Soil Loss Estimation: Measurement and Universal Soil Loss Equation Formula

Soil Loss Estimation: Measurement and Universal Soil Loss Equation!

Soil losses occur from areas subjected to different types of erosion. The soil loss varies with the type of erosion and it is also influenced by all the factors that affect erosion. Along with the soil losses, loss of plant nutrients also occurs.

Estimation of soil losses under a given set of conditions is necessary to evaluate the various alternative treatments as related to the amount of expected soil loss. Again estimation of soil losses from large areas and their subsequent movement and deposition in streams, rivers and any reservoirs constructed on these rivers is also needed in multipurpose river valley projects.

Measurement of Soil Losses:

Measurement of soil losses from area under controlled condition is necessary in order to precisely know about the influence of different land management practices on soil loss. Such studies help in obtaining information for developing relationships which can be used for predicting or estimating the soil loss under a different set of conditions.

Runoff Plots:

Runoff plots are isolated areas of known size used to measure the losses of soil and water due to sheet erosion. Both field size plots and relatively small rectangular plots of the size 1/100 to 1/50 of acre (1/250 to 1/125 hectare) are commonly used. The runoff from these plots is measured using a suitable measuring device, commonly a V-notch or a H-flume of suitable capacity or even by volumetric measurements.

The measurement of sediment outflow from the runoff plots is done in one of the two following ways. A multi-slot divisor is used to collect only a part of the runoff into a masonry tank.

Water samples from the tank are taken and the sediment quantity is estimated from these samples. The main disadvantage in this method is that the collecting tanks are to be made comparatively large in order to hold the runoff expected through the multi-slot divisor.

The Coshocton wheel silt sampler (developed at an experimental station at a place known as Coshocton in USA) can be used for collecting water samples from the runoff passing through a multi-slot divisor or directly through a measuring device like the H-flume.

The device consists of a wheel made of thin circular plate to which eight vanes are attached. A sampling head with a narrow opening called slot is mounted radially on this wheel. The slot is kept raised sloping down towards the outside of the wheel.

A cylindrical pan below the water wheel receives the sampled portion of the runoff. An outlet pipe from the pan leads to a collecting tank where the sample runoff gets collected. When the discharge from the measuring flume falls on the wheel its impact rotates the wheel.

With each revolution of the wheel, the slot cuts across the jet from the flume and extracts a small portion of the flow. The extracted portion is collected in the storage tank through the conduit.

An expression for the relation between rate of flow over the sampler and the flow collected by the sampler is derived as follows –

A jet of water with flow Q, width x, thickness y and velocity v (Fig. 20.7) is allowed to fall over a slot with length greater than y and with width w. The discharge Q₁ into the slot is –

The ratio R is called the sampler ratio. It is the ratio of the slot width to