Literature Review On Soil Erosion

Literature review On res publica eating awayThe Latin word erodere, (meaning to gnaw away) is the transmission line of the word wearing away (Roose, 1996). dry shoot down Erosion is the personal remotion of rise up skank by respective(a) agents, including f on the wholeing pelting confuses, piss full stoping everyplace the change profile and gravitational pull (Lal 1990). The basis intelligence Society of the States defines eroding as the wearing away of the degrade fold by running piss, wind, ice or new(prenominal) geo lumberical agents, including such routinees as gravitational creep (SCSA, 1982). Physical eating away involves the detachment and transportation of indissoluble fault particles (sand, silt and thoroughgoing topic). Removal of soluble physical as dissolved substances is called chemical substance corrosion and this whitethornbe variationerd by surface outpouring or subsurface diminish where the water moves from one social class to an other(prenominal) within the dry primer coat profile (Lal 1990). match to ASCE, 1975, the physical processes in discolouration corroding include detachment of priming particles, their transportation and subsequent evidence of discolouration sediments filiation by raindrop impact and runoff over the domain surface. Rainfall is the most master(prenominal) detaching agent (Morgan and Davidson 1986 Lal, 1990) followed by overland scat in entraining turd particles (Lal 1990).The process of filth eating away occurs in trine main steps, detachment of colly particles, transportation and deposition of speck particles dispirited gear by raindrop impact and runoff over the stigma surface (ASCE 1975 Morgan and Davidson, 1986, Lal 1990) followed by overland flow in entraining belowcoat particles (Lal, 1990). Soil eating away abases nastiness productivity by physical acquittance of top contend, reduction in grow depth and release of water. In contrast colly, realm de pletion means privation or decline of blur fertility collect to crop removal or removal of nutrients by eluviations from water passing through the defect profile (Lal, 1990). Sedimentation however, characters off site personal set up like debasement of basins, accumulation of silts in water reservoirs and burial of low-lying productive argonas and other problems (Lal, 1990). Sediments is the main cause of pollution and eutrophication (Lal, 1990). fit in to Lal 1990, imperfection degradation may be caused by accele numberd primer wearing, depletion through intensive land use, deterioration in soil structure, changes in soil pH, take away, salt accumulation, get up up of toxic elelments such as aluminum or zinc, luxuriant inundation leading to reduced soil conditions and poor aeration.Soil Erosion is the most serious and least reversible form of land degradation (Lal, 1977 El-Swaify, Dangler and Amstrong, 1982). Soil corroding and soil way out , correspond to Lal (1 990) throw off unfortunate effects on agriculture because they deplete the soils productivity and diminish the resourse base.2.2 Soil Erosion ProcessGeologic corrosion crowd out be caused by a number of instinctive agents including rainfall, flowing water and ice, wind and the the kitty suit of soil bodies under the action of gravity which cause the unsnarled or dissolved earthy and rock materials to be removed from a place and eventually deposited to a new location (Lal,1990 Morgan and Davidson, 1986). The Soil Science Society of America (SCSA, 1982) described geologic erosion as the form or natural erosion caused by geologic processes acting over long tips and aft(prenominal)mathing in the wearing away of mountains, the building up of flood tide plains, coastal plains. Etc. The slow and constructive natural soil erosion process has been signifi lavtly accele pointd by human activities of poor body politic practices, overgrazing, ground clearing for construction, logg ing and mining (Lo, 1990). Accelerated erosion not only alludes the soil but too the environment and is the primary cause of soil degradation (Lal, 1990). Agriculture has been identified as the primary cause of accelerated soil erosion (Pimentel, 1976).2.3 Soil Characteristics in the TropicsExtremes of humour and wide variety of p arent materials cause great contrast of soil properties in the tropics from soils in other temperate regions. In the tropics soils are steeply variable and diverse like the vegetation (Sanchez and Buoi, 1975 train Wambeke, 1992). The main soil references are alfisols, oxisols, ultisols and inceptisols (El-Swaify, 1990). Tropical soils low in weatherable minerals and introductory cations (sodium, calcium, magnesium, and potassium) evented from continuous weathering of rear materials (Lo, 1990). The ability of these soils to keep plant nutrients is largely dependent on the humus satisfy found in plant bio plurality and the fundamental social fun ction (Rose,1993). The inactivity of soil mineral constituents (kaolin and sesquioxides) in these soils, causes deficiency in crop nutrients, lowers the capacity to retain basic cations, limits active relationship with ingrained social function and excessively immobilizes phosphates and related anions, a condition which are laid-backly toxic to plant grow (Lo, 1990). Crop production in equatorial soils are constrained by primarily aluminum- derived soil acidity and sterileness but chiefly their physical properties are favourable (El-Swaify, 1990). Tropic soils welcome moderate to high permeability under natural conditions, but supersensitised to slaking and development of impermeable sauciness upon action of raindrops and as a result runoff affixs with continuous snug (Lal, 1982). This crusting cause insignifi butt endt reduction of filtration rate, change order of magnitude water runoff which leads to acceleration of soil erosion (Falayl and Lal, 1979).It is weighty to put down however that heavy and intense rains cause austere erosion in the tropics (Morgan, 1974 Wilkinson 1975 Amezquita and Forsythe, 1975 Lal 1976 Aina, Lal and Taylor, 1977 Bois, 1978 Sheng 1982).2.4 Soil Erosion on Steep SlopeAccording to Lal 1990, Steeplands refer to lands with a hawk incline great than 20%. It is of the essence(p) to note however that flat undulating lands score a great authority for crop production and untaught development. Due to the possibility of soil erosion and the problem of mechanization, the steep areas are considered fringy for agriculture production (Lal, 1990).The tight topography in steepland agriculture restricts mechanizations of operations therefore, reducing all agricultural activities (land preparation, cultivation and harvesting), limiting the farmer in home and efficiency. Inputs such as fertiliser and pesticides have to be carried manually by the farmer. As a resulted they are used scarcely. Observably either increase in the use of these agricultural inputs provide result in decline in he farmers pelf from the generally lower agricultural field (Benvenuti, 1988). For all these reasons steepland farmers tend to constrict in high value crop production of limited scale (Ahmad, 1987 Ahmad 1990). It is important to note however that farmers prefer steepslopes ascribable to cultural elapse cultivation, planting and harvesting can be done in an honest fashion (Williams and Walter, 1988). Futher more(prenominal) than subsistence farmers are found on steep slopes because of more favourable environmental conditions such as lower temperatures, reduced diseases and higher(prenominal) reliability of rainfall. (Hurni, 1988).In the tropics, removal of forest vegetation causes excessive leaching and accelerated soil nutrient button. Being highly weathered soil types , their contained minerals generally have poor ability to retain sorbed nutrients against leaching. mud soils with high residualmiron content s are considered superior in resistance to runoff caused soil erosion thus, soils emanated from basic igneous rocks and red soils developed from chalky rocks are strongly aggregated due to the cementing property of iron oxides, hence, soil erosion is expected to be little than for most other soils. to a fault soils developed from fragmentary volcanic materials with andic properties are resistant to soil erosion (Sheng, 1986 Ahmad, 1987 Ahmad, 1990 Lal, 1990). Soils formed from shales, schists, phyillites and sandstones are considered highly erodible. Soils produced from these rocks are high in twain sand or silt fraction, and ashes minerals and iron oxides are generally insufficient as cementing agents for a stable-structured soil. These parent materials are generally bounteous in muscovite occurring in all soil particle- size of it fractions. Micah-rich soils are weak-structured, and thus raindrops can intimately dislodged the weak aggregates, while the clay fraction outspr ead in water. The resulting mica flakes settling on their flat axes in the water film on the soil surface causes soil crusting. The administration of soil crusts further restricts water entry into the soil (Ahmad and Robin, 1971 Sumner, 1995), resulting to presidency of a untold