Soil loss estimation

 Over the last few decades, a cooperative effort between scientists and users to update the USLE has resulted in the development of RUSLE. The modifications incorporated in USLE to result in the RUSLE are mentioned as under (Kenneth et.al. 1991): Computerizing the algorithms assists the calculations. New rainfall-runoff erosivity term (R). Development of a seasonally variable soil erodibility term (K). A new approach for calculating the cover management term (C) with the sub-factors representing considerations of prior land use, crop canopy, surface cover and surface roughness New slope length and steepness (LS) algorithms reflecting rill to inter-rill erosion ratio The capacity to calculate LS products for the slopes of varying shapes New conservation practices value (P) for rangelands, strip crop rotations, contour factor values and subsurface drainage.

 The equation involves the procedure for assigning the values of different associated factors on the basis of practical concepts. Therefore, there is the possibility to introduce some errors in the selection of the appropriate values, particularly those based on crop concepts. Normally R and K factors are constants for most of the sites/regions in the catchment, whereas, C and LS vary substantially with the erosion-controlled measures, used. The following are some of the limitations of the USLE:  Empirical The USLE is a totally empirical equation. Mathematically, it does not illustrate the actual soil erosion process. The possibility to introduce predictive errors in the calculation is overcome by using empirical coefficients.  Prediction of Average Annual Soil Loss This equation was developed mainly on the basis of average annual soil loss data; hence its applicability is limited for estimation of only the average annual soil loss of the given area. This equation computes less v...

 Wischmeier (1976) reported that the USLE may be used to predict the average- annual soil loss from a field-sized plot with specified land use conditions (Mitchell and Bubenzer 1980). The assumptions associated with the USLE are as follows (Goldman et. al. 1986; Novotny and Chesters 1981; Foster 1976; Onstad and Foster 1975): The USLE is an empirically derived algorithm and does not mathematically represent the actual erosion process. The USLE was developed to estimate long-term, average-annual, or seasonal soil loss. Unusual rainfall seasons, especially higher than normal rainfall, and typically heavy storms may produce more sediment than estimated. The USLE estimates soil loss on upland areas only; it does not estimate sediment deposition. Sediment deposition generally occurs at the bottom of a slope (i.e., change in grade) where the slope becomes milder. The USLE estimates sheet, rill, and inter-rill erosion and does not estimate channel or gully erosion. Gully erosion, caused b...

 There are three important applications of the universal soil loss equation. They are as follows:  It predicts soil loss;  It helps in the identification and selection of agricultural practices; and  It provides recommendations on crop management practices to be used. USLE is an erosion prediction model and its successful application depends on the ability to predict its various factors with a reasonable degree of accuracy. It is based on a considerably large experimental database relating to various factors of USLE. Based on 21 observation points and 64 estimated erosion values of soil loss obtained by the use of USLE at locations spread over different regions of the country, soil erosion rates have been classified into 6 categories. Areas falling under different classes of erosion are shown in Table. Table. Distribution of various erosion classes in India (Source: K Subramanya, 2008)

 LS is the slope length-gradient factor. The topographic factor is used to account for the length and steepness of the slope. The longer the slope, the greater is the volume of surface runoff, and the steeper the slope, the greater is its velocity. LS is 1.0 on a 9% slope and for a 22.1-meter long plot.

 The soil erodibility factor (K) in the USLE relates to the rate at which different soils erode. Under the conditions of equal slope, rainfall, vegetative cover, and soil management practices, some soils may erode more easily than others due to inherent soil characteristics. The direct measurement of K on unit runoff plots reflects the combined effects of all variables that significantly influence the ease with which soil is eroded or the particular slope other than 9% slope. Some of the soil properties which affect the soil loss to a large extent are the soil permeability, infiltration rate, soil texture, size and stability of soil structure, organic content, and soil depth. These are usually determined at special experimental runoff plots or by the use of empirical erodibility equations which relate several soil properties to the factor K. The soil erodibility factor (K) is expressed as tons of soil loss per hectare per unit rainfall erosivity index, from a field of 9% slope and ...