Understanding soil erosion by water to improve soil conservation
Soil erosion by water is still the number one threat to sustainable crop production on upland soils while flooding caused by excessive runoff from uplands threatens lowland soils. High rainfall in recent years exposed cropping practices that do not provide sufficient protection, leading to visible soil erosion. In this article, we discuss the different types of soil erosion by water, the significance of visible soil erosion, and conservation practices used to treat different types of erosion.
The United States has been the global leader in soil erosion research and application of conservation practices. This is due in no small measure to the visionary leadership of the Father of Soil Conservation, Hugh Hammond Bennett, who galvanized political will to garner resources to address the threat of soil erosion. This caused the U.S. Government to recognize erosion as a national menace in the 1930s and led to the passage of federal laws allowing for the study of soil erosion and provision of assistance to farmers to implement conservation practices. President Franklin D. Roosevelt famously stated, “The nation that destroys its soils destroys itself.” New institutions were formed, starting with the Soil Erosion Service in the Department of the Interior in 1933 that became the Soil Conservation Service in the U.S. Department of Agriculture in 1935 (now called the Natural Resources Conservation Service). Action also ensued at the local level with formation of Soil Conservation Districts to speed up adoption of conservation practices and provide long-lasting support through grassroots community action.
From the beginning, soil conservation was to be based on science, leading to development of the Universal Soil Loss Equation (USLE), published in the early 1960s, the first comprehensive erosion prediction model used by thousands of conservation professionals all over the United States (Wischmeier & Smith, 1978). The USLE estimates average annual sheet and rill erosion (A) as the product of six factors: R— rainfall/runoff erosivity; K—soil erodibility; L—slope length; S—slope steepness; C—cover management practices; and P—supporting practices. For USLE model application, 23 benchmark soils were selected across the nation to determine K. Unit plots with identical slope steepness (9%) and length (72.6 ft) were tilled up-and-down slope to eliminate weeds and keep the soil from crusting (Wischmeier & Smith, 1978; Renard et al., 1997). Additionally, field rainfall simulation experiments on 55 Corn Belt soils were conducted in the 1960s to provide more data to use in development of the soil erodibility nomograph, which allows rapid estimation of K for soils based on their texture, structure, and permeability (Wischmeier & Smith, 1978).
Over the last 40 years, two revisions of the USLE were produced (RUSLE and RUSLE2) to make the model computer based, allow for seasonality in parameterization, and include new data from research. RUSLE and RUSLE2 are scheduled to be replaced in 2020 by the process-based WEPP (Water Erosion Prediction Project) model (Flanagan & Nearing, 1995; Flanagan et al., 2007; Flanagan et al., 2012). With each prediction technology update, a more accurate water erosion prediction estimate can be made using field-specific data. One important improvement of WEPP compared with USLE and RUSLE and RUSLE2 is that WEPP can estimate ephemeral gully erosion in addition to sheet and rill erosion.
Types of Water Erosion
Four types of erosion on upland are distinguished: (1) interrill or sheet erosion, (2) rill erosion, (3) ephemeral gully erosion, and (4) classic gully erosion. These types are recognized because of the different processes that impact them and the conservation practices used to control them (Table 1).
