1
SOIL LOSS
Erosion Processes
Each year billions of tons of soil are lost in the United States through erosion. Compared with the magnitude of the problem, the basic cause – raindrops may seem insignificant. Yet falling raindrops strike the ground with surprising, cumulative force. With no vegetative cover or mulch to absorb the impact, rain is especially erosive on bare soil. To understand the problem, let us look at some of the mechanisms of erosion (Figure 1).
Rainfall
Soil erosion is the detachment of particles from the soil mass and their transport through the watershed system. When it rains, drops up to 6 millimeters in diameter bombard the soil surface at impact velocities of up to 20 miles per hour. In general, the heavier the rainfall, the larger the drops.
The constantly pounding raindrops dislodge soil particles and aggregates and splash them up to 3 feet away. When rain hits vertically on a flat surface, the splash is equal in all directions. On a slope, more of the splash goes downhill than uphill. In a wind-driven rainfall, splash movement depends on slope and wind direction.
A twofold problem often occurs: a rainfall may be too intense to be absorbed, and it may also seal off the surface, thus further reducing infiltration of the water. If water could always filter into the soil, the splashing of particles would be of minor concern. In many cases, however, water collects in low places, eventually overflows, and then begins to travel downhill, carrying soil particles with it.
A raindrop falling on a thin sheet of water detaches soil particles more readily than one falling on dry soil. Splash erosion increases with surface water depth, but only up to a depth about equal to the raindrop diameter. Once the water becomes deeper, the splash effect is reduced.
Water flowing off the soil surface provides the mechanism for transporting particles loosened by rainfall. Although described as sheet flow, this type of flow seldom occurs in an uninterrupted sheet. Usually the water detours around clods, spills out of small depressions, and in general moves with sluggish irregularity. Even so, the water is able to carry soil particles.
This transport ability is influenced by the energy level of the flow, which in turn is dependent on the depth of flow and slope of the land. Flat areas have little or no runoff; consequently, no transport occurs. Runoff from steeper areas flows at greater velocities and may have considerable transport capability.
Rills and gullies
Under certain conditions, water from sheet flow (inter-rill) areas will run together, forming small rivulets or rills. This type of flow usually occurs only a small percentage of a field, but because the flow is concentrated, it can cause some erosion. The rills thus created leave small channels. Energy levels of water flowing in rills vary somewhat, depending on the depth of flow
Figure 1 Types of Erosion. Note that types are limited to slope positions shown.
and slope of the channel. Long, steep slopes allow rivulets with considerable erosive power to develop.
In many soils, rill flow detaches less material than does splash erosion. However, while a rill is forming, raindrops continue to detach soil within shallow rills and from the surrounding soil surface. Rill flow has an exceptional capacity to transport the detached particles. Because the flow is concentrated, much material can be transported within these small channels. A few soils are very susceptible to rill erosion; thus any rill flow that develops can easily detach soil particles or aggregates.
Rills gradually join together to form progressively larger channels, with the flow eventually proceeding to some established streambed. Some of this flow becomes great enough to create gullies.
Rills and gullies often progress upstream at a head-cut or overfall (small waterfall). As the pool below the overfall enlarges, the turbulent water undercuts the overfall; eventually the soil sloughs off and is transported downstream. Through similar processes, the banks of streambeds can be undercut and eroded if flow velocities are excessive.
Sediment
Sedimentation from soil or other materials carried by moving water may occur with sheet, rill, gully, and stream flow. Natural or artificial dams are a prime place for runoff to collect. Large particles tend to settle in quiet pools formed at these sites. When the water is slowly released, much of the material is left behind as sediment. Ponding is apt to occur in small depressions or above contour controls in inter-rill areas. It may also occur above small debris dams formed from residue in rills and gullies, terrace channels, or reservoirs in large streams. Also, dense vegetation can reduce the flow velocity, thereby allowing soil material to be deposited. Effects of this process are sometimes seen in grassed waterways where the center gradually fills in with sediment.