greater volume of runoff water, a condition which leads to further sedition of soil aggregates and transport of colloidal soil material (Ahmad, 1987 Ahmad 1990). Soil crust restricts gaseous exchange leading to anaerobic soil conditions, denitrification, toxic effects due to ethylene production, and mechanical impedance to seedling emergence (Ahmad 1987 Ahmad, 1990).Steep slope cultivation can cause certain in stableness in the ecological system with twain onsite and offsite detrimental impacts (El-Swaify, Garnier and Lo, 1987). Soil, climate, land use and farming systems affect the extent and the degree of severity of soil erosion. However, regard slight of soil and climatical conditions, intensivel y used steeplands in densely populated regions experience severe soil erosion problem.Land use influences the degree of severity of soil erosion on steeplands. Uncontrollable grazing or over grazing, exensive and black cultivation, diversified cropping are responsible for severe soil erosion in unprotected arable lands (Roose, 1988 Liao et al 1988). Ahmad (19871990) reportd soil loss of approximately great hundred t0 180 tonnes per hectare in Tobago Trinidad. In Australia, annual soil loss of cc t/ha to 328 t/ha has ben reported from slope dulcorate cane plantations in central and north Queensland (Sallaway, 1979 Mathews and Makepeace 1981). in that respect are dickens types of soil erosion associated with the Caribbean region, land slipping and gullying. Land slipping is a manifestation of mass movement associated with steepland agriculture and the severity being strongly influenced by the parent materials. Land clearing (example deforestation) and crop production can influenc e land slipping particularly in the early set apart of the wet season when the cleared soil wets faster due to vividness of the soil above rock. Serious dislocations, crop loss and destruction of any mechanical anti erosion devices can result from this form of mass movements. Due to drastic changes in hydrological conditions experienced by land course prone already to slipping and cleared for agriculture for the first quantify land slippage would be of customary experience (Ahmad 1987 Ahmad 1990).Gullying is another common form of soil erosion that occurs on steep land bcause of the terrain involved. This is more common on sandy soils, volcanic soils and vertisols, which are all porous materials. Soils easily attain saturated conditions upon the fast entry of water, consequently breaking the material and ultimately, leading to the formation of gullies. Agricultural activities enables this soil erosion in steeplands by allowing rapid soil wetting upon the start of the wet season . Farming activities though unsuitably oriented field boundaries, foot tracks and the lack of provision for disposal of surface water are slightly main causes of gullying, even on soils not prone to this tpe of steepland soil erosion (Ahmad 1987Ahmad 1990).Since steeplands are traditionally considered marginal for agricultural crop production, most research on soil erosion and soil conservation has been done on either flat land or rolling land with a maximum slope of or so 20%(Lal, 1988).2.5 Factors Affecting Soil ErosionThe causes of soil erosion have been intensively discussed during the past 40 years. Soil erosion is a natural process that is enhanced by human activity (Richter, 1998) and occurs in all landscapes and under different land uses. In addition to human activities, soil erosion processes are similarly caused by morphometric characteristics of the land surface, the erosive forces of rainfall and the erodibility of soils and soil surfaces.When rainwater reaches the s oil surface it will either visualize the soil or run off. runoff occurs when the rainfall meretriciousness exceeds the percolation capacity of the soil. urine erosion is the result of the spreading action of rain drops, the transporting power of water and similarly the vulnerability of the soil to spreading and movement (Baver and Gardner, 1972). The effects of soil erosion is as well classified definition of gullies and description of gully development is given by Morgan (1996), as substantially as Hudson (1995) who additionally focuses on individual cases of the development of gullies. Toy et al (2002) give comminuted definitions of soil erosion features and processes such as sheet erosion and inter- running play erosion, rill erosion, as headspring as ephemeral and permanent gully erosion.Rill erodibility depends both controlly and indirectly on soil properties such