Table 1. Interrill, rill, ephemeral gully, and classic gully erosion and control practices
| Process | Definition | Control practices on non-irrigated cropland (with NRCS conservation practice code) |
|---|---|---|
| Interrill (“sheet”) erosion | The process of soil detachment by the impact of raindrops, transport by shallow sheet flow, and delivery to rill channels. | Conservation cover (327) Crop rotation (328) Cover crop (340) Forage and biomass planting (512) Mulching (484) Multi-story cropping (379) No-till (329) Reduced till (345) |
| Rill erosion | The removal of soil by concentrated water running through little streamlets or headcuts. Rills are less than 4 inches deep, are obliterated by tillage, and typically form in different locations from year to year (Figure 1). | Buffer strips (332) Conservation cover (327) Cover crop (340) Contour farming (330) Critical area planting (342) Diversion (362) Field border (386) Grade stabilization structure (410) Grassed waterway (412) Hedgerow planting (422) Land smoothing (466) Mulching (484) No-till (329) Reduced till (345) Row arrangement (557) Strip cropping (585) Terrace (600) Vegetative barrier (601) |
| Ephemeral gully erosion | Erosion in temporary channels usually obliterated by tillage. Ephemeral gullies are typically 4-18 inches deep and will re-form in the same locations every year due to topographic flow convergence (Figure 2). | Conservation cover (80) Crop rotation (328) Cover crop (340) Diversion (362) Grade stabilization structure (410) Grassed waterway (412) Stripcropping (585) Structure for water control (587) Terrace (600) Underground outlet (620) Water and sediment control basin (638) |
| Classic gully erosion | Erosion in channels that are too deep to cross with farm equipment. They are deeper than 18 inches and permanent unless repaired (Figure 6). | Access control (472) Access road (560) Critical area planting (342) Dike (356) Diversion (362) Fence (382) Grade stabilization structure (410) Grassed waterway (412) Hillside ditch (423) Land smoothing (466) Lined waterway or outlet (468) Pond (378) Rock barrier (555) Structure for water control (587) Subsurface drain (606) Terrace (600) Underground outlet (620) Vertical drain (630) Water and sediment control basin (638) |
The term interrill erosion is now preferred over the term sheet erosion because erosion rarely takes place in sheets. Interrill erosion is the beginning of the erosion process, and it is caused by raindrops striking the soil surface, splashing soil particles into the air and causing shallow overland flow that carries the soil to rills. The raindrops that strike shallow overland flow enhance the flow’s turbulence, increasing its ability to transport detached sediment to a nearby rill. The processes affecting soil splash are the intensity and duration of rainfall events, the amount of exposed soil, and the structural stability of the soil. Practices that are most effective to control interrill erosion are those that increase soil coverage with mulch or living vegetation to avoid raindrops hitting the soil surface and those that help improve soil structural stability and consolidation of the surface, so soil is not as easily dislodged from the matrix.
Rill erosion occurs when runoff concentrates in small rivulets that cut into the soil surface to depths of no more than 4 inches. Rill erosion is no longer due to raindrop impact but to the scouring action of running water. Rills are captured within the standard length of the USLE plot and are also modeled as part of RUSLE(2) and WEPP calculations. Rills are generally of uniform spacing and dimension, run parallel down the slope but may converge, and are easily obliterated by tillage. They usually reappear in a different location from year to year, are generally short, and may end at a concentrated flow channel, terrace, or flattening slope. In long-term no-till, of course, if rills form, they cannot be obliterated by tillage and therefore do not change in location and may grow into gullies if nothing is done to address the reason for their formation. Practices that help reduce rill erosion include those that improve infiltration such as long-term no-till with continuous residue or living cover (Table 2), cover crops, deep-rooted bio-drilling crops, close-cropped crops, planting on the contour (which slows down runoff, so it has more opportunity to infiltrate), heavy mulch cover, including perennial sod crops in the rotation, subsurface drainage, and avoiding soil compaction.
Table 2. Effect of residue cover on interrill and rill erosion reduction compared with conventional clean tillage
| Residue cover, % on any day | Erosion reduction, % while residue is present |
|---|---|
| 10 | 30 |
| 20 | 50 |
| 30 | 65 |
| 40 | 75 |
| 50 | 83 |
| 60 | 88 |
| 70 | 91 |
| 80 | 94 |
Shortening slope length is another important way of reducing rill erosion. This can be done by alternating strips of sod- or close-cropped crops with row crops planted on the contour (strip cropping), terracing (cropped earthen structures that run along the contour at regular intervals), diversions (similar to terraces but planted to perennial sod and not harvested), and contour buffer strips (narrow sod strips along the contour that alternate with crop strips). One important rule of thumb for field personnel is that if rill erosion is seen on hillsides, the erosion rate is generally above 14 tons/ac per year (Wischmeier & Smith, 1978), which is at least two to three times more than what is currently deemed “tolerable.”
