Types of Water Erosion
The four types of water erosion are as follows:
- Inter-rill erosion—the movement of soil by rain splash and its transport by this surface flow.
- Rill erosion—erosion by concentrated flow in small rivulets.
- Gully erosion—erosion by runoff scouring large channels (deeper than 1 foot).
- Streambank erosion—erosion by rivers or streams cutting into banks.
The term “sheet erosion” is still frequently used, but omits the concept of rainsplash and conveys the erroneous concept that runoff commonly occurs as a uniform sheet. Since soil-management affects inter-rill and rill erosion, we will focus on these in the following discussion.
The threat of inter-rill and rill erosion is affected by the amount and intensity of rainfall, the erodibility of the soil, the slope length and steepness, cropping and management factors, and erosion control practices. The USDA-NRCS uses book values for erosivity and erodibility and combines this with field observations and farmer information about management practices to estimate the average annual soil loss on a field.
The USDA-NRCS uses the Revised Universal Soil Loss Equation (RUSLE) to calculate soil loss by erosion as a function of five factors:
A = R × K × LS × C × P
- A = annual soil loss (tons/a/yr)
- R = erosivity of rainfall (function of total rainfall and rainfall intensity)
- K = erodibility of the soil (function of soil texture, soil organic matter, and soil structure)
- LS = slope length/steepness
- C = cropping and management factors (e.g., crops grown, canopy cover, residue cover, surface roughness)
- P = erosion control practices (contour tillage and planting, strip-cropping, terracing, subsurface drainage)
The impact of raindrops on the soil surface is the beginning, and most important part, of the erosion process. The extent of erosion caused by rainfall (erosivity) depends on the size and velocity of raindrops and the amount of precipitation. Gentle, drizzly rain is not very erosive, whereas fierce thunderstorms and hurricanes are very erosive. High-intensity storms produce larger drops that fall faster than those of low-intensity storms and therefore have greater potential to destroy aggregates and dislodge particles from the soil matrix. Although the same total amount of rain may fall, a short, high-intensity rainfall event causes much more erosion than a long, low-intensity storm. Total average precipitation does not vary much across Pennsylvania, but the intensity of rainfall does. Thunderstorms and hurricanes with accompanying high-intensity rainfall tend to hit the southeastern part of the Commonwealth more frequently, leading to higher erosion threat in the southern than northern parts of Pennsylvania. Most erosive precipitation events usually occur in the late summer and early fall. Soils that are bare during this period are under extreme risk of soil erosion. Bare soil (especially if planted to wide-spaced crops such as corn) is also extremely vulnerable to erosion before canopy closure in the spring.
Soils differ in their susceptibility to erosion (erodibility) depending on natural and human factors. Erodibility is influenced by many factors, some of which vary during the year and/or vary with soil management:
- Erodibility of a soil increases with a decrease in aggregate stability. Clay and organic matter help improve aggregate stability and reduce erodibility.
- Living or dead roots also increase aggregate stability and decrease erodibility.
- Erodibility decreases with an increase of large sand grains and rock fragments because these large particles are not easily moved with water.
Soil conservation personnel use standard erodibility values published for each soil series in their county based on typical soil texture, structure and organic matter contents.
Since soils are continuously formed from parent material, it is commonly accepted that a low level of erosion will not compromise soil productivity. NRCS personnel use tolerable soil loss levels (T), which vary per soil type, to indicate the maximum rate of soil erosion that can be allowed while still permitting crop productivity to be sustained indefinitely. Levels of T are a function of root development, gully prevention, on-field sediment problems, seeding losses, reduction of soil organic matter, and loss of plant nutrients. The level of T varies from 3 to 5 tons per acre per year for most soils in Pennsylvania. Deep soils with subsoil characteristics favorable for plant growth have greater T levels than soils with shallow root zones or high percentages of shale at the surface. The reason is that the shallow soils and soils with high contents of rock fragments lose their productivity more quickly than deep soils without rocks fragments.
Controlling Water Erosion
The two types of water erosion that can be controlled by soil management practices are inter-rill and rill erosion. Engineering structures such as grassed waterways and streambank reinforcement help control other types of water erosion.
Cropping and management practices used to control erosion include previous management and cropping, the protection of vegetative canopy to the soil surface, and surface cover and roughness. Generally, the following most important crop management practices will help decrease water erosion:
- Maintain crop residue cover above 30 percent until crop canopy closure.
- Alternate summer crops with winter crops and perennial crops
- Use cover crops during periods when the soil would have insufficient residue.
Contour Farming and Strip-Cropping
Additional protection from water erosion is provided by contour farming and contour strip-cropping. Contour farming implies that crops are planted nearly on the contour. The benefit of this practice is greatest on moderate slopes (2 to 6 percent) when crops are planted in tilled soil where ridge height is 2 to 3 inches. However, even in no-till contour farming can reduce erosion if residue cover is marginal and ridge height is 2 inches or more.
Contour strip-cropping involves alternating strips of high-residue cover or perennial crops with strips with low residue cover. The strips should be laid out close to the contour, which is not always possible in rolling landscapes. Strip width is usually between 75 and 120 feet. Soil that erodes from the bare or low-residue strips is deposited in strips with high residue or dense vegetation because runoff velocity is decreased. This practice is most useful if the soil is tilled, or if the soil is left bare during part of the year in no-till. In today’s cropping systems the difference in cover between strips is frequently minimal, which reduces the effectiveness of this practice. If high-residue cover (greater than 30 percent at all times) is maintained in no-till systems, contour farming and contour strip-cropping are usually not necessary.
As slope length and steepness increase, runoff and soil loss also increase. Changing slope steepness can be changed by the construction of level terraces as is common in southeastern Asia. However, changing slope steepness with management practices is relatively uncommon in Pennsylvania. Slope length can be changed by installing terraces and diversions that divert runoff.
Terraces are cross-slope channels that control erosion on cropland and are built so that crops can be grown on them. Storage terraces hold water until it can be absorbed by the soil or released to stable outlet channels or through underground outlets. Storage terraces are usually designed to drain completely in 48 hours to avoid waterlogging within the terrace. Gradient terraces are channels designed almost perpendicular to the natural field slope that collect runoff water and carry it to a stable outlet like a waterway. Diversions are similar to terraces, except that they are permanently vegetated with grass. They are used on steeper slopes where a terrace would be too expensive or difficult to build, maintain, or farm. They can also be used to protect barnyards or farmsteads from runoff.
Other erosion-control practices help maintain water quality but are not immediately relevant to maintain soil productivity on working cropland. The following practices are helpful in reducing sediment and nutrient load in surface waters even though they do not directly improve soil quality:
- Contour buffer strips—permanently vegetated strips located between larger crop strips on sloping land.
- Field borders—bands or strips of permanent vegetation at the edge of a field.
- Filter strips—strips or areas of permanent vegetation used to remove sediment, organic materials, nutrients, pesticides, and other contaminants from runoff.
- Riparian forest buffers—areas of trees and/or shrubs along streams, lakes, ponds, or wetlands.
- Vegetative barriers—narrow permanent strips of stiff-stemmed, tall, dense perennial vegetation established in parallel rows perpendicular to the dominant field slope.
- Grassed waterways—natural or constructed swales where water usually concentrates as it runs off a field.
- Streambank protection: structures such as fences and stable crossings to keep livestock out of the streams as well as streambank stabilization with rocks, grass, trees, shrubs, riprap, or gabions.