as heap density, natural carbon and clay content, clay mineralogy, cations in the exchange complex , soil pH and experimental conditions such as wet content, aging of prewetted soil and quality of eroding water (Rapp,1998). Govers (1990) found that runoff erosion resisitance of a loamy material was extremely sensitive to variation in the initial moisture content and to a lesser extent to changes in stack density.The process of water erosion can be separated into devil components, rill and interrill erosion (Young and Onstad, 1978). Interrill erosion (sheet erosion) is mainly caused by raindrop impact and removes soil in a thin almost imperceptible layer (surrogate, 1989). In interril erosion the flow of water is generally unconfined, except mingled with soil clods and covers much of the soil surface. As the velocity of flow increases the water incises into the soil and rills forms (Evans,1980).Rill erosion begins when the eroding capacity of the flow at some point exceeds the ability of the soil particles to resistant detachment by flow (Meyer cited by Rapp, 1998). Soil is det ached by headcut advance from knickpoints (De Ploey, 1989 Bryan, 1990), rill sailplaning sloughing and hydraulic shear stress (Foster cited by Rapp, 1998) as well as by slumping by undercutting of side walls and scour hole formation (Van Liew and Saxton, 1983). These processes are usually combined into a detachment prediction comparability as a function of medium shear stress (Foster cited by Rapp, 1998). When the rills develop in the landscape, a three to five fold increase in the soil loss commonly occurs (Moss, Green and Hutka 1982 and Meyer and Harmon 1984).2.5.1 Vegetative FactorsThe effects of vegetation can be classified into three catergoriesThe interception of raindrops by the cover (DHuyvetter, 1985). Two effects are associated with this. Firstly, part of the intercepted water will gasify from the leaves and stems and thus reduce runoff. Secondly, when raindrops strike the vegetation, the nil of the drops is dissipated and there is no direct impact on the soil surfac e. The interception percentage depends on the type of crop, the growth stage and the number of plants per unit area.A well distributed, close growing surface vegetative cover will slow down the rate at which water flows down the slope and will as well reduce constriction of water (DHuyvetter, 1985). As a result, it will ebb the erosive action of running water.There is also the effect of roots and biological activity on the formation of stable aggregrates, which results in a stable soil structure and increased percolation that reduces runoff and decreases erosion (DHuyvetter, 1985). Increased permeability also reduces erosion as a result of in increased water percolation due to better drainage. Stables aggregrates in the topsoil also counteract crusting.2.5.2 Rainfall FactorsRaindrop size, shape, duration of a coerce and wind speed interactions controls the erosive power of rainfall (DHuyvetter, 1985). The erosivity of rainfall is explicit in terms of kinetic energy and is affec ted by various factors.According to Wischmeier and Smith (1965), the long suit of rainfall is closely related tally e kinetic energy, according to the regression equationE = 1.213 + 0.890 log IWhereE = the kinetic energy (kg.m/m2.mm)I = rainfall intensity (mm/h)Raindrop size, dissemination and shape all influence the energy momentum of a rainstorm. Laws and Parson (1943) reported an increase in median drop size with increase in rain intensity. The relationship between mean drop size (D50) and rainfall is given byD502.23 I 0.182 (inch per hour).The median size of rain drops increases with low and medium intensity fall, but declines slightly for high intensity rainfall (Gerrard, 1981). The kinetic energy of an rainfall event is also related to the velocity of the raindrops at the time of impact with the soil (DHuyvetter, 1985). The outdistance through which the rain drop must fall to maintain storage velocity is a function of drop size. The kinetic energy of a rainstorm is related to the terminal velocity according to the equationEk = IV2/2Where Ek = energy of the rain stormI = IntensityV= Velocity of raindrop forrader impactEllison (1945) developed an equation showing that the relationship between the soil detached, terminal velocity, drop diameter and rainfall intensityE = KV4.33 d1.07 I0.63Where E = relative full(a) of soil detachedK = soil eternalV = velocity of raindrops (ft/sec)d = diameter of raindrops (mm)I = rainfall intensity2.5.2.1 Effect of rainfall