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PENGAWETAN TANAH DAN AIR(SOIL AND WATER CONSERVATION)MATA KULIAH DIT KLAS FThe term erosion describes the transport of soil constituents by natural forces, primarily water and wind.

Erosion is very damaging to soil fertility because mainly nutrient-rich surface soil is removed, and of that, predominantly the ne and light fractions clay and organic matter leaving behind the more inert sand and gravel. The ne and light fractions remain in suspension longer (in air or water) and so are carried further2Soil formation and soil erosion are two natural and opposing processes. Many natural, undisturbed soils have a rate of formation that is balanced by a rate of erosion. Under these conditions, the soil appears to remain in a constant state as the landscape evolves. Soil ErosionThere are two main types of erosion: geologic and accelerated erosion.

Geologic erosion is a normal process of weathering that generally occurs at low rates in all soils as part of the natural soil-forming processes. It occurs over long geologic time horizons and is not inuenced by human activity. The wearing away of rocks and formation of soil proles are processes affected by the slow but continuous geologic erosion.

In contrast, soil erosion becomes a major concern when the rate of erosion exceeds a certain threshold level and becomes rapid, known as accelerated erosion. This type of erosion is triggered by anthropogenic causes such as deforestation, slash-and-burn agriculture, intensive plowing, intensive and uncontrolled grazing, and biomass burning.

Drivers of Soil ErosionAnthropogenic activities involving deforestation, overgrazing, intensive cultivation, soil mismanagement, cultivation of steep slopes, and urbanization accelerate the soil erosion hazard.

Land use and management, topography, climate, and social, economic, and political conditions inuence soil erosion. The leading three causes of accelerated soil erosion are: deforestation, overgrazing, and mismanagement of cultivated soils.

About 35% of soil erosion is attributed to overgrazing (penggembalaan), 30% to deforestation, and 28% to excessive cultivation (FAO, 1996).

WATER EROSIONSeveral types of water erosion have been identified: raindrop (splash), sheet, rill, and gully or channel. Raindrops falling on bare soil detach particles and splash them up into the air. When there is little water at the soil surface, the water tends to run over the soil surface as a thin sheet (erosi lembar), causing sheet erosion. Rill (erosi alur) and gully erosion (erosi parit) are due to the energy of water that has concentrated and is moving downslope. As water concentrates, it first forms small channels-called rills-followed by greater concentrations of water. Large concentrations of water lead to gully formation. Some of the highest soil erosion rates occur at construction sites where the vegetation has been removed and the soil is exposed to the erosive effects of rain, as shown in Figure 7.1.

The tendency of water to collect into small rills that coalesce to form large channels and produce gully erosion is also shown in Figure 7.1

Predicting Water Erosion Rates on Agricultural Land

The rate of soil erosion on agricultural land is affected by rainfall characteristics, soil erodibility, slope characteristics, and vegetative cover and/or management practices.

These quantative data form the basis for predicting erosion rates by using the Universal Soil Loss Equation (USLE) developed by Wischmeier and Smith

A = R K L S C PA = R K L S C PThe USLE equation is where A is the computed soil loss per unit area as tons per acre, R is the rainfall factor, K is the soil-erodibility factor, L is the slope-length factor, S is the slope-gradient factor, C is the cropping-management factor, and P is the erosion-control practice factor.

The equation is designed to predict water erosion rates on agricultural land surfaces, exclusive of erosion resulting from the formation of large gullies.Control and management of soil erosion are important because when the fertile topsoil is eroded away the remaining soil is less productive with the same level of input. While soil erosion can not be completely curtailed, excessive erosion must be reduced to manageable or tolerable level to minimize adverse effects on productivity.

Magnitude and the impacts of soil erosion on productivity depend on soil prole and horizonation, terrain, soil management, and climate characteristics. Soil Erodibility

The Soil Loss Tolerance ValueThe soil loss tolerance value (T) has been defined in two ways. This is an indication of the lack of concensus among soil scientists as to what approach should be taken with T values or how much erosion should be tolerated. First, the T value is the maximum soil erosion loss that is offset by the theoretical maximum rate of soil development, which will maintain an equilibrium between soil losses and gains.Second, the T value is the maximum average annual soil loss that will allow continous cropping and maintain soil productivity without requiring additional management inputs. Fertilizers are used to overcome the decline in soil fertility caused by erosion.

