Total suspended sediment (TSS) concentrations and their nutrient composition were studied in discharges from seven contiguous small watersheds on the Atlantic Coastal Plain in Maryland for up to 22 yr. All watersheds were equipped with V-notch weirs and volume-integrating, flow-proportional samplers. Spot samples were also taken for analysis of TSS concentrations and the composition of dissolved and particulate nutrients at known water discharge rates. Interannual variations in annual and seasonal precipitation during this study spanned approximately the range of 160-yr weather records in the vicinity. Mean annual TSS fluxes were 263, 546, 134, and 92 kg ha^–1 for the area-weighted mean of the overall Rhode River watershed, and for subwatersheds that were primarily-cropland, completely forested, and grazed, respectively. TSS fluxes were highest in the summer, followed by spring, winter, and fall. Regressions of TSS flux vs precipitation were used to calculate TSS fluxes for seasons and years with average, above and below the average precipitation. TSS flux from the Rhode River watershed was 12-fold higher in very wet years than in very dry years and 271-fold higher in very wet summers than in very dry summers. At base flow, TSS had high nutrient content, but as flow rates increased TSS nutrient content rapidly declined. At base flow, most of the total-P, TPi, TKN, and organic-C was in the dissolved phase, while at higher flows, most of these nutrients were in the TSS phase.