intensity on runoff and soil lossAccording to Morgan (1995), soil loss is closely related to rainfall partly through the detaching power of raindrops striking the soil surface and the contribution of rain to runoff. If rainfall intensity is less than the infiltration capacity of the soil, no surface runoff occurs and the infiltration rate would equal the rainfall intensity (Horton, 1945) as sited by Morgan (1995). If the rainfall intensity exceeds the infiltration capacity, the infiltration rate equals the infiltration capacity and the excess rainfall forms surface runoff.According to Morgan (1995), when the soil is unsaturated, the soil matric potential is negative and water is held in the capillaries due to matrics suction. For this reason, under saturated conditions sands may produce runoff very quickly although their infiltration capacity is not exceeded by the rainfall intensity. Intensity partially controls hydraulic conductivity, increasing the rainfall intensity may cause conductivity to rise so that although runoff may have formed promptly at relatively low rainfall intensity, higher rainfall intensities do not always produce greater runoff (Morgan, 1995). This mechanism explains the reason why infiltration rates sometimes increase with rainfall intensities (Nassif and Wilson, 1975).2.5.3 Soil FactorsAccording to Baver et al, (1972), the effect of soil properties on water erosion can be in two ways Firstly, certain properties destine the rate at which rainfall enters the soil. Secondly, some properties affect the resistance of the soil against dispersion and erosion during rainfall and runoff.The particle size distribution is an important soil property with regards to erodibility. Generally it is found that erodible soils have a low clay content (DHuyvetter, 1985). Soils with more than 35% clay are often regarded as being cohesive and having stable aggregates which are resistant to dispersion by raindrops (Evans, 1980). Evans also stated that sands and coarse loamy sands are not easily eroded by water due to its high infiltration rate. In contrast soils with a high silt or fine sand fraction are very erodible.Erodibility of soil increases with the coincidence of aggregates less than 0.5mm (Bryan, 1974). Factors which contribute to aggregate stability include organic matter content, root secretions, mucilaginous gels formed by break down of organic matter, the binding of particles by sesquioxides and the presence of a high Ca concentratio n on the exchange sites of the colloids instead of a high sodium content (DHuyvetter, 1985).The depth of erosion is obstinate by the soil profile (Evans, 1980). According to Evans soil horizons below the A horizon or plough layer are often more compact and less erodible. The texture and chemical composition of the sub surface horizon can also have an adverse effect. Normally deep gullies can be cut if the parent material is unconsolidated. If resistant bedrock is near the surface only rills will develop. Soil rich in surface stones are less fictile to erosion (Lamb, 1950 and Evans, 1980). Stones protect the soil against erosion and also increase the infiltration of the flowing water into the soil.The antecedent soil moisture and the surface inclemency are both regarded by Evans (1980) as important soil factors touching erosion. The ability of a soil to accept rainfall depends on the moisture content at the time of the rainfall event.2.5.3.1 Factors affecting aggregate stabilitySo il structure is determined by the shape and size distribution of aggregates. Aggregrate size and strengthe determine the physical properties of a soil and its susceptibility to division due to water forces. Their stability will have a crucial effect on soil physical properties (Lynch and Bragg, 1985). The main binding materials crowing stable aggregates in air dry state are the glueing agents in organic matter (Chaney and Swift, 1984 Tisdale and Oades, 1982) and sesquioxides (Goldberg and Glaubic, 1987).2.5.3.1.1 Aluminium and Iron OxidesThe soil used by Kemper and Koch (1966) contained relatively little free iron, although it did contribute to aggregrate stability. Their data show a sharp increase of free iron from 1 to 3%. Goldberg and Glaubic (1987) cerebrate that Al-oxides were more effective than Fe-oxides in stabilizing soil structure. Al-oxides have a greater proportion of sub-micrometer size particles in a sheet form as opposed to the spherical form of Fe-particles.Shain berg, Singer and Janitzky (1987) compared the effect of aluminum and iron oxides on the