In this article we will discuss about:- 1. Introduction to Water Erosion Risk 2. Assessment of Water Erosion Risk 3. Factors 4. Methods.
Introduction to Water Erosion Risk:
The assessment of relative risk of erosion to bare soil involves consideration of slope class erodibility. Previous vegetative covers are not taken into consideration for water erosion risk analysis. Also, it is not important whether the land was cleared from its vegetative cover for residential or agricultural purposes.
The important factors affecting water erosion risks are mentioned as under:
i. Slope class
ii. Soil erodibility; and
iii. Soil resistance to detachment.
Erosion gets increase with increase in slope gradient. The shape of slope also affects the rate of soil loss to some extent.
The slopes have been classified into five classes, given as under:
1. Class-1 – Level-very gentle 0-3%
2. Gentle – 3-10%
3. Moderately inclined – 10-20%
4. Moderately steep – 20-30%
5. Steep-very steep – 30%
The soil erodibility is a complex type of land property affecting the erosion potential. There is very large difference between soils regarding their susceptibility to water erosion. These differences may be in terms of resistance to detachment of soil particles, e.g., structural breakdown due to raindrop impact or sheet of water flow and the infiltration characteristics, which allow absorbing the rainwater as it falls on the ground surface; and thus limiting the runoff. In this way, the soil erodibility reflects two subsidiary qualities; they are the soil resistance to detachment and rainfall acceptance.
The resistance of soil to detachment or breakdown largely depends on the texture, structure and dispersability. The soil texture affects the soil resistance, surface structure and aggregate stability, is an important parameter. The other factors which affect the resistance to detachment or breakdown, are the surface gravel and stone cover, soil surface condition and surface structure.
For land capability studies the assessment of water erosion risk to bare soil is very important requirement. The water erosion risk is defined as the intrinsic susceptibility of land to water erosion. The climate, landform and soil factors decide it. The land use and land management factors are not considered for this purpose.
The erosion risk denotes to an intrinsic quality of the land, whereas hazard refers to the combination of risk and land use/management factors. Water erosion is the process, in which the soil particles are detached and transported from one point to another on the land surface by the action of rainfall, runoff, seepage and/or ice. The splash, sheet, rill, gully, stream bank and tunnel erosions are the common forms of water erosion.
The most satisfactory method for assessing the erosion hazard is the soil loss predicting models, derived by taking into account the effects of climate, soil erodibility, slope, slope length, vegetative cover and soil conservation practices. Amongst all the USLE, i.e. Universal Soil Loss Equation is considered to be very useful empirical model for this purpose. The USLE predicts the rate of soil loss on the basis of several factors (R, K, LS, C and P), simply by multiplying them together.
The associated parameters to the rainfall and runoff, soil erodibility, topography (slope length and gradient), vegetative cover/land use and conservation practices followed in the area, are evaluated. In USLE the rainfall and runoff factor are determined by the method of EI-30 index. The soil erodibility factor K of USLE can be determined by using the nomograph. The nomograph computes the K based on the amount of very fine sand and silt (0.002 to 0.1 mm) to the sands (0.10 to 2.0 mm), organic matter, soil structure and permeability.
The slope class and soil erodibility are the parameters, mainly considered for assessment of relative risk of soil erosion to bare soil. Previously existing vegetative covers are not considered; and it is also not being important whether the vegetative cover was removed from the land surface for residential or for agricultural purposes.
The slope classes and soil erodibility parameters are outlined as under:
1. Slope Class:
The slope steepness and its length significantly affect the soil loss. The slope shape also affects the soil loss.
For assessment of soil erosion/loss the slope steepness are mainly grouped in following five classes:
2. Soil Erodibility:
It is a complex type land quality. In general, there is intrinsic differences between the soils regarding their susceptibility to water erosion. The soil erodibility basically represents the soil resistance to detachment. Because of this reason, the soil erodibility is the product of two subsidiary qualities, i.e. the soil resistance to detachment and rainfall acceptance.
The resistance of soil to detachment or breakdown largely depends on soil texture, structure and dispersability. The rainfall acceptance denotes the ability of soil to absorb rainwater as it falls; and to prevent the runoff formation. This factor is important for determining the water erosion.
The water erosion risk can be determined on the basis of slope class and soil erodibility rating. The soil erodibility rating is determined based on the soil resistance to detachment, and runoff producing factors, i.e., the rainfall acceptance, which can be obtained by using the land resource survey data or land characteristics. The ratings of soil resistance to detachment or structural breakdown by water, rainfall acceptance and soil erodibility rating are given in Table 3.18 and 3.19, respectively; and the schematic outline of method is presented in Table 3.20.
The procedure follows following steps:
Step 1 – Determine the rainfall acceptance index using Table 3.21. The values given in Table 3.21 are based on NERC method with slight modification in slope, depth and permeability classes.
Step 2 – Determine the soil resistance rating, using Table 3.18.
Step 3 – Combine the rainfall acceptance index with the soil resistance rating to determine soil erodibility rating, using Table 3.19. Note that, the soil erodibility rating should be modified as per footnote given in the Table 3.19.
Step 4 – Determine the slope class and combine with the soil erodibility rating to determine the water erosion risk, using Table 3.22. Note that, the determined value of water erosion risk needs to be modified as per footnote given in the Table 3.22.
The permeability above ‘impermeable’ layers is explained as under:
The rain acceptance indices (winter rain) shown in the Table 3.19 are explained under: