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II. LITERATURE REVIEW
II.1.
FACTORS AND CAUSES OF LAND DEGRADATION
Factors of land degradation are the biophysical processes and attributes that determine the kind of degradation processes. These include land quality as affected by its
intrinsic properties of climate, terrainlandscape, vegetation, and biodiversity, especially soil biodiversity. Causes of land degradation are the agents that determine the rate of
degradation. These are biophysical, socioeconomic, and political forces that influence the effectiveness of processes and factors of land degradation Eswaran et al., 2001;
Zerabruk, 2003. Basically soil erosion by water is an interaction of two items, the water and the soil.
However, there are subsidiary factors that can influence the interaction between the two items Zerabruk, 2003. Hence, the rate and magnitude of soil erosion by water is
dominantly controlled by the following factors:
A. Rainfall erosivity and runoff
Both rainfall and runoff factors must be considered in assessing a water erosion problem. Erosivity is the capacity of rainfall to cause erosion. It is a function of rainfall
intensity, rainfall amount, erosive rain, rainfall duration, and kinetic energy of rain Shestha, 2002; Zerabruk, 2003. The soil lost due to rainfall is directly related to the
duration and intensity. As the intensity increases so does the diameter of each raindrop. In addition, as the size of each raindrop increases, so does the amount of energy
transferred to the soil when it hits the soil surface. Thus, the more energy a particular raindrop possesses, the greater erosive capability it has. Surface runoff occurs when
rainfall exceeds a soil’s maximum saturation level and all surface depression storage is filled to capacity. The amount of runoff can be increased if infiltration is reduced due to
soil compaction Zerabruk 2003.
B. Soil erodibility
Erodibility of a soil is its vulnerability or susceptibility to erosion. The factors which affect erodibility of a soil fall into three broad groups namely physical features of soils,
such as aggregate stability, particle size distribution, base minerals, organic carbon content, clay mineralogy, infiltration capaciity, pore size, pore stability and moisture
holding capacity of soil, topographic features and management of the land Shestha, 2002; Zerabruk, 2003. Generally, soils with faster infiltration rates, higher levels of
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organic matter and improved soil structure have a greater resistance to erosion. Sand, sandy loam and loam-textured soils tend to be less erodible than silt, very fine sand and
certain clay textured soils. Tillage and cropping practices, which lower soil organic matter levels, cause poor soil structure, and result compacted soils that contributed to increases
in the soil erodibility. Soil erodibility index can be derived from nomographs or determined using different methodes such as the simple field test.
C. Slope gradient and length
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