Erosion and sediment delivery in Iowa
Soil export from agricultural watersheds in Iowa is an ongoing natural phenomenon that in recent history, due to significant unnatural increases, has become a foremost environmental concern. Since European-style agricultural land management began in the mid-19th century, delivery of soil sediments from the upper Midwest to the Gulf of Mexico has caused tremendous damage to aquatic ecosystems in terms of eutrophication leading to hypoxia. In the 1950’s and 1960’s, much research was conducted to quantify soil export from watersheds to streams. During that time, the “Sediment Delivery Ratio” (SDR) which is the proportion of soil sediment delivered to streams to the amount of soil eroded from a given watershed was defined. That research led to the development of a widely used “Soil Erosion and Sediment Delivery” report published by the USDA, NRCS in 1998 which quantified SDRs based on the size of a given watershed and its respective Iowa landform region. Limited information on the precise methodologies used to develop that report and the development of new data collection technologies has prompted this dissertation which sought to critically evaluate all aspects of SDRs with the recognition that modern technologies and information had the potential to drive a methodological shift in the way they are documented, determined, and ultimately justified regardless of watershed natural history, size, or modern land use. Results show that accurate estimates of SDRs are possible for unique watersheds of varying size using new estimates of soil erosion, stream total suspended solids, and stream discharge. Results further show that since conservation efforts began in earnest in the mid-twentieth century, significant progress has been made to lower SDRs in Iowa. New research must now focus on delineating contributions to SDRs from individual sources like sheet/rill erosion, gullies, and streambanks.