All three processes of detachment, transport, and sedimentation occur during an erosive rainfall event. The extent is determined by the amount and intensity of rainfall, topography of the land surface, vegetative cover, and character of the soil.
Soils and ground cover
Each type of soil has its own inherent susceptibility to the forces of erosion, in large part because of chemical composition and organic matter content. Although large-grained materials are easily detached by raindrop splash or flowing water, they are not easily transported. On the other hand, fine soils such as clays and fine silts that bond together tightly are not easily detached, but once free they are transported with little difficulty. For this reason, fine materials can be carried considerable distances, whereas larger particles are deposited somewhere along the flow path.
Mulch and vegetative covers play an important role in hindering the erosion process. For example, mulch lying directly on the ground and completely covering the soil surface absorbs the force of a falling raindrop and thus eliminates splash erosion.
Canopy covers will also reduce drop erosion to a great extent. Close growing plants catch raindrops and keep them from hitting the soil directly. Much of the water runs down the plant stem, although some of it runs off the leaves. Falling on bare soil, these drops cause a small amount of detachment, but since they have not fallen far enough to reach an erosive velocity of any significance, detachment is less than with no canopy cover.
Trees provide less protection for bare soil because of the greater height from which the drops fall. However, forests usually contain protective ground cover in the form of leaf or needle mulch.
Not only do ground covers intercept raindrops and keep them from detaching soil particles, but these covers also prevent soil compaction, which restricts infiltration of water into the soil. With greater infiltration there is less runoff. However, some runoff with transport capacity will occur.
Even when no particles are detached by raindrop splash, the flow itself, forming larger and larger rivulets, can eventually loosen particles. By slowing down the velocity of flowing water, vegetation is helpful in reducing flow erosion. In a highly susceptible soil, some rill erosion may occur beneath the mulch cover, but the flow is impeded and the degree of erosion reduced.
Prediction
Many factors, among them rainfall, soil, topography, and vegetative cover, affect the erosion process. At present the Universal Soil Loss Equation (USLE) and the Revised USLE (RUSLE), the advanced computer version of the USLE, are the most widely accepted methods of estimating soil loss from land surfaces. The equation includes the effects of rainfall erosivity, soil erodibility, slope, slope length, ground cover, and erosion control practices. Although empirical, the USLE provides the best estimates available for these complex phenomena. Using the USLE, one can estimate soil loss from a field and recommend alternative cultural practices for bringing excessive erosion to within tolerable limits.
Factors That Influence Erosion
The erosion potential of a site is principally determined by five factors; the erodibility of the soil, vegetative cover, topography, climate and land use. Although the factors are interrelated as determinants of erosion potential, they are discussed separately for ease of understanding.
Soils
Physically, a soil is a porous mixture of inorganic particles of various sizes, decaying organic matter, air, and water. Each soil is a unique mixture of these materials that determines its four fundamental properties: soil texture, soil structure, organic matter content, and soil permeability. The vulnerability of soil to erosion, known as its erodibility, is a result of these four properties that act to either influence infiltration (the movement of water into the ground) or to resist soil detachment and transport by rainfall and runoff. The soil erodibility factor (K) is a measure of the soil’s susceptibility to erosion by water.
Soil texture describes the relative proportion of particles of different sizes in a given soil. This is also often described as the proportions of sand, silt, and clay in the soil. A greater content of large particles means a soil with a coarse texture. It also allows water infiltrate readily, reducing erosion. Soils that contain large proportions of silt and very fine sand are most erodible. Clay, like organic matter, acts to bind particles and limit erodibility. But while clays tend to resist erosion, once eroded they are easily transported by water.
Organic matter originates from decomposing plant material that becomes incorporated into the soil. It is responsible more than any other single factor for the stability of soil aggregates. In addition, the presence of organic matter in the soil increases water-holding capacity, and may increase infiltration rate. Such soils high in organic matter have a more stable structure and resist raindrop detachment.
Soil structure is determined by the shape and arrangement of soil particles. A stable, sharp, granular structure absorbs water readily, resists erosion by surface flow, promotes plant growth. Clay soils or compacted soils have slow infiltration capacities that increase runoff rate and create severe erosion problems.
