Can Cover Crops Reduce Flooding?

By Salam Murtada, Floodplain Hydrologist – DNR Floodplain Program

The Watershed Modeling Group in the DNR Division of Ecological and Water Resources has been evaluating best management practices (BMPs), mainly in agricultural areas, to determine their effectiveness in reducing surface water run-off. The group recently evaluated BMPs that included using cover crops.  Cover crops include a range of vegetation options typically planted in row crop areas between the harvest and planting seasons to cover the normally barren soils. 

Cover crops provide many benefits. They are currently used to improve water quality and soil health. They also reduce surface water runoff and potential soil erosion by increasing the soil’s ability to infiltrate water (hydraulic conductivity). Cover crops also increase organic matter in the soil, which improves the water storage capacity in the root zone. 

Other benefits of cover crops include: reducing nitrogen and phosphorus reaching our streams; providing nutrients for crops; maintaining soil moisture for crops; supporting habitat; and pest management and weed control.

The use of cover crops can also reduce soil loss, which can be especially important for water quality during the pre-growing season when the ground is otherwise fallow and vulnerable to extreme rainfall events. Soil loss is a “lose-lose” situation for the farmers and our natural resources.  Soil loss carries valuable nutrients and applied fertilizers from the field, and can contaminate our streams.

But can cover crops also help control flooding? 

The answer is yes, particularly during non-growing seasons. In a watershed study involving the 323 square mile Shakopee River Watershed, the group simulated using cover crops on all the row crop areas (mainly corn and soybeans), which comprise approximately 68% of the watershed. The study did not convert any current agricultural areas to other land uses (such as prairie). Some of these areas do use drain tiles, however, the effects of drain tiles on peak flows will be addressed in a future article.

The group used a hydrological model called Gridded Surface Subsurface Hydrologic Analysis (GSSHA) model. GSSHA can distribute the flow across the watershed using grids and route it through the river network. The model simulates complex spatial interactions between the surface and subsurface soil layers, including the water table, based on the soil conditions specified in each grid cell. In “long simulations,” where the model was run to simulate the effects of cover crops over several years of historical rainfall events, the group observed peak flow and run-off volume reductions, especially during the non-growing season. The results also showed the effects of hydraulic conductivity (infiltration) and surface roughness (vegetation) were important during shorter periods of rain, and the effects of factors such as evapotranspiration were more important in reducing runoff over longer periods. Hydraulic conductivity affects the rate of infiltration whereas surface roughness affects the routing of surface water across the gridded landscape.

 

But what happens when we apply the 1% annual chance rainfall event?

After modeling the 1% annual chance (aka 100-year storm) synthetic storm event based on the NOAA Atlas 14 rain intensity, the group compared the results using existing conditions versus simulated cover crops. For the growing season, the peak flow reduction was only around 4%. However, for the non-growing season, the peak flow reduction was much more significant, at 30%, which raised the following questions:

• How could using cover crops cause this dramatic peak flow reduction?
• Why was the reduction more significant for the non-growing fallow-ground conditions, when compared with the growing season?
• What process or factors contributed to the reduction?

Knowing that the rate of the 1% annual chance precipitation would still overwhelm the rate of infiltration despite the increase in hydraulic conductivity due to cover crop, we needed to further investigate which of the two main factors caused the 30% reductions, hydraulic conductivity or surface roughness. The long term effects of evapotranspiration did not apply in this case.

What factors contributed to the significant reductions in peak flow?

Based on the Shakopee Watershed study, we can say that cover crops can provide flood control benefits in addition to water quality improvements, especially if applied extensively. Rate of infiltration and surface roughness are important for both water quality improvements and flood control; but the rate of infiltration plays the bigger role for water quality, whereas surface roughness plays the bigger role for flood control.

For more information about cover crops:

• https://extension.umn.edu/soil-and-water/cover-crops
• https://www.sare.org/Learning-Center/Topic-Rooms/Cover-Crops