Slope steepness and length are considered when planning sheet (interrill) and rill erosion control. On steep hillslopes (with greater than 10% grade), more of the total erosion often results from rill rather than interrill erosion. On slopes flatter than 3% grade, more of the total erosion results from interrill rather than rill processes. Slope length for rills usually does not exceed 400 ft although on rare occasions, it can be up to 1,000 ft such as in the case of water running down rows planted up and down very long slopes (Figure 1).
Long-Term No-Till Stops Interrill and Rill Erosion
No-till is very effective in reducing interrill and rill erosion (Mannering & Burwell, 1968; Hughes et al., 1980). Living cover and dead mulch reduce soil splash and maintain the soil’s infiltration capacity by preventing the impact of raindrops from sealing the soil surface. Increased surface organic matter content in continuous no-till helps improve aggregate stability and surface porosity (Duiker et al., 2015). Increased populations of anecic earthworms (earthworms living in deep vertical burrows) help increase infiltration (Shipitalo et al., 1994). Vegetation (crop residue or living crops) forms numerous barriers, decreasing overland flow velocity and increasing the time of concentration.
Nonetheless, not tilling alone may not be enough. Continuous use of no-till is important, especially if low-residue crops are part of the rotation. No-till soybeans after corn or established hay usually have more than 60% cover, reducing sheet and rill erosion by almost 90%. But if soybean stubble is plowed or even only disturbed with a field cultivator or vertical tillage tool after harvest, soil erosion potential is high in the crop planted after it. Whereas soybean residue is short lasting, corn or small-grain residue can often still be seen in a no-till corn crop planted after soybeans, contributing to lower interrill and rill erosion potential compared with rotational no-till. Field erosion research has found that when winter cover crops are turned under in the spring, their effectiveness at reducing water erosion was less and of shorter duration than the full-year rotation meadows in USLE studies (Wischmeier & Smith, 1978). The effectiveness of cover crops to control interrill and rill erosion is therefore maximized if they are used in continuous no-till systems.
One reason for rills in long-term no-tillage may be soil compaction by heavy farm equipment. There may be surface compaction caused by harvesting equipment or subsoil compaction such as a plow pan from past tillage that is limiting infiltration. A penetrometer (Figure 3) can be used to identify the severity and depth of subsoil compaction. One-time use of a deep-tillage tool to break this compaction layer and subsequent use of hay or deep-rooted cover crops like sweet clover or forage radish in the crop rotation can improve infiltration and reduce soil erosion.
Erosion research has shown the effectiveness of hay crops as part of the crop rotation for soil erosion control. Killing a meadow with herbicide followed by no-till planting of the row crop results in much lower soil erosion than if the meadow is plowed (Lindstrom et al., 1998). Some of the first no-till corn in the USA was planted into a killed pastured hay field, resulting in minimal soil erosion in corn (Meyers, 2018; Triplett et al., 1964). High biological activity in long-term sod and an active soil food web lead to high porosity and aggregate stability, high infiltration, and low erosion (Jemison et al., 2019). It is unfortunate that modern farming systems exclude ruminant livestock from the landscape that could use these grasses for fodder, so that they could be re-integrated in continuous no-till systems. A new interest in integrating grazing livestock in row-crop production offers potential to reconsider this trend (Franzluebbers & Stuedeman, 2008; Sulc & Franzluebbers, 2014).
