Water Erosion: Types And Causes

Water erosion is the removal of soil material by water. The process may be natural or augmented by human action. The following factors influence the rate and magnitude of soil erosion caused by water.

 

 

 

•    Greater the intensity and duration of a rainstorm, higher the potential for erosion. The impact of raindrops on the soil surface can disperse aggregate material and break down soil aggregates. 

 

•    Lighter aggregate materials, such as very fine sand, silt, clay, and organic matter, are easily removed by raindrop splash and runoff water; larger sand and gravel particles require more raindrop energy or runoff amounts.

 

•    During short-duration, high-intensity thunderstorms, soil movement (raindrop splash) is usually greatest and most noticeable. Although the amount of soil loss caused by long-lasting and less-intense storms is not usually as spectacular or noticeable as that caused by thunderstorms, it can be significant when compounded over time.

 

•    When there is excess water on a slope that cannot be absorbed into the soil or is trapped on the surface, surface water runoff occurs. Increased runoff is caused by reduced infiltration due to soil compaction, crusting, or freezing. 

 

•    Agricultural land runoff is highest in the spring, when the soils are typically saturated, the snow is melting, and the vegetation cover is minimal.

 

 

•    Soil erodibility is a measurement of a soil's ability to resist erosion based on its physical characteristics. Texture is the most important factor in erodibility, but structure, organic matter, and permeability also play a role. 

 

•    Soils with faster infiltration rates, higher organic matter levels, and improved soil structure are more resistant to erosion in general. Silt, very fine sand, and certain clay-textured soils are less erodible than sand, sandy loam, and loam-textured soils.

 

•    Increases in soil erodibility are caused by tillage and cropping practices that reduce soil organic matter levels, cause poor soil structure, or result in soil compaction. Compacted subsurface soil layers, for example, can reduce infiltration and increase runoff. 

 

•    Infiltration is also reduced by the formation of a soil crust, which tends to “seal” the surface. A soil crust may reduce the amount of soil loss from raindrop impact and splash on some sites, but a corresponding increase in runoff water can contribute to more serious erosion problems.

 

•    The erodibility of a soil is also affected by past erosion. Because of their poorer structure and lower organic matter, many exposed subsurface soils on eroded sites are more erodible than the original soils. Lower nutrient levels in subsoils lead to lower crop yields and, in general, poorer crop cover, resulting in less crop protection for the soil.

 

 

•    The higher the risk of erosion, the steeper and longer the slope of a field. Because of the greater accumulation of runoff, soil erosion by water increases as the slope length increases. 

 

•    Due to increased water velocity, which allows for a greater degree of scouring, combining small fields into larger ones often results in longer slope lengths with increased erosion potential (carrying capacity for sediment).

 

 

•    If the soil has no or very little vegetative cover of plants and/or crop residues, the risk of soil erosion increases. Plant and residue cover shields the soil from raindrop impact and splash, slows runoff water flow, and allows excess surface water to infiltrate.

 

•    The effectiveness of plant and/or crop residues in reducing erosion is determined by the type, extent, and quantity of cover. The most efficient in controlling soil erosion are vegetation and residue combinations that completely cover the soil and intercept all falling raindrops at and close to the surface (e.g., forests, permanent grasses). Residues that have been partially incorporated and residual roots are also important because they provide channels for surface water to enter the soil.

 

•    The effectiveness of any protective cover is also determined by the amount of protection available at different times of the year in relation to the amount of erosive rainfall that falls during these times. 

 

•    Crops that provide a full protective cover for a significant portion of the year (e.g., alfalfa or winter cover crops) can reduce erosion significantly more than crops that leave the soil bare for a longer period of time (e.g., row crops), especially during periods of high erosive rainfall, such as spring and summer. 

 

•    Crop management systems that emphasize contour farming and strip-cropping techniques can help to reduce erosion even more. Leave a residue cover greater than 30% after harvest and over the winter months to reduce most erosion on annual row-crop land, or inter-seed a cover crop (e.g., red clover in wheat, oats after silage corn).

