Erosion and sediment delivery in southern Iowa watersheds: Implications for conservation planning

Abstract

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-20^th 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.