Soil permeability refers to a soil’s ability to transmit air and water. Soils with high permeability rates reduce runoff by allowing more water to infiltrate. Well-graded gravels and gravel sand mixtures typically have high permeability rates and are less susceptible to erosion from rainfall and surface runoff.
Surface cover
Vegetation is the most effective means of stabilizing soils and controlling erosion. Vegetation controls erosion because it:
· Intercepts rainfall and shields the soil surface from the impact of falling rain;
· Provides a rough surface that reduces the velocity of surface runoff, promotes infiltration, forces deposition of suspended soil particles, and disperses flow;
· Physically hold soil particles in place thus resisting detachment by raindrops or surface runoff;
· Removes water from the soil via transpiration thus increasing the soil’s capacity to absorb water.
The type and condition of ground cover influences the rate and volume of runoff from a given site. By limiting the removal of existing vegetation and limiting the duration of exposed soil, soil losses from erosion can be drastically reduced. Vegetated sites that exhibit a large erosion potential should be disturbed only as a last resort.
Although vegetation offers the best general protection against erosion, nonvegetative ground covers such as mulches, stone aggregates, and paving also protect the underlying soil from erosion. Impervious surfaces, however, do not permit infiltration and, thereby decrease the time of concentration for runoff. The result is large peak flow and increased potential for stream and channel erosion. Runoff must, therefore, be appropriately controlled or it will cause erosion problems off-site.
Topography
Topographic features directly affect erosion potential. Watershed size and shape, for example, affect runoff rates and volumes. As slope length and degree of slope increase, runoff velocity and the potential for sediment transport increase. Slope orientation can also be a factor in determining erosion potential. For example, exposed south-facing soils are hotter and drier, which may make the vegetation less tolerant to disturbance and more difficult to reestablish. Topographical depressions such as swales and channels concentrate surface flow, increase flow velocities, and are susceptible to scouring.
Climate
Climatic factors affecting erosion are precipitation, temperature, wind, humidity, and solar radiation. Temperature and wind affect evaporation and transpiration rates and therefore, indirectly, the water holding capacity of soils. The frequency, intensity, and duration of rainfall and the size of the area on which the precipitation falls are fundamental factors in determining the amount of runoff produced. Frequent and intense storms, especially those of longer duration, pose a serious erosion risk. Seasonal changes in temperature, as well as variations in rainfall, help to define the high erosion risk period of the year.
Land use
Disruptive land use is a primary cause of accelerated erosion and is particularly the case on many Army sites. Disruptive activities accelerate the natural erosion process because they expose soils to precipitation and surface runoff. Training sites are commonly completely denuded and often their soils are so highly disrupted that they exhibit little remaining structure and consequently are unable to resist the erosive forces of raindrops, splash, and runoff. Tremendous soil losses can be expected from untreated disrupted areas. A list of selected land use/disturbance activities that foster erosion follows:
· Removal and loss of topsoil during construction or site preparation expose underlying less previous and more erodible material that critically hinders natural colonization or revegetation of the site.
· Establishment of heavily used training areas on sites with steep slopes, highly erodible soils or sites subject to flooding will inevitably lead to severe erosion problems.
· Construction of roads and tank trails that run up and down slopes rather than conforming to the contours of the land.
· The absence of properly prepared and maintained waterways along roadways and the routine scraping of road shoulders promotes erosion by destroying soil structure and eliminating vegetation.
Sedimentation
Sedimentation is the deposition of soil particles that have been transported by water and wind. The quantity and size of the material transported increases with the velocity of the runoff water or wind. Sedimentation occurs when the water in which the soil particles are carried is sufficiently slowed for a long enough period to allow particles to settle out. Heavier particles, such as gravel and sand, settle out sooner than finer particles, such as clay. The length of time a particle stays in suspension increases as the particle size decreases. The smallest clay particles, known as colloidal clays, stay in suspension for very long periods and contribute significantly to water turbidity.