Advanced Modeling of Soil Erosion, Sediment Delivery and Nutrient Export from Iowa Watersheds
Soil sediment and sediment bound phosphorus export from agriculturally dominated watersheds is a global crisis that has far-reaching, potentially severe environmental and economic impacts. Multiple sources are known to contribute to surficial exports from these watersheds including in-field sheet and rill erosion, ephemeral gully erosion and streambank erosion. However, accurately quantifying suspended sediment and P export from watersheds is difficult.The Water Erosion Prediction Project (WEPP) may be utilized to help quantify sediment yields from agricultural fields. Sensitivity analysis and validation indicated that WEPP has high runoff and sediment yield accuracy for small watersheds. Still, scaling up WEPP Watershed to a HUC-12 scale may be difficult. The Daily Erosion Project (DEP) may be used to scale up WEPP, but these new outputs must be ground-truthed with field-collected data.
This project will help to identify and quantify the effects of ephemeral gullies on sediment and P loss. Ultimately, this project will work towards delineating sources of sediment and P export, i.e., hill slopes or ephemeral gullies, from Iowa watersheds, using Walnut Creek watershed in Jasper County, Iowa, as a test for soil erosion, sediment and nutrient export modeling.
The University of Iowa will perform a field investigation to collect and analyze actual soil sedimentation and export data at individual subcatchments within the study area to estimate sediment bound P exported from the watershed and calibrate related models. The USDA-Agricultural Research Service will provide and conduct an annual drone-based LiDAR field investigation in multiple land-use areas within the watershed to use as input data for the WEPP model. Iowa State University will complete WEPP models for the study area and scale up results using the DEP framework.
The project kicked off with a team planning meeting in August 2020. In November, the research team met in Walnut Creek watershed and confirmed the location of three study area catchments/agricultural fields. Due to COVID shutdowns, the expected arrival date of the ARS LiDAR capable drone was delayed and flights were not conducted in the fall as originally anticipated. A pre-thaw flight is planned for February 2021 and another before planting in April 2021.
During this reporting period, a Python script was developed that will take current Daily Erosion Project (DEP) inputs of rainfall, elevation, soils and ACPF management, along with a point location of interest (monitoring station, gully head), to generate a DEP flowpath that simulates the hillslope flowing to that point. This will enable modeling of that hillslope in WEPP and validating model estimates versus measured data. This work was recently completed – researchers have not been able to validate any STRIPS2 watersheds yet. This was the first step in building a WEPP watershed implementation within DEP. WEPP watershed estimates runoff and erosion across multiple hillslopes and routes this to a ‘channel’ where channel erosion and deposition may be estimated. These WEPP/DEP watershed estimates will be much closer to estimates of delivery to a stream or waterbody.