A Tool for Predicting Soil Erosion Reductions with Prairie Strips
By Elise Miller
When a farmer and/or farmland owner decides to install prairie strips on a field to reduce erosion and improve water quality, a few questions can arise. For instance: How should the prairie strips be designed, and where should they go? How much benefit I expect?
The STRIPS team has found that prairie strip conservation practice has substantial benefits, especially in terms of sediment loss and water quality. But since each farm has different soil types, topography, crop management practices, and climate conditions that have important effects on soil and water movement, the team also is investigating how to help farmers get a more personalized prediction of the benefits they can expect, based on the characteristics of their particular farm.
To see if this kind of prediction was possible, I explored using a model called the Water Erosion Prediction Project, or WEPP. I used data on soil, topography, and weather from experimental catchments with and without prairie strips at Neal Smith National Wildlife Refuge in the WEPP “hillslope” model. I ran 8-yr simulations, and compared the results to actual field data collected from the experimental catchments.
First, I ran the model straight “out of the box” to see if it confirmed the team’s prior findings that prairie strips reduce sediment export. It did! The simulations indicated decreased runoff, sediment export off the field, and displaced soil within the field compared to fields with only row crops. Then I adjusted parameters in the model, based on recommended values from existing literature and data the team had collected from other prairie strips, to try to improve the prediction. After this process, the model produced estimates that were very similar to what was seen on the Neal Smith fields themselves.
The most accurate estimates were for sediment export. While the model slightly overestimated the amount of sediment exported in each simulation compared to measured results from the field, the estimated percent reduction in sediment export on catchments with prairie strips compared to catchments with only row crops was nearly identical for WEPP simulations (97%) and field data (96%). Simulated runoff and measured runoff both varied greatly from site to site. Both the model simulations and field data indicate that prairie strips decrease runoff, although -- because of the high variability within theses data -- this effect was significant only in the simulations and not for field-collected data. Prairie strips also decreased soil displacement or movement within the field itself, according to the model. This is consistent with data collected on commercial farms with prairie strips (M. Liebman, Iowa State University, unpublished data), although the model seems to overestimate the size of this effect. On the whole, these results were encouraging, especially the sediment export predictions!
For more complex-shaped watersheds, more exploration would help determine the best way to estimate soil and water benefits. For some of the fields I studied, the estimates changed somewhat depending on what part of the watershed I used to represent a typical slope in the model. For other fields, this didn’t make a large difference. It is possible that for watersheds with more complex shapes, it would be better to use another version of WEPP -- the “watershed” model -- that can account for more variability and multiple directions of water flow. Another important question is how to improve the model’s accuracy in crop sections. The simulations I ran seemed to overestimate sedimentation on fields that contained only crops and no prairie strips. Other factors, such as soil parameters and crop parameters, may be important to explore.
If WEPP can reasonably represent erosion processes on fields with existing prairie strips, it is a promising tool for planning future implementations of prairie strips! It could potentially give farmers and farmland owners an adequate estimate of what benefits to expect from their designs. If the model is sensitive enough, it could potentially help farmers decide where on the slope to install their prairie strips and inform their design decisions. For instance, the simulations seem to show that the first 10 feet or so of a prairie strip captures most of the sediment, and after that the amount captured tapers off. Knowing how wide a prairie strip needs to be on a hillslope with a certain soil type and steepness could help farmers and farmland owners budget their acres most effectively.
Elise Miller interned in the LESEM Lab during the summers of 2016 and 2017. She wrote her Stanford University Honors Thesis in Earth Systems on quantifying erosion reduction with prairie strips. You can read her thesis here.