hydraulic conductivity of a sandy soil.2.5.3.1.2 essential MatterOrganic matter can bind soil particles together into stable soil aggregates. The stabilizing effect of organic matter is well documented. Little detailed information is available on the organic matter content required to sufficiently strengthen aggregates with ESP values greater than 5 or 7, and containing illite or montmorrillionite, so as to prevent their dispersion in water (Smith, 1990). High humus content makes the soil less susceptible to the unfavourable influence of sodium (Van den Berg, De Boer, Van der Malen, Verhoeven, Westerhof and Zuur, 1953). Kemper and Koch (1966) also found that aggregate stability increased with an increase in the organic matter content of soils. A maximum increase of aggregate stability was found with up to 2% organic matter, after which aggregate stability increased very little with further incr eases in organic matter content.2.5.3 Slope FactorsSlope characteristics are important in determining the standard of runoff and erosion ( DHuyvetter, 1985). As slope gradient increases, runoff and erosion usually increases (Stern, 1990). At low slopes due to the low overland flow velocities, detachment of soil particles from the soil surface into the water layer is due to detachment alone (Stern, 1990). Additionally, at low slope gradients, particles are disperse into the air in random directions unlike the case with steeply sloping land where down slope splash occurs (Watson and Laflen, 1985).As slope gradient increases, the ability for surface runoff to entrain and transport sediments increases rapidly until the entrainment by the surface runoff becomes dominant contributing to sediment transport (Stern, 1990). Foster , Meyer and Onstad (1976) presented a conceptual model that showed that at lower slopes, interill transport determined erosion, while at steeper slopes, raindrop detachment determined it. Th invariant bed characteristics of sheet flow transport tend to be replaced by channels because of instability and turbulent flow effects (Moss, Green and Hutka, 1982).There are many empirical relationships relating soil transport by surface wash to slope length and slope gradient. Zingg (1940) showed that erosion varied according to the equationS = X1.6 tanB1.4Where S = soil transport cm/yrX = slope length (m)B = slope gradient (%)Studies conducted by Gerrard (1981), showed that plane and convex slopes did not differ significantly in the amount of soil lost by surface runoff, but concave slopes were less eroded.Some researchers such as Zingg (1940) and Mc Cool et al (1987) indicated that soil erosion increases exponentially with increase in slope gradient. The relationship is indicated after Zing (1940) by E = aSb where E is the soil erosion, S is the slope gradient (%) and a and b are empirical constants. The value of b ranges from 1.35 to 2.0. The othe r relationship between erosion and slope gradient for inter-rill erosion is given by Mc Cool et al (1987)E = a sin b Q+CQ is the slope angle in degreesA,b and C are empirical constants.However, even if the effect of slope gradient on erosion is well recognized, several studies indicate that the power relationship between slope gradient and soil loss over predicts interrill erosion rate by as much as two or more times (Torri, 1996Fox and Bryan, 1999), and the relationship is better described as linear.2.8 Soil Erosion Impacts2.8.1 Soil Physical PropertiesProgressive soil erosion increases the magnitude of soil related constraints for crop production. These constraints can be physical, chemical and biological. The important physical constraints caused by erosion are reduced rooting depth, loss of soil water storing capacity (Schertz et al 1984 Sertsu, 2000), crusting and soil compaction and harden of plinthite (Lal, 1988). Erosion also results in the loss of clay colloids due to inv idious removal of fine particles from the soil surface (Fullen and Brandsma, 1995). The loss of clay influences soil tilth and conformity. Exposed subsoil is often of massive structure and harder consistency than the aggregated surface soil (Lal, 1988).Development of rills and gullies may change the micro-relief that may make use of farming machinery backbreaking. Another effect of erosion is that the manangement and time of farm operations.2.8.2 Soil Chemical PropertiesSoil erosion reduces the fertility experimental condition of soils (Morgan, 1986 Williams et al., 1990). Soil chemical constraints and nutritional problems related to soil erosion include low CEC, low