Soil erosion is a gradual process that occurs when the actions of water, wind, and other factors eat away and wear down the land, causing the soil to deteriorate or disappear completely. Soil deterioration and low quality of water due to erosion and run off has often become a severe problem around the world. Many times the problems become so severe that the land can no longer be cultivated and is abandoned. The key to minimizing soil erosion and saving the farm lands is the farmer himself. Ultimately, he is the one who must reduce the level at which erosion sediments are dislodged from his cropland. This program will discuss the erosion process, its effects on crops and the environment, and the Best Management Practices that can be implemented to limit or contain soil movement from the land.
TYPES OF EROSION
Soil erosion can be divided into two very general categories:
+ Geological erosion: Geological erosion occurs where soil is in its natural environment surrounded by its natural vegetation. This has been taking place naturally for millions of years and has helped create balance in uncultivated soil that enables plant growth. A classical example of the results of geological erosion is the Grand Canyon.
Accelerated erosion is the type that will be covered in most depth. It includes such problems as
+ wind erosion
The action of wind on exposed sediments and friable rock formations causes erosion (abrasion) and entrainment of sediment and soil. Eolian action also forms and shapes sand dunes, yardangs (streamlined bedrock hills) and other landforms. Subsurface deposits and roots are commonly exposed by wind erosion. Wind can also reduce vegetation cover in wadis and depressions, scattering the remains of vegetation in interfluves. Stone pavements may result from the deflation (removal) of fine material from the surface leaving a residue of coarse particles. Blowouts (erosional troughs and depressions) in coastal dune complexes are important indicators of changes in wind erosion. The potential for deflation is generally increased by shoreline erosion or washovers, vegetation die-back due to soil nutrient deficiency or to animal activity, and by human actions such as recreation and construction.
SIGNIFICANCE: Changes in wind-shaped surface morphology and vegetation cover that accompany desertification, drought, and aridification are important gauges of environmental change in arid lands. Wind erosion also affects large areas of croplands in arid and semi-arid regions, removing topsoil, seeds and nutrients.
HUMAN OR NATURAL CAUSE: Eolian erosion is a natural phenomenon, but the surfaces it acts upon may be made susceptible to active wind shaping and transport by human actions, especially those, such as cultivation and over-grazing, that result in the reduction of cover vegetation.
ENVIRONMENT WHERE APPLICABLE: arid and semi-arid lands
TYPES OF MONITORING SITES: Dune fields, coastlines, desert surfaces
SPATIAL SCALE: patch to landscape / mesoscale to regional
METHOD OF MEASUREMENT: Field observations, aided by airphotos and field surveys. Changes in vegetation cover can be monitored using historical records, sequential maps, air photos, satellite images, and by ground survey techniques.
FREQUENCY OF MEASUREMENT: Every 5-20 years
LIMITATIONS OF DATA AND MONITORING: The effect of wind erosion on different rock types and landforms (with contrasted aerodynamic shapes) varies, so that it is not easy to assess the degree of erosion of a complex landscape.
APPLICATIONS TO PAST AND FUTURE: Differential erosion by wind in the past may be detected through study of buried soil horizons developed on ancient erosional surfaces, which formed during dry (wind erosion) to wet (soil formation) climatic cycles.
POSSIBLE THRESHOLDS: Sediment erosion and transport takes place within a specific range of wind speeds, depending on grain size, degree of cementation and compaction, moisture content, and vegetation cover.
Differential erosion by wind in the past may be detected through study of buried soil horizons developed on ancient erosional surfaces, which formed during dry (wind erosion) to wet (soil formation) climatic cycles.
+ Water erosion:
Raindrops can be a major problem for farmers when they strike bare soil. With an impact of up to 30 mph, rain washes out seed and splashes soil into the air. If the fields are on a slope the soil is splashed downhill which causes deterioration of soil structure. Soil that has been detached by raindrops is more easily moved than soil that has not been
detached. Sheet erosion is caused by raindrops. Other types of erosion caused by rainfall include rill erosion and gullies.
caused by rain and poor drainage. Three types of erosion are:
o Sheet erosion: Sheet erosion is defined as the uniform removal of soil in thin layers from sloping land. This, of course, is nearly impossible; in reality the loose soil merely runs off with the rain.
o Rill erosion: Rill erosion is the most common form of erosion. Although its effects can be easily removed by tillage, it is the most often overlooked. It occurs when soil is removed by water from little streamlets that run through land with poor surface draining. Rills can often be found in between crop rows.
o Gully erosion: Gullies are larger than rills and cannot be fixed by tillage. Gully erosion is an advanced stage of rill erosion, just as rills are often the result of sheet erosion.
BRIEF DESCRIPTION: Erosion, the detachment of particles of soil and surficial sediments and rocks, occurs by hydrological (fluvial) processes of sheet erosion, rilling and gully erosion, and through mass wasting and the action of wind. Erosion, both fluvial and eolian (wind) is generally greatest in arid and semi-arid regions, where soil is poorly developed and vegetation provides relatively little protection. Where land use causes soil disturbance, erosion may increase greatly above natural rates. In uplands, the rate of soil and sediment erosion approaches that of denudation (the lowering of the Earth’s surface by erosional processes). In many areas, however, the storage of eroded sediment on hillslopes of lower inclination, in bottomlands, and in lakes and reservoirs, leads to rates of stream sediment transport much lower than the rate of denudation.
