WATER EROSION
Types of Water Erosion
The four types of water
erosion are as follows:
1. Inter-rill
erosion: the movement of soil by rain splash and its transport by this
surface flow.
2. Rill
erosion: erosion by concentrated flow in small rivulets.
3. Gully
erosion: erosion by runoff scouring large channels (deeper than 1 foot).
4. 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
Where:
§ 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)
Controlling Water Erosion
Soil Management
Practices
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 Practices
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 perennial grass or close-growing 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 to control
erosion.
Terraces and Diversions
As slope length and
steepness increase, runoff and soil loss also increase. 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
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.