Water Erosion

On a global scale, water erosion is the most severe type of soil erosion (Fig. 1.1). It occurs in the form of splash/interrill, rill, gully, tunnel, streambank, and coastal erosion.

When precipitation rates exceed the water inltration rates, both raindrop impact and water runoff can cause soil detachment and transport. Water erosion is a dominant form of erosion in humid, and sub-humid, regions characterized by frequent rainstorms. It is also a problem in arid and semiarid regions where the limited precipitation mostly occurs in the form of intense storms when the soil is bare and devoid of vegetal cover.

A gully formed by water erosion

One of the spectacular types of water erosion is the concentrated gully erosion which can cause severe soil erosion even in a single event of high rainfall intensity. Excessive gully erosion can wash out crops, expose plant roots, and lower ground water table while adversely affecting plant growth and landscape stability.

In the USA, soil erosion by gully erosion has been measured at 100Mg ha1 yr1 and represents about 21275% of the interrill and rill erosion (USDA, 1996). In mountainous terrains and structurally fragile soils subjected to intense rains, total erosion from gullies can be as high as that from other types of erosion.Gullying is a major source of sediment and nutrient loss. It causes drastic alterations in landscape aesthetics and removes vast amounts of sediment. Sedimentation at the lower end of the elds in depressional sites can bury crops, damage eld borders, and pollute water bodies. Gullies dissect the eld and exacerbate the non-point source pollution (e.g., sediment, chemicals) to nearby water sources. Gullies undercut and split croplands and alter landform features and watercourses.

Wind Erosion

Wind erosion is a widespread phenomenon, especially in arid and semi-arid regions.It is a dominant geomorphic force that has reshaped the earth. Most of the material carried by wind consists of silt-sized particles. Deposition of this material, termed as loess, has developed into very fertile and deep soils. The thickness of most loess deposits ranges between 20 and 30m, but it can be as thick as 335m (e.g.,Loess Plateau in China). Extensive deposits of loess exist in northeastern China, Midwestern USA, Las Pampas of Argentina, and central Europe. Water Erosion Wind Erosion

Consequences of Soil ErosionAccelerated soil erosion causes adverse agronomic, ecologic, environmental, and economic effects both on-site and off-site. Not only it affects agricultural lands but also quality of forest, pasture, and rangelands. Cropland soils are, however, more susceptible to erosion because these soils are often left bare or with little residue cover between the cropping seasons. Even during the growing season, row crops are susceptible to soil erosion. The on-site consequences involve primarily the reduction in soil productivity, while the off-site consequences are mostly due to the sediment and chemicals transported away from the source into natural waters by streams and depositional sites by wind.On-site Problems

The primary on-site effect of erosion is the reduction of topsoil thickness, which results in soil structural degradation, soil compaction, nutrient depletion, loss of soil organic matter, poor seedling emergence, and reduced crop yieldsRemoval of the nutrient-rich topsoil reduces soil fertility and decreases crop yield. Soil erosion reduces the functional capacity of soils to produce crops, lter pollutants, and store C and nutrients. One may argue that, according to the law of conservation of matter, soil losses by erosion in one place are compensated by the gainsat another place. The problem is that the eroded soil may be deposited in locations where either no crops can be grown or it buries and inundates the crops in valleys.Off-site ProblemsWater and wind erosion preferentially remove the soil layers where most agricultural chemicals (e.g., nutrients, pesticides) are concentrated. Thus, off-site transport of sediment and chemicals causes pollution, sedimentation, and silting of water resources. Sediment transported off-site alters the landscape characteristics, reduces wildlife habitat, and causes economic loss. Erosion also decreases livestock production through reduction in animal weight and forage production, damages water reservoirs and protective shelterbelts, and increases tree mortality. Accumulation of eroded materials in alluvial plains causes ooding of downstream croplands and water reservoirs. Soil erosion also contributes to the projected global climate change. Large amounts of C are rapidly oxidized during erosion, exacerbating the release of CO2 and CH4 to the atmosphere (Lal, 2003).

Strategi Pengawetan Tanah dan Air

Perubahan Penggunaan Lahan, Perubahan iklim, Runoff dan Erosi tanah

Perubahan Iklim dan Erosi Tanah