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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
Naturally, high gradient of slope, the greater the amount of soil loss from erosion by water will be. Soil erosion by water also increases as the slope length increases due to
the greater accumulation of runoff. Slope gradient and length can be determined from field estimation using clinometers, compas and measuring tape, topographic maps, and
DTM Shestha, 2002; Zerabruk, 2003.
D. Vegetationcrop cover
Soil erosion potential is increased if the soil has no or very little vegetative cover of plants andor crop residues. Plant and residue cover protects the soil from raindrop impact
and splash, tends to slow down the movement of a surface runoff and allows excess surface water to infiltrate. The erosion-reducing effectiveness of a plant andor residue
cover depends on the type, extent and quantity of cover.
E. Conservation measures
Certain conservation measures can reduce water erosion. Tillage and cropping practices, as well as land management practices, directly affect the overall soil erosion
problem and solution on a farm.
II.2. P
OTENTIAL
S
OIL
L
OSS
E
STIMATION
B
ASED ON THE
U
NIVERSAL
S
OIL
L
OSS
E
QUATION
USLE
The USLE is a simple multiplicative model to calculate potential soil loss. The USLE can be written as:
A = R K LS C P with:
A – computed spatial average soil loss per unit of area, expressed in thaa. R – rainfall-runoff erosivity factor – the rainfall erosion index plus a factor for
any significant runoff from snowmelt.
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K – soil erodibility factor – the soil-loss rate per erosion index unit for a specified soil as measured on a standard plot, which is defined as a 22.1 m
length of uniform 9 slope in continuous clean-tilled fallow. L – slope length factor – the ratio of soil loss from the field slope length to soil
loss from a 22.1 m length under identical conditions. S – slope steepness factor – the ratio of soil loss from the field slope gradient
to soil loss from a 9 slope under otherwise identical conditions. C – cover-management factor – the ratio of soil loss from an area with
specified cover and management to soil loss from an identical area in tilled continuous fallow.
P – support practice factor – the ratio of soil loss with a support practice like contouring, strip cropping, or terracing, to soil loss with straight-row farming up
and down the slope.
II.3. R