 

 

•    Tillage operations affect the potential for soil erosion by water, depending on the depth, direction, and timing of ploughing, the type of tillage equipment used, and the number of passes. In general, the less vegetation or residue cover at or near the surface is disturbed, the more effective tillage is at reducing water erosion. Water erosion can be reduced by using minimum till or no-till practices.

 

•    Tillage and other practices that are carried out up and down field slopes create pathways for surface water runoff and can hasten soil erosion. Cross-slope cultivation and contour farming techniques reduce surface water runoff concentration and soil movement.

 

 

 

•    The movement of soil caused by raindrop splash and runoff water is known as sheet erosion. It usually happens in a straight line on a flat slope and goes unnoticed until the productive topsoil has been lost. The eroded soil is deposited at the bottom of the slope or in low areas. 

 

•    Other indicators include lighter-colored soils on knolls, changes in soil horizon thickness, and low crop yields on shoulder slopes and knolls.

 

 

•    Surface water runoff concentrates, forming small but well-defined channels, resulting in rill erosion. 

 

•    When the soil has been washed away, these distinct channels are called rills when they are small enough to not interfere with field machinery operations. Rills are filled in every year as part of tillage operations in many cases.

 

 

•    Gully erosion is a more advanced form of rill erosion in which surface channels are eroded to the point where they become a tillage nuisance. Gully erosion is causing large amounts of topsoil and subsoil loss on some farms in central India each year. 

 

•    Surface water runoff, which causes gully formation or enlargement, is usually caused by poor outlet design for local surface and subsurface drainage systems. 

 

•    Sloughing and slumping (caving-in) of bank slopes is caused by the soil instability of gully banks, which is usually associated with groundwater seepage. Such failures are most common in the spring, when the soil water conditions are ideal for the problem.

 

•    If corrective measures are not designed and built properly, gully formations can be difficult to control. Control measures must take into account the source of the increased water flow across the landscape and be capable of directing runoff to a proper outlet. Gully erosion causes significant land to be taken out of production and creates dangerous conditions for farm machinery operators.

 

 

•    Surface water runoff and subsurface drainage systems are discharged through natural streams and constructed drainage channels. The gradual undercutting, scouring, and slumping of these drainage ways is known as bank erosion. Bank erosion can be caused by poor construction practices, lack of maintenance, uncontrolled livestock access, and cropping too close together.

 

•    Bank erosion is also exacerbated by poorly constructed tile outlets. Some are inoperable because they lack a rigid outlet pipe, have an insufficient or non-existent splash pad, or have outlet pipes that have been damaged by erosion, machinery, or bank cave-ins. Loss of productive farmland, undermining of structures such as bridges, increased need to clean and maintain drainage channels, and washing out of lanes, roads, and fence rows are all direct effects of bank erosion.

 

 

 

•    The consequences of water-induced soil erosion go beyond the loss of valuable topsoil. The loss of natural nutrients and applied fertilisers has a direct impact on crop emergence, growth, and yield. Erosion can disturb or completely eliminate seeds and plants. Organic matter from the soil, residues, and any applied manure are relatively light and can be easily transported off the field, especially during the spring thaw. Pesticides could also be carried away from the site by eroded soil.

 

•    The loss of soil can affect the quality, structure, stability, and texture of the soil. The breakdown of aggregates, as well as the removal of smaller particles or entire layers of soil or organic matter, can weaken the structure and alter the texture. Textural changes can affect the soil's water-holding capacity, making it more vulnerable to extreme weather like drought.

 

 

•    The off-site effects of water-induced soil erosion are not always as obvious as the on-site effects. Eroded soil deposited down slope prevents or delays seed emergence, Burys small seedlings, and necessitates replanting in affected areas. Sediment can also accumulate on down-slope properties, causing road damage.

 

•    Sediment in streams and watercourses can hasten bank erosion, clog stream and drainage channels, fill reservoirs, harm fish habitat, and degrade downstream water quality. Pesticides and fertilizers contaminate or pollute downstream water sources, wetlands, and lakes, as they are frequently transported with eroding soil. Controlling “non-point” pollution from agricultural land is an important consideration because of the potential severity of some of the off-site effects.