Soil erosion

Erosive processes

Water-induced erosion can take various forms depending on climate and topography. The force of rainfall striking a land surface unimpeded by vegetation or man-made structures is sufficient to raise 15 cm (6 inches) of material from an A horizon nearly 1 metre (39 inches) into the air. The impact of raindrops breaks the bonds holding soil aggregates together and catapults the particles into the flowing water from surface runoff. Wholesale removal of soil particles by the sheet flow of water (sheet erosion) or by flow in small channels (rill erosion) accounts for most of the water-induced soil loss from exposed land surfaces. More spectacular but less prevalent types of erosion are gully erosion, in which water concentrates in channels too deep to smooth over by tilling, and streambank erosion, in which the saturated sides of running streams tumble into the moving water below. The same forces at work in streambank erosion are seen in soils on hillslopes that become thoroughly saturated with water. Gravity, able to overcome the cohesive forces that hold soil particles together, can cause the entire soil profile to move downslope—a phenomenon called mass movement. This movement may be either slow (soil creep), rapid (debris flow or mudflow), or sometimes catastrophic (landslide).

The mechanisms involved in wind erosion depend on soil texture and the size of soil particles. Dry soil particles of silt or clay size can be transported over great distance by wind. Larger particles that are the size of fine sand, 0.05 mm (0.002 inch) to 0.5 mm (0.02 inch) in diameter, can be vaulted as high as 25 cm (10 inches) into the air, then drop to the ground after a short flight, only to rebound under the continual driving force of the wind. Coarser sand particles are not lifted, but they can tumble along the land surface. The major cause of wind erosion is the jumping motion of the smaller soil particles, a process called saltation. The texture of the windblown surfaces of these soils becomes coarser, making them less chemically reactive and less able to retain plant nutrients or trap pollutants. In arid regions, wind erosion often produces a gravelly land surface known as desert pavement.

Rates of soil erosion

Soil erosion and deposition are natural geomorphic processes that give shape to landforms and provide new parent material for the development of soil profiles. These processes become soil conservation issues when the rate of erosion greatly exceeds the rate expected in the absence of human land use—a situation referred to as accelerated erosion. Rates of normal soil erosion have been estimated from measurements of sediment transport and accumulation, mass movement on hillslopes, and radioactive carbon dating of landforms. They range from less than 0.02 to more than 10 metric tons per hectare (0.01 to 4.5 tons per acre) of soil lost annually. In comparison the rates of natural soil formation range from 0.2 to 9 metric tons per hectare per year. The average annual rate of normal soil erosion is nearly 1 metric ton per hectare (0.45 ton per acre), while that of natural soil formation is nearly 0.7 metric ton per hectare (0.3 ton per acre). Broad variation is the rule, but rates of soil loss exceeding 10 metric tons per hectare annually signal accelerated erosion. It is important to note that this accelerated soil loss is equivalent to less than 1 mm (0.04 inch) of soil depth, making erosion damage very difficult to observe over short time spans.

When climate and topography are fixed and soil cover is varied, the rate of soil loss by water erosion has a predictable and dramatic dependence on vegetation. Irrespective of location, erosion losses are usually very small from forestland or permanent pastureland, moderate to high from land planted with grain crops, and very high from clean-tilled orchards, vineyards, and land planted with row crops, as shown in the figure.

Resistance to erosion

The ability of soils to resist water and wind erosion depends on their texture and topographic characteristics. Clay-rich soils resist erosion well because of strong cohesive forces between particles and the gluelike characteristics of humus. Both loam and sandy soils are moderately resistant to erosion—the former because they have sufficient clay content to hold the particles together, the latter because their high permeability limits the amount of surface runoff that can wash soil particles away, while their larger particle size makes them too heavy to be easily entrained (transported) in flowing water. Silty soils, on the other hand, exhibit the least resistance to erosion because their permeability is low (resulting in more surface runoff), and their particle size is neither small enough to promote cohesion nor large enough to prevent entrainment. Soils on steep, long slopes are much more susceptible to erosion than those on shallow, short slopes because the steeper slopes accelerate the flow of surface runoff.

The development of soil conservation strategies requires knowledge of actual and acceptable rates of soil erosion. A practical measure of soil resistance to erosion used by pedologists in the United States is the soil loss tolerance (T-value, or T-factor). This quantity is defined as the maximum annual rate of soil loss by erosion that will permit high soil productivity for an indefinite period of time. Operationally, the concept is interpreted as the maximum annual loss from the A horizon that does not reduce the thickness of the rooting zone significantly over millennia.

Guidelines have been developed by the U.S. Natural Resource Conservation Service to assist field estimations of the T-value based on texture, topography, and depth to bedrock or to a root-impeding layer (hardpan) in a soil profile. Deep, coarse-textured soils are assigned a T-value of 11.2 metric tons per hectare (5 tons per acre), fine-textured soils have a T-value of 9 metric tons per hectare (4 tons per acre), and shallow soils or those with an impeding layer are assigned T-values in the range of 2.2–6.7 metric tons per hectare (1–3 tons per acre), depending on texture. Unfavourable slope characteristics are used to modify these values downward as experience may warrant.