Universal Soil Loss Equation: Applications & Limitations | Soil Management

In this article we will discuss about:- 1. Applications of the Universal Soil Loss Equation 2. Limitations of the Universal Soil Loss Equation 3. Conversion.

Applications of the Universal Soil Loss Equation:

There are following three main applications of the universal soil loss equation, given as:

1. It predicts the soil loss.

2. It helps in selection of the agricultural practices; and

3. It provides recommendations on crop management practices to be used for soil conservation.

Prediction of Soil Loss:

For a given condition of the field regarding its soil type, slope length, slope steepness, cropping system and climatic characteristics, the numerical values of each factor associated to the equation can be assessed and multiplied together to predict the soil loss. This gives the amount of soil-loss (t/ha/y) for a particular condition of the field.

Selection of Agricultural Practices:

In USLE the left hand side parameter represents the maximum soil loss which is likely to take place from the field; and right hand side consists of several factors, in which someone cannot be controlled while some can be controlled. The uncontrolled factors are the rainfall erosivity index, slope length and steepness factors. Similarly, the controlled factors are the crop management practices and conservation practices factors.

These two controlled factors (i.e. C and P) can be changed by using different cropping systems and different conservation practices such as ploughing, terracing, bunding, contouring, strip cropping etc. In other words, for reducing the level of soil loss from the watershed, the type of crops, cropping system, tillage operations and possible agronomical measures, i.e. contouring, and strip cropping etc., can be selected.

Limitations of the Universal Soil Loss Equation:

The USLE involves field observations for determining the values of different associated factors. This causes probability to get introduce some errors in selection of appropriate practices. Particularly, care should be taken for choosing the values of those factors, which are based on the crop. The value of factor-R is the function of rainfall intensity; so it is not constant. While the K factor is constant for most of the sites in the catchment, but the factors C and P vary substantially with the erosion controlled measures, used.

Apart from above, there are several other limitations regarding use of USLE, given as under:

1. It is Empirical:

The USLE is totally empirical relationship for computing soil loss. Theoretically, it does not include the actual soil erosion process. There is always likely to introduce predictive errors in the calculation. For computing the value of factor R, many individuals have proposed several methods. Moreover, they are not general and more difficult to fit them to a specified set of data available.

2. It Predicts Average Annual Soil Loss:

The universal soil loss equation is developed for predicting annual soil loss, hence its applicability is limited to estimate only average annual soil loss of the given area. As per various reporting’s this equation computes less value than the measured, especially when rainfall occurs at high rate. Also, it does not estimate the soil loss, storm-wise.

3. It does not Compute Gully Erosion:

The universal soil loss equation (USLE) is used for assessing the sheet and rill erosion, but not used for predicting the gully erosion. The gully erosion, caused by concentrated water flow is not counted by this equation, and as such can result greater volume of soil loss.

4. It does not Compute Sediment Deposition:

This equation estimates only soil loss, but not the soil deposition. The amount of deposition of sediment at bottom of the channel is being less as compared to the total soil loss taking place from the entire watershed. As soon as the runoff from a sloppy area reaches the toe of the slope or into a channel of lesser slope, a large amount of soil particles are deposited. The total amount of eroded soil carried by the runoff gets decrease as the length of flow path increases. The USLE can be used for computing the sediment storage volume required for sediment retention structures.

Also, it can be used as a tool for deciding the conservation measure for predicting the potential sediment storage need, particularly where sediment basins on construction sites serve a small area ranging from 5 to 100 areas or 2 to 40 ha and runoff does not travel farther distance and basin is intended to serve as settling area. In addition to the above, one more demerit of USLE is that, if drainage system on a site is improperly controlled and gully erosion is in extensive form, then this equation greatly underestimates the sediment storage requirement for construction of retention structure.

Apart from above practical limitations, this equation also has some conceptual limitations; for example – there is a considerable interdependency between the associated variables, and some are counted twice also. For instance, rainfall influences the factor R and C, and terracing affects the L and P factors.

The other interested points about USLE are outlined as under:

(i) The significance of slope steepness in the areas of intense rainfall is ignored.

(ii) The computation of rainfall erosion index based on the drop-size distribution of rains, is not there.

(iii) The runoff amount and its velocity significantly affect the soil loss from the land, is not taken into account. In order to overcome this phenomena Foster, Mayer and Onstad (1973) have suggested to replace the factor ‘R’ by an energy term ‘W’, which is the function of rainfall and runoff energy. The ‘W’ can be computed by the following equation –

W = 0.5 R + 15 . Q . q_p^1/3 … (21.20)

Where, R = rainfall erosivity factor

Q = storm runoff (inch)

q_P = storm peak runoff rate (inch/hour)

To use USLE, a care should be taken to estimate the contribution of hill slope erosion for basin sediment yield, because it does not incorporate sediment delivery ratio. This equation cannot be applied for predicting the soil loss from an individual storm, because the equation was derived to estimate the annual soil loss based on annual rainfall amount. The use of this equation should be avoided for the locations, where the values of different factors associated to the equation have not been determined, yet.

Weighing of USLE Factors for Watershed Applications:

The universal soil loss equation was developed for small size fields to determine the soil loss, but when it is used for computing the sediment yield (not the soil loss) from watersheds, then all factors except ‘R’ are required to change in weighted form. Furthermore, if the land use or conservation practices followed in the watershed are different in different areas, then erosion should always be calculated individually for each area. In this case, it is required to modify the factors KLSCP of USLE.

The modifications are explained as under:

1. Factor K:

The soil-erodibility factor for watershed application is determined by weighting the K value of each soil of the watershed, according to the area involved under different soil types. It is weighted as below –

In which, K_i is the soil erodibility factor for an individual soil i; DA_i is the area under individual soil i; DA is the total area of the watershed and n is the total number of soil types in the watershed.

2. Factor LS:

It is determined based on the values of length of overland flow (L) and slope of land segments (S) of the watershed. The slope length is the length of overland flow of the watershed. To determine it, a rectangular watershed with one channel in the center extending to the watershed length, is assumed.

The watershed width is determined as the area divided by the channel length. Since, the channel is located in the centre of watershed, so the length of overland flow will be equal to the half of the watershed width. Thus, the value of overland flow length will be as –

In which, L is the length of overland flow or slope length; DA is the drainage area of watershed and LCH is the total length of channels in the watershed. Williams et. al (1972) mentioned that, although this equation was derived for a simple rectangular watershed with only one channel, however it also gives better approximation of length of overland flow for the complex watershed.

The value of slope gradient (S) is determined from the topographic map of the watershed. The average slope between each contour is determined by the following equation –

where, S_i is the average land slope for area i between contours j and j + 1; H is the difference in elevation between contours LC_j and LC_j+1 and DA_i is the drainage area between the contours j and j + 1.

Once, the average percent slope of area i is determined, the average watershed slope is computed by weighting the incremental slopes between the contours, according to the area falling between the contours. The following relationship can be used to compute the average slope of watershed –

In which, all the factors are already defined. After determining the weighted values of L and S of the watershed, the factor LS (topographic factor) of USLE is computed by using the following equation –

3. Factor C:

The weighted value of C of watershed is determined by weighting the C values of each crop for different management levels according to the area covered under respective crop and management levels. It is computed by the following equation –

In which, C_i is the crop management factor of crop i; DA_i is the drainage area of crop i under a given manage­ment level.

4. Factor P:

To determine the weighted value of erosion control practices factor for watershed application, only cultivated area of the watershed is taken into consideration. The entire cultivated area can be divided into three categories, namely – (i) straight rows (ii) straight rows with grassed waterways; and (iii) terraces. Considering these categories of cultivated land, the following equation can be used for computing the weighted P value –

In which, SR is the portion of watershed under straight rows; SRWW is the portion of watershed under straight rows and grassed water ways; P_t is the erosion control practice factor for terracing and T is the terraced area of watershed.

