Representing soil health practice effects on soil properties and nutrient loss in a watershed-scale hydrologic model

Authors: EVENSON, GREY - The Ohio State University; Osterholz, William - Will; SHEDEKAR, VINAYAK - The Ohio State University; King, Kevin; MEHAN, SUSHANT - The Ohio State University; KALCIC, MARGARET - The Ohio State University

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2022
Publication Date: 2/19/2022
Citation: Evenson, G.R., Osterholz, W.R., Shedekar, V.S., King, K.W., Mehan, S., Kalcic, M.M. 2022. Representing soil health practice effects on soil properties and nutrient loss in a watershed-scale hydrologic model. Journal of Environmental Quality. https://doi.org/10.1002/jeq2.20338.
DOI: https://doi.org/10.1002/jeq2.20338

Interpretive Summary: Models that simulate watershed-scale processes are important tools for understanding how agricultural management practices influence water quality. One such model, the Soil Water Assessment Tool (SWAT), has proven useful in understanding management effects on water quality in the Lake Erie watershed. However, a remaining weakness of the model is that it does not fully represent how management practices like cover crops and no-till will affect soil health - and how changes in soil health may influence water quality. In this study we implemented improvements to SWAT model to represent changes in soil health resulting from no-till and cover crops. The modifications to soil properties such as organic matter content and water holding capacity resulted in mixed effects on water quality: the predicted amount of nitrogen and total phosphorus entering Lake Erie were reduced, but dissolved reactive phosphorus increased. However it was also noted that the direct effects of cover crops and no till on water quality were much more significant than the modifications made to soil properties. This research demonstrates an improved representation of no-till and cover crop practices in the SWAT model that could be used to improve predictions of how these practices influence water quality in Lake Erie as well as other regions. Additionally, the results highlight the importance of gathering additional data on soil health and water quality to verify the trends observed in the model.

Technical Abstract: Watershed-scale hydrologic models are commonly used to assess the water quality effects of agricultural conservation practices that improve soil health (e.g., cover crops and no-till). However, models rarely account for how these practices (i.e., ‘soil health practices’) impact soil physical and functional properties such as water holding capacity and soil aggregate stability, which are likely to affect water quality. We introduce a method to represent changes in soil physical and functional properties caused by soil health practices in the Soil and Water Assessment Tool (SWAT) model. We used the SWAT model’s default representation of winter cover crops and no-till and modified soil descriptive parameters to depict soil health practice impacts on soil properties. We assumed that the soil health practices would increase soil organic carbon (SOC), a principal indicator of soil health, by 0.01 g C g-1 of soil and then estimated changes in other soil properties (e.g., water holding capacity) using SOC-based predictive equations and preceding literature. Results indicated that our soil property modifications had statistically significant impacts on simulated hydrology and nutrient loss, though outputs were more substantially affected by the model’s default representation of cover crops and no-till. Results also indicated that soil health practices can reduce nitrogen and total phosphorus loss but may increase dissolved reactive phosphorus loss. Our representation of soil health practices provides a more complete estimate of practice efficacy but underscores a need for additional observational data to verify results and guide further model improvements.