Another reason for eliminating tillage is the interaction between water and tillage erosion (Li et al., 2008). Clay knobs are often stripped of topsoil by tillage erosion, and this loosened soil is then deposited at the lower end of the field where runoff easily removes it and carries it to a nearby ditch or stream. Yield monitoring and recent soil test soil organic matter (SOM) data reveal much lower SOM levels and crop yield on critical slopes (Figure 5) than the field average (J. Grigar, personal communication). Field measurements of tillage erosion rates on critical slopes often exceed the predicted water erosion estimates for average annual soil loss rates (Gullickson, 2017). Tillage erosion is often greater than expected due to today’s large equipment that pulls tillage tools up and down slope at high speeds, moving high SOM topsoil downslope and resulting in lower residue production on highly degraded slopes. Soil organic matter helps bind soil into water-stable aggregates that resist erosion, improve infiltration, and reduce runoff. True erosion rates are often underestimated on clay knobs by the erosion model predictions where SOM is depleted.
Ephemeral Gully Erosion
Gully erosion is not captured by USLE or RUSLE calculations. Beasley (1972) described gully erosion as: “surface channels eroded to where they cannot be smoothed by normal tillage operations.” However, John Laflen, USDA-ARS Engineer at the National Soil Erosion Research Lab, refined the Beasley definition of gully erosion using the depth of erosion for the Ephemeral Gully Erosion Model (EGEM). This resulted in two definitions for gully erosion: ephemeral gully erosion and classic gully erosion (Laflen & Shaw, 1988). Ephemeral gullies can be partially or entirely filled in by tillage operations, just like rills, but are deeper (between 4 and 18 inches), wider, and longer. Another difference between rills and ephemeral gullies is that the latter usually form at the same place in the landscape in well-defined, shallow depressions in natural drainageways that may end in a classic gully. Ephemeral gullies typically form when several rills join together in a dendritic (tree-like) pattern. The interaction between tillage and ephemeral gully erosion is especially insidious because soil that fills in the ephemeral gully comes from the adjacent area and gradually leads to loss of topsoil in an area as wide as 100 ft or more. The loss of topsoil in ephemeral gullies is usually to tillage depth (Laflen & Shaw, 1988).
Ephemeral gully erosion can also be a serious problem in no-till systems. When they are ignored, they become deeper and deeper until they turn into classic gullies. If ephemeral gullies are repaired merely by filling them in with soil without any change in management, this loose soil can easily be washed away in the future, and soil loss becomes very high. Laflen identified three factors that control ephemeral gully depth: (1) peak discharge, (2) volume of runoff, and (3) tillage depth (Laflen & Shaw, 1988). The increasing working depth of modern tillage equipment and greater tractor horsepower available to producers has caused the need to recognize ephemeral gully erosion as an intermediate between rill and classic gully erosion. Where 40 years ago, it was hard to obliterate ephemeral gullies, current 600-HP tillers pulling 60-ft-wide field cultivators can fill the gullies in. In the last 10 years, the annual till-in of ephemeral gullies by large equipment has become common in the moraine hills of western Michigan and in fact, across the Midwestern United States.
Classic Gully Erosion
Classic gully erosion (Figure 6) results in channels deeper than 18 inches that cannot be filled in with “normal” tillage operations (Laflen & Shaw, 1988). Classic gully erosion generally occurs in well-defined drainageways, tends to form a dendritic pattern, and usually includes head cutting, slumping of unstable banks, and well-defined sidewalls. The soil is transported by flowing water, and soil may be eroded to the depth of profile (instead of tillage depth such as in the case of ephemeral gully erosion). Once gullies form, major earthmoving is required to fill them back in, but most important, the cause for their formation needs to be addressed. Gullies form due to large quantities of high-velocity runoff, so all the upland practices we have talked about before should be addressed first to reduce runoff as much as possible. Then, it may still be necessary to stabilize the area where the gully formed by planting permanent vegetation, such as a grassed waterway. Some type of grade control structure (e.g. drop-box, sediment retention dam, etc.) is also commonly needed to safely convey surface runoff in the waterway to the lower elevation of an off-site stream or ditch bed.
