2.2   LITERATURE REVIEW The effect of soil erosion is a worldwide phenomena particularly because of its adverse effects on man. As a result a lot of literature exists on these issues. A review of the ones relevant for this study will be discussed.

1. 1. 1. EFFECTS OF SOIL EROSION/ CAUSES

Soil erosion has been identified as a serious environment problem with a multiplicity of social and economic consequences. Soil erosion is a disastrous form of environment degradation whose effects is multi- dimensional. Accord to Michael Pidiwirmy (2010), past erosion has an effect on a soil erodibility for a number of reasons, many exposed subsurface soils on eroded site tend to be more erodible than the original soil, because of their poorer structure and lower organic matter. The lower nutrient levels often associated with subsoil contribute to lower crop yield and generally poorer crop cover which in turn provides less crop protection for the soil. He further expantiate by saying that the implications of soil erosion extend beyond the removal of valuable topsoil, crop emergence, growth and yield are directly affected through the loss of natural nutrient and applied fertilizers with the soil. Sediment can be deposited on down slope properties and can contribute to road damage. Michael Pidwirmy (2010) Ofomata (1986) has explicitly stated some of the effects to include loss of properties both home and farm crops, loss in finances and impediment to Urbanization. It should be emphasized that the totality of these negative consequences contributes negatively to production. In addition, damage to vital urban infrastructure such as roads and communication links include adverse effects on sources of water supply for domestic use. According to Aziegbe (2004) one of the major effects of soil erosion in our environment is the development of dip-pits. The causes of soil erosion have been intensively discussed during the past 40 years. Soil erosion is a natural erosion process that is enhanced by human activity Richter (1998) and occurs in all Landscapes and under different land uses. Beside the influence of human activities, soil erosion processes are also caused by morphometric characteristics of the land surface, the erosive force of rainfall and the erodibility of soil and soil surface. Toy et- al (2002) gave detailed definition of soil erosion features and processes such as sheet erosion and inter rill erosion, rill erosion, as well as ephemeral and permanent gully erosion. They also describe the influence of changing land use on stream channel erosion. Hugg (1982) defines sheet flood, sheet- wash and sheet flow in terms of hydrologic and geomorphic based classification system

1. Sheet flood is unconfined floodwater moving downhill
2. Sheet flow is a high- frequency, low magnitude overland flow
3. Sheet wash is superseded by the more meaningful term, rain- wash, which is defined as the washing action of rain on slopes.

The cause and processes of soil erosion are connected as it effects the environment. Intensity of soil erosion is mainly influenced by three factors

1. Erosivity of water
2. Erodibility of soil
3. Human activities.

Physical aspects of erosivity forces of water are independent of the locally prevailing climate conditions.  In reality, different climatic condition reveals deferent erosivity forces of rainfall. Van Dijk et –al (2002) critically review published studies of rainfall intensities and kinetic energy in order to derive a generally predictive exponential equation. Next to erosivity by rainfall drops (splash effects) forces by surface runoff (overland flow) are part of the erosivity. Saturated overland flow and surface runoff occur both in tropical semi-arid and semi-humid landscapes. The erodibility of soil has not been rigorously defined (Bryan et-al 1989) – Bryan (2000) highlights the importance of the inherent resistance of soil to erosion process. Result of his research show that many components of erosion response, such as threshold hydraulic conditions for rill erosion, rill network configuration and hill slope sediment delivery, are strongly affected  by spatially variable and temporally dynamic soil properties (Bryan 2000) Veihe (2002) examines the spatial variable of erodibility of soil types based on a case study in Ghana. The estimation of factor from soil type can in general be problematic because soil classifications are often not based on parameters reflecting erodibility. Erodibility of tropical soil is highly dependent on grain size distribution, clay content and organic carbon content, which influence the stability of soil aggregates. Lebissonais (1996) identifies four main mechanisms by which soil aggravates break down slaking, differential swelling, raindrop impact and physio-chemical dispersion caused by osmotic stress. Barthes and Roose (2002) analyze topsoil aggregate stability and compared these results to susceptibility to erosion. Mbagwu and bazzoffi (1998) investigate the resistance of dry soil aggregate against rain drops. Valnis et- al (2005) correlate inter- rill erosion to aggregate instability, rainfall intensity and slope gradient.