Soil sediment export from agricultural watersheds is a major environmental concern and is a primary contributor to nonpoint source sediments in streams. The purpose of this study was to quantify total suspended solid (TSS) export and current sediment delivery ratios (SDRs) in four southern Iowa watersheds and evaluate how existing and potential best management practices (BMPs) have affected SDRs. Our study updated estimates of SDRs that were previously developed using mid-20th century data and largely unknown methods. We estimated TSS export using continuous turbidity measurements and total phosphorus (P) data and measured discharge data to calculate TSS loads. By comparing annual TSS export to watershed-scale soil erosion estimated with the Revised Universal Soil Loss Equation (RUSLE) model, we calculated annual SDRs for the study watersheds and found that current SDRs were significantly lower than previous estimates. This new analysis provides an exceptional story of conservation progress in our study watersheds over the past four decades. Further, they are likely a worst-case scenario for sheet and rill eroded sediment export since TSS export does not distinguish among other sediment sources, such as stream bank and gully erosion. Based on the extent of BMP implementation in the watersheds and the potential for future BMPs determined using the Agricultural Conservation Planning Framework (ACPF) toolbox, we found that there is only limited potential for further reducing TSS export using additional in-field practices. Hence, we believe that further work toward reducing TSS export in these Iowa watersheds should be shifted to reducing contributions from other TSS sources including from streambed, bank, and gully erosion.

中文翻译：

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爱荷华州南部流域的侵蚀和泥沙输送：对保护规划的意义

农业流域的土壤沉积物出口是一个主要的环境问题，并且是溪流中非点源沉积物的主要贡献者。这项研究的目的是量化爱荷华州南部四个流域的总悬浮固体（TSS）出口和当前泥沙输送比（SDR），并评估现有和潜在的最佳管理实践（BMP）如何影响SDR。我们的研究更新了以前使用20世纪中叶的数据和鲜为人知的方法开发的SDR估算值。我们使用连续的浊度测量以及总磷（P）数据和实测排放数据估算TSS出口量，以计算TSS负荷。通过将修正的通用土壤流失方程（RUSLE）模型估算的年度TSS出口量与流域规模的土壤侵蚀进行比较，我们计算了研究流域的年度特别提款权，发现当前的特别提款权大大低于先前的估计。这项新的分析为我们过去四十年来研究流域的保护进展提供了一个非同寻常的故事。此外，由于TSS出口无法区分其他沉积物来源，例如溪流和沟壑侵蚀，因此它们可能是片状和小孔侵蚀性沉积物出口的最坏情况。根据流域实施BMP的程度以及使用农业保护规划框架（ACPF）工具箱确定的未来BMP的潜力，我们发现使用其他现场实践进一步减少TSS出口的潜力有限。因此，