Sediment budget closure during runoff-generated high flow events in the South Amana sub-watershed, Ia

Event-based sediment budgets were developed in a small agricultural sub-watershed using radionuclide tracers in conjunction with traditional monitoring techniques. The result of these budgets quantified the flux of material from each sediment source in the sub-watershed. The first step in the study was to quantify the net flux of material through the watershed outlet for runoff events. Results indicated that a pronounced clockwise hysteresis effect occurred during all studied events. The cause of the hysteresis effect was attributed to exhaustion of the upland (i.e., hillslopes and floodplains) sediment source. The hysteresis effect was dampened during an extreme flash flood event that caused overbank flow. This dampening was attributed to an increase in upland sediment mobilization resulting from overbank flow. Results of the event-based monitoring were compared against a previously developed sediment rating curve that assumed a power-law relationship between suspended sediment transport and water flow rates. The results indicate that the power-law relationship grossly under predicted the sediment flux over each runoff event. A tracing technique was utilized to establish the relative contributions from the uplands and the stream channel (i.e., channel banks and bed). This technique used the relationship between the naturally occurring radionuclide tracers ⁷Be and ²¹⁰Pb_xs to differentiate eroded upland soils from channel-derived sediments in the suspended sediment loads. A simple two end-member unmixing model was used to determine the relative contribution from each source. Results indicate that the upland source was the dominant contributor to the suspended load early in the runoff events, but channel contributions were more prevalent at later stages, reinforcing the conclusion drawn from the hysteresis observation. Further analysis of the results indicated that the uplands contributed the majority of the material to the suspended load because the sediment transport rate during the early stages of the event was much larger than during the later stages. Therefore, watershed managers wishing to minimize non-point source pollution resulting from erosion should first focus their efforts on reducing erosion of upland soils.