Conversion of Universal Soil Loss Equation (USLE) to SI Units:

The Universal Soil Loss Equation (USLE) is used to estimate the soil loss occurring due to water erosion. Originally, the values of factors associated to the equation were in F.P.S. Nowadays, the F.P.S. has been changed; everywhere SI unit is used for scientific explanations. Keeping this in view, the conversion of USLE factors in SI unit from F.P.S. becomes essential. The conversion procedure has been described by Foster et. al. 1981 as under –

The Universal Soil Loss Equation is given by –

A = R K LS CP

In which, all the factors except LS, C and P have dimensions. Table 21.12 presents the dimensions of USLE’s factors.

The basis for conversion of different USLE factors into SI unit is described below:

1. Soil Loss (A):

The term ‘A’ of universal soil loss equation (USLE) represents the annual soil loss per unit area per unit time. Since, the factors LS, C and P are dimensionless, hence unit of A is the multiplication of the units of factor R and K, only. The SI unit of A is the “kg per sq. m or metric tons per hectare”.

The time unit of ‘A’ depends on the time period of ‘R’, which is average annual for a calendar year. In modelling applications, sometimes to express the soil loss as “grams per sq. m. is convenient”. To convert metric tons per hectare into grams per sqm, a multiplying factor ‘100’. is used. Similarly, the multiplying factors for other factors to be used for ‘A’ are cited in Table 21.13.

2. Erosivity R:

This factor is the sum of individual storm erosivity value (EI) of qualifying storm over a time period, usually year or an average crop stage. Storms of less than 13 mm and separated from other rain periods by more than 6 hours are not considered for computation, unless as much as 6 mm of rainfall is there in 15 minutes. The R is the product of storm energy (E) and maximum 30-minutes rainfall intensity (I₃₀), i.e.

Where, e_k is the rainfall energy per unit rainfall and ΔV_k is the k-th increment of the storm hyetograph divided into ‘p’ parts. The ‘e_k‘ (unit energy) is the function of rainfall intensity. The simplified form of energy equations are given as under –

This equation yields the rainfall energy in terms of mega joules per hectare per mm of rainfall (MJ/ha. mm). In Table 21.5, the multiplying factors to convert the storm energy (E), storm erosivity (EI) and annual erosivity (R) from U.S. customary units to SI unit, are given.

3. Soil Erodibility Factor (K):

It expresses the rate of soil loss per unit ‘R’ or ‘EI’ for a specified soil, is measured on a unit plot which has 22.1 m as length and 9% uniform slope, under continuously clean-tilled fallow condition.

This definition reveals that, the ‘K’ has unit of mass per area per erosivity. In SI system, the unit of ‘K’ is metric ton-hectare hour per hectare, mega joule, millimeter (t.ha.h/ha.MJ.mm). The multiplying factor for ‘K’ to get in SI unit from U.S. Customary unit is used as 0.1317 (Table 21.13).

4. Slope Length and Steepness Factor (LS):

The factors ‘L’ and ‘S’ are dimensionless ratios of soil loss, measured from a given slope to that from a unit plot, keeping all other conditions, same. The prediction equation for slope length factor is given as –

In which, λ and λu are the lengths of given slope and of unit plot, respectively, and ‘m’ is an exponent, which depends on the slope steepness. Since, the ratio (λ/λu) is dimensionless, hence its conversion is not applied.

5. Cover-Management (C) and Supporting Practices Factor (P):

The factors C and P are also the ratio of soil loss under given cover, management and supporting practices to that from a field in continuous fallow with periodic tillage to control weeds and break the crusts, keeping all other conditions unchanged. These two factors (C & P) are dimensionless, and require no use of conversion.