Erosion prediction knowledge can be a powerful tool to assist farmers to restore soil productivity if the parameters that affect soil and tillage erosion are planned to help increase SOM using real field data. Using field observations, yield monitor data, soil penetration resistance readings, and soil testing for the current SOM level on critical eroding slopes combined with the right sequence of conservation practices can begin healing our soils from the “top down.” However, to control ephemeral and classic gully erosion, other conservation practices like water and sediment control basins, grassed waterways, or vegetative barriers (Figure 7) need to be designed and implemented. Hence, the art and science of combining conservation planning and agronomy with engineering practices can act to restore and enhance soil health and productivity.
Soil Conservation Planning for the Dominant Critical Area
Soil conservation professionals design conservation plans based on the “dominant critical area” concept to meet the tolerable soil loss level (T). Fields typically contain flat upland, sloping, depositional, or bottomland areas with different soil types and erodibility. Erosion rates vary in these areas, and the steeper slopes may be more erosive. Because farmers typically manage whole fields to the same cropping sequence and residue management, the conservation planner designs a conservation plan for the dominant critical area (Lightle & Weesies, 1995). The dominant critical area is an area representing at least 10% of the field (or 20% for fields smaller than 25 ac) that requires the most conservation. The thought is that it would not be proper to design a conservation plan for a field based on a flat part of the field although it may be the largest section. It recognizes that planning based on average slope steepness, length, and erodibility would undertreat certain areas of the field that have greater erosion potential. The dominant critical area concept also recognizes that treating an entire field based on the erosion prediction of a critical slope, which may represent the highest erosion threat, could be impractical and represent undue cost to the farmer if this section represented a small part of a field. However, it is recommended to split a small, highly erodible area off from the entire field and put it in permanent cover or treat it separately with more stringent conservation practices.
Planning for ‘T’
Conservation planners develop plans to meet the “tolerable soil loss level” (T). The idea behind T is that some limited level of soil erosion is acceptable if it does not affect the productivity of the soil in question in the long term. In theory, T should not exceed the rate of soil formation from parent material. However, soil formation rates are mostly unknown, and it was considered impossible to bring soil loss rates down to known rates of soil formation with soil management techniques that were available in the 1960s when tolerable soil loss rates were established (Wischmeier & Smith, 1978). Therefore, soils were assigned T values based on the thickness of the topsoil (Wischmeier & Smith, 1978). The lower the T factor, the more vulnerable a field is to productivity loss by erosion. The range for T is 1 to 5 tons/ac per year. At T soil erosion is considered sustainable, so the soil can maintain its current productivity. Nonetheless, the timeline considered seems to be more in terms of decades instead of centuries. That can be shown by considering that a shallow soil from the same parent material has a lower T than its deep cousin. Therefore, it is assumed that a deep soil can lose more soil without a reduction in productivity.
For example, a shallow Opequon silt loam soil in Pennsylvania derived from limestone parent material has a T of 1 ton/ac per year, but a deep Hagerstown silt loam derived from the same parent material and found in the same field has a T of 5 tons/ac per year. However, eventually the deep soil would lose so much soil that it would become a shallow soil. This is already becoming a reality that results in reduction of T for deep soils subject to high soil loss rates (although less than T). For example, Blount soil in Lenawee County, MI on 3-8% slopes has decreased SOM from tilling up and down hill and now has a T value of 3 instead of 5 tons/ac per year because of a severely eroded A horizon. It is clear that soil regeneration from parent material is significantly lower than a T of 5 tons/ac per year, and therefore, long-term sustainability is still compromised if soil loss equals T on our productive soils. In fact, a paper published in 1982 suggested that soil formation was no more than 0.2 tons/ac per year (Logan, 1982). It has been more than 30 years that the concept of T has been questioned (Johnson, 1987), but nonetheless, it is still used as the design criterium for conservation plan development.
All this goes to say that the threat of soil erosion by water is much more severe than normally thought. And farmers and landowners who are serious about conserving their soils for future generations should strive to reduce the annual soil loss to no more than 1 ton/ac per year. We now have the technology to do it, so let’s put it into effect.
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