UVM Ext. Agronomy Outreach Professional
As farmers, nutrient management planners and soil conservationists, many of us deal with the estimated loss of soil from fields. We often use a very important tool called the Revised Universal Soil Loss Equation (commonly referred to as RUSLE2). If you have a nutrient management plan, you know about RUSLE2. This tool, however, only estimates soil loss in the form of sheet and/or rill erosion. This is the gradual and sometimes unnoticeable erosion that sheets off fields or that forms small, uniformly spaced and sized channels (less than 4 inches deep). With proper crop rotations, reduced tillage, good cover cropping, good organic matter and even proper manure applications, we can manage for this erosion fairly simply and inexpensively.
Gullies, on the other hand, are the “unaccounted for” erosion that can have a major impact on soil loss, soil health, water quality, and crop yields. Gullies are water formations with increased intensity to sheet and rill erosion, and can also exacerbate sheet/rill erosion. While we have all seen photos of giant gullies big enough to consume a tractor, those tend to be rare. However, the gullies in Vermont farm fields are no less impactful on our landscape. According to an older, but interesting analysis from USDA-NRCS in 1997, they estimated that (19 years ago), roughly 6.1 tons/acre of soil loss per year was attributed to gully erosion, making up roughly 58% of the total sediment lost through water erosion annually (the remaining 4.5 tons/acre/year was from sheet and rill erosion).
Types of Gullies
Ephemeral gullies recur in the same area each time they form, can be partially or totally erased or filled in with tillage, and frequently form in well-defined depressions or natural drainage in a field. As described by the USDA –NRCS (1997), “most ephemeral gullies occur on fields with highly erodible soils, little or no crop residue cover or where crop harvest disturbs the soil.” They are associated with water flow in areas where runoff is great, including snow-melt runoff like that experienced in the Northeast.
True or ‘classic’ gullies are “channels too deep for normal tillage operations to erase.” (NRCS, 2015). They may get bigger in subsequent years, but can also stabilize and become more permanent drainage channels. They tend to start as ephemeral gullies that were left untreated. They can also start as a result of tillage, for example adjacent to a dead furrow. Or they may start at the edges of established grassed waterways or buffers that were inadequately sized or not maintained.
In this pictured example, a gully started upland as an ephemeral gully, but when it reached a dead furrow, this larger scale channel formed. You can see how quickly a gully like this can be an even more significant contributor of soil loss than typical sheet and rill erosion. Depending on how the field is managed a gully like this can account for two to four times the sheet and rill erosion from an entire 25-acre field. It’s hard to tell, but in the picture you can see the field had been cover cropped and no-till planted to corn, but it was too late to prevent the ultimate result. This gully has subsequently been repaired and now has a diversion at the upland slope to prevent its reoccurrence.
Management Implications
This type of significant erosion has many costs associated with it: water quality degradation, decreased yields, and the sometimes significant costs to repair (potentially tens of thousands of dollars). The cost of fixing and maintaining an area where a classic gully has formed can be drastically more expensive and time intensive than preventing them from forming. Once a gully begins forming, additional measures will need to be implemented. Continuing to till and level out an ephemeral gully every year only introduces more soil into the drainage area for erosion.
Conservation practices to prevent gullies include grassed waterways, cover crops, crop rotation and no-till. These practices relate to not re-tilling the gully area, maintaining residue on the soil surface, keeping soil covered and preventing erosion from starting in the first place.
Management Strategies:
Grassed Waterways are constructed channels that are planted with fast growing grass species that are mowed regularly to reduce sedimentation. These waterways convey the water to a stable outlet where it will not cause erosion. They not only significantly reduce erosion, but are located in the areas of the field where drainage wants to occur anyway and tend to not be very productive. Once installed, they can be permanent with proper maintenance.
Conservation Crop Rotation is a management practice that simply changes the rotation pattern of the field in question. In dairy forage systems this includes reducing the number of years of corn production, and rotating into a perennial sod.
Cover Crops are close growing crops (grasses, legumes, forbs) planted to provide protection from soil erosion on annually cropped fields in the times between cash crop growth. In addition to other conservation benefits, they provide significant decrease in erosion.
No-Till otherwise known as Residue Management is the limiting or elimination of soil disturbance to maintain plant residues on the soil surface all year. By not tilling, soil is not exposed to erosion and it is more stable and able to infiltrate more water and support equipment operations without disturbance. In conjunction with cover cropping, it may eliminate the need for grassed waterways or other more expensive conservation practices, if the gully erosion has not already become a serious problem.
Other soil conservation practices such as strip cropping and contour plowing on slopes can help prevent gully formation.
An existing classic gully will need repair. This is a big ticket item. It often requires significant machine time, may need stone or pipe, and often includes a water diversion structure to prevent it from forming again. These can cost more than $20,000 per gully to repair.
Gully erosion is the not so hidden, but unaccounted for, source of erosion in our watersheds. It is detrimental to our waterways, our cropland and pastures, and the sustainability of our farms. Take an afternoon and take a look around your fields. Do you see any gullies forming? Do you see where gullies could potentially form? See a gully in need of repair? Visit your local NRCS office and get help, either stopping gullies before they start or fixing existing gully problems.
+ Estimations based on field observations and NRCS erosion calculations based on dimensions, frequency and soil type.
References:
America’s Private Land. A Geography of Hope, United States Department of Agriculture—Natural Resources Conservation Service, Washington, DC (1997), p. 39 (https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs143_012458.pdf)
Gordon, Lee M., et al. Modeling long-term soil losses on agricultural fields due to ephemeral gully erosion, Journal of Soil and Water Conservation, Volume 63, Issue 4, 1 July 2008, Pages 173-181.
Poesen, J., et al., Gully erosion and environmental change: importance and research needs, CATENA, Volume 50, Issues 2–4, 1 January 2003, Pages 91-133.
Valentin, C., J. Poesen, Yong Li, Gully erosion: Impacts, factors and control, CATENA, Volume 63, Issues 2–3, 31 October 2005, Pages 132-153.
USDA-NRCS Wisconsin Field Office Technical Guide, Section 1-General Resource References. Ephemeral and Classic Gully Erosion Worksheet. August 2015 (https://efotg.sc.egov.usda.gov/references/public/WI/Gully_Erosion_Prediction.pdf)
USDA-NRCS Vermont Field Office Technical Guide, Section 4 – Conservation Practices.
Do you have questions about soil conservation practices? Would you like to conduct a trial on your farm? Contact Kirsten [802-388-4969 ext. 347, kirsten.workman@uvm.edu]