Soil loss estimation

The USLE was modified by Williams in 1975 to MUSLE by replacing the rainfall energy factor (R) with another factor called as ‘runoff factor’. The MUSLE is expressed as Y = 11.8 (Q x qp)^0.56 K(LS) CP where, Y = sediment yield from an individual storm (in metric tones), Q = storm runoff volume in m3, and qp = the peak rate of runoff in m3/s. All other factors K, (LS), C, and P have the same meaning as in USLE. The values of Q and qp can be obtained by appropriate runoff models. In this model, Q is considered to represent the detachment process and qp is the sediment transport. It is a sediment yield model and does not need a separate estimation of sediment delivery ratio and is applicable to individual storms. Also, it increases sediment yield prediction accuracy. From a modeling point of view, it has the advantage that daily, monthly, and annual sediment yields of a watershed can be modeled by combining appropriate hydrological models with MUSLE.

 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 ...

 It refers to the rainfall erosion index, which expresses the ability of rainfall to erode the soil particles from an unprotected field. It is a function of the physical characteristics of rainfall. Since the measurement of R is complicated, it is best estimated from certain indices called Erossivity indices viz EI30, KE>25 mm/hr, and AIm. EI30: Studies have shown that soil loss by splash, overland flow, and rill erosion is related to a compound index of kinetic energy and the maximum 30 minutes rainfall intensity. The I30 is the greatest average intensity experienced in any 30 minute period during a storm. KE> 25 mm/hr: It is based on the fact that little erosion takes place at low intensities. At low intensity, rain is composed mainly of small drops, falling with low velocity, and hence low energy. The intensity of 25 mm/hr can be taken as a threshold value separating erosive and non-erosive rain. AIm: It is the product of the amount of rain (A) and maximum intensity (Im) o...

USLE has been designed to predict soil loss from agricultural land under various agroclimatic and topographical situations. Since soil erosion is influenced by many different variables, the essence of USLE is to isolate each variable and reduce its effect to a number, so that when these numbers are multiplied together, the answer is the amount of soil loss. The USLE is an erosion prediction model for estimating long-term averages of soil erosion from sheet and rill erosions from a specified land under specified conditions (Wischmeier and Smith, 1978). The equation (USLE) is presented as below. A = R × K × L × S × C × P where, A = soil loss per unit area in unit time, t ha-1 yr -1, R = rainfall erosivity factor which is the number of rainfall erosion index units for a particular location, K = soil erodibility factor - a number which reflects the susceptibility of a soil type to erosion, i.e., it is the reciprocal of soil resistance to erosion, L = slope length factor, a ratio which comp...

The control of erosion is essential to maintain the productivity of soil and to improve or maintain downstream water quality. The reduction of soil erosion to tolerable limits necessitates the adoption of properly planned cropping practices and soil conservation measures. Several methods exist for the measurement of soil loss from different land units. These include the measurements from runoff plots of various sizes for each single land type and land use, small unit source watersheds, and large watersheds of mixed land use. However, to estimate soil erosion, empirical and process-based models (equations) are used. Universal Soil Loss Equation (USLE) is an empirical equation. It estimates the average annual mass of soil loss per unit area as a function of most of the major factors affecting sheet and rill erosions. Estimating soil loss is considerably more difficult than estimating runoff as there are many variables, both natural such as soil and rainfall, and man-made such as adopted ...