Contribution of Streambank Erosion to Total Phosphorus Loads in Agricultural Watersheds

Excessive loss of phosphorus to fresh water systems can lead to eutrophication and nuisance algal blooms and development of hypoxic conditions in coastal water bodies, including the Gulf of Mexico. It is therefore important to develop an accurate estimate of P loads exported from the landscape at local and regional scales. In this study, annual total phosphorus (TP) loads at 48 Iowa watersheds were estimated using two USGS load estimation programs (LOADEST and WRTDS) for the 1999 to 2017 period. Mean and median TP yields varied considerably, ranging from 0.4-7.7 kg/ha and 0.32 to 3.75 kg/ha, respectively. A methodology was developed to identify severely eroding segments located along 3rd to 6th order river systems using high-resolution Light Detection and Ranging (LiDAR) topographic data. Mapping results indicate that more than 35,000 km of streambanks are severely eroding in Iowa, representing approximately 41% of all 3rd to 6th order streams in the state. Results indicate that P contributions from streambanks contribute significantly to annual P export from agricultural watersheds.