When runoff occurs, less water enters the ground, thus reducing crop productivity. Soil erosion also reduces the levels of the basic plant nutrients needed for crops, trees and other plants, and decreases the diversity and abundance of soil organisms. Stream sediment degrades water supplies for municipal and industrial use, and provides an important transporting medium for a wide range of chemical pollutants that are readily sorbed on sediment surfaces. Increased turbidity of coastal waters due to sediment load may adversely affect organisms such as benthic algae, corals and fish.
SIGNIFICANCE: Soil erosion is an important social and economic problem and an essential factor in assessing ecosystem health and function. Estimates of erosion are essential to issues of land and water management, including sediment transport and storage in lowlands, reservoirs, estuaries, and irrigation and hydropower systems. In the USA, soil has recently been eroded at about 17 times the rate at which it forms: about 90% of US cropland is currently losing soil above the sustainable rate. Soil erosion rates in Asia, Africa and South America are estimated to be about twice as high as in the USA. FAO estimates that 140 million ha of high quality soil, mostly in Africa and Asia, will be degraded by 2010, unless better methods of land management are adopted.
HUMAN OR NATURAL CAUSE:Erosion is a fundamental and complex natural process that is strongly modified (generally increased) by human activities such as land clearance, agriculture (ploughing, irrigation, grazing), forestry, construction, surface mining and urbanization. It is estimated that human activities have degraded some 15% (2000 million ha) of the earth’s land surface between latitudes 72. N and 57. S. Slightly over half of this is a result of human-induced water erosion and about a third is due to wind erosion (both leading to loss of topsoil), with most of the balance being the result of chemical and physical deterioration.
ENVIRONMENT WHERE APPLICABLE: Potentially any land surface, but especially where disturbed for any reason, and sloping areas mantled with soil or loose sediment.
TYPES OF MONITORING SITES: Representative sites in uplands and bottomlands.
SPATIAL SCALE: patch (watershed) to landscape / drainage basin to continental
METHOD OF MEASURE MENT:Standard techniques, using erosion pins to detect soil creep or sheet and rill erosion, painted-rock lines and other sediment tracers to determine soil movement, cliff-recession and headcut markers, Young pits, repeated profile and slope measurements, and repeat photography using reference points. Repeat measurements of water and sediment collected in permanently installed hillslope troughs provide seasonal, annual and longer-term estimates of erosion and storage along hillslope profiles. Rates of soil erosion can be estimated using erosion-prediction equations developed during the last four decades. Among these algorithms are the Universal Soil Loss Equation (and its recent update the Revised Universal Soil Loss Equation), the Water Erosion Predict ion Project model, and the European Soil Erosion Model.
FREQUENCY OF MEASUREMENT: Seasonally, annually to once per decade, depending on local conditions and parameter measured.
APPLICATIONS TO PAST AND FUTURE: Knowledge of past soil erosion rates under undisturbed conditions provides a basis for understanding downstream and downslope landforms and processes. Where surface disturbance has occurred, information about present and possible future erosion rates furnishes a basis for reducing the adverse effects of accelerated soil erosion. In particular, measurements of erosion resulting from agricultural disturbance provide the means for developing technology to minimize loss of topsoil and maximize crop productivity over extended periods.
POSSIBLE THRESHOLDS: Gully erosion may become pronounced following cyclic periods of local to regional deposition, during which a critical threshold slope for drainageways is developed. When these threshold slopes are exceeded, the bottomlands adjacent to channels or drainageways may become unstable and subject to erosion. The slope angle above which instability occurs depends on local conditions of water and sediment distribution and on particle sizes of the sediment subject to transport. One result is a natural alternation of gully filling and evacuation of sediment, especially in arid areas over decadal periods. Another result may be intense rill and gully erosion where land use has reduced or destroyed soil cover (vegetation, litter, rock fragments) or has increased runoff and its erosive effects.
Commission on Applied Geomorphology, 1967. Field methods for the study of slope and fluvial processes. Revue de Geomorphologie dynamique : 152-58.
Foster, G.R., & L.J. Lane, 1987. User requirements – USDA Water Erosion Prediction Project (WEPP) . NSERL Report 1, U.S. Department of Agriculture, Agricultural Research Service, West Lafayette, IN: National Soil Erosion Research Laboratory.
Osterkamp, W.R. & S.A.Schumm 1996. Geoindicators for river and river-valley monitoring. In Berger, A.R. & W.J.Iams (eds). Geoindicators: Assessing rapid environmental changes in earth systems :83-100. Rotterdam: A.A. Balkema (see also paper by Lancaster).
Renard, K.G., G.R. Foster, G.A. Weesies, D.K. McCool & D.C. Yoder 1995. Predicting soil erosion by water: a guide to conservation pla nning with the revised universal soil loss equation (RUSLE) . Agricultural Handbook 703, Washington DC: U.S. Department of Agriculture.
Wolman, W.G. & H.C. Riggs 1990. Surface water hydrology. The Geology of North America vol. 0-1, Boulder, Colorado: Geological Society of America. (especially paper by Meade, R.H., T.R. Yuzyk & T.J. Day, Movement and storage of sediment in rivers of the United States and Canada, p255-280).
RELATED ENVIRONMENTAL AND GEOLOGICAL ISSUES: Land degradation. Deposition of eroded soil particles with sorbed contaminants can endanger entire ecosystems along continental margins, in estuaries, wetlands and bottomlands, and on other areas of low slope angle. Soil erosion both affects and is affected by vegetation and crop