plant nutrients (NPK) and trace elements (Lal, 1988 Fullen and Brandsma, 1995). Massy et al (1953) reported an honest loss of 192 kg of organic matter, 10.6 kg of N and 1.8kg per ha on a Winsconsin soils with 11% slope. Sharpley and Smith (1990) reported that the mean annual loss of total P in runoff from P fertili zed watersheds is equivalent to an average of 15%, 12% and 32% of the annual fertilizer P use to wheat, mixed crop grass and peanut sorghum rotation practices respectively. Researchers (Massy et al 1953 Lal, 1975) have also reported extensive loss of N in eroded sediments.2.8.3 ProductivityQuantifying the effects on crop consequences is a difficult task. It involves the evaluation of interactions between soil properties, crop characteristics and climate. The effects are also cumulative and not observed until long after accelerated erosion begins. The degree of soil erosions effects on crop yield depends on soil profile characteristics and trouble systems. It is difficult to establish a direct relationship between rates of soil erosion and erosion induce soil degradation on the one hand and crop yield on the other (Lal, 1988).It is well know that soil erosion can reduce crop yields through loss of nutrients, structural degradation and reduce of depth and water holding capacity (Ti milin et al, 1986 Lal,1988). Loss of production in eroded soil further degrades its productivity which in turn accelerates soil erosion. The cumulative effect observed over a long period of time may lead to irreversible loss of productivity in shallow soils with hardened plinthite or in soils that respond to expensive management and additional inputs (Lal,1988).2.8.4 Off Site Effects of Soil Erosion.Effects of erosion include siltation of rivers, crop failure at low lying areas due to flooding, pollution of waterbodies due to the various chemicals brought by the runoff from different areas. some(prenominal) studies reported the significance of the off site effects of soil erosion on land degradation (eg. Wall and ven Den,1987 Lo, 1990 Robertson and Colletti, 1994 Petkovic et al, 1999)Rainwater washes away materials that prepare from fertilizers and various biocides (fungicides, insecticides, herbicides and pesticides) which are applied in large concentrations. They reappear in gre atr quantities in the hydrosphere polluting and contaminating the water environment (Zachar,1982Withers, and Lord, 2002 Verstraeten and Poesen, 2002). Chemical pollution of water mainly by organic matter from farm fields causes rapid eutrophication in waterways (Zachar, 1982Zakova et al, 1993 Lijklema, 1995).2.8.5 Soil Erosion ModelsModelling soil erosion is the process of mathematically describing soil particle detachment, transport and deposition on land surfaces (Nearing et al, 1994). Erosion models are used as predictive tools for assessing soil loss and project planning. They can also be used for understanding erosion processes and their impacts (Nearing et al 1994). There are three main types of models, empirical or statistical models, conceptual models and physically ground models (Morgan 1995, Nearing et al 1994, Merritt et al 2003). It is important to note however that there is no sharp difference among them.2.8.5.1 physically Based ModelsThese models are based on solving fundamental physical equations describing stream flow and sediment and associated nutrient generations in a peculiar(prenominal) catchment (Merritt et al ., 2003). They are developed to predict the spatial distribution of runoff and sediment over land surfaces during individual storms in addition to total runoff and soil loss (Morgan, 1995). Physically based models are also called process based models (Morgan, 1995) as they rely on empirical equations to determine erosion processes. These models use a particular differential equation known as the continuity equation which is a statement of conservation of matter as it moves through space over time. The common physically based models used in water quality studies and erosion include The areal Non-Point Source Watershed Environment Response Simulation (ANSWERS) (Beasley et al., 1980), Chemical Runoff and Erosion from Agricultural Management Systems (CREAMS) (Knisel, 1980), Griffith University Erosion System Template (GUEST) (Misra a nd Rose, 1996), European Soil Erosion Model (EUROSEM) (Morgan, 1998), Productivity, Erosion and Runoff, Functions to Evaluate Conservation Techniques (PERFECT) (Littleboy et al., 1992) and Water Erosion Prediction Project (WEPP) (Laflen et al., 1991).2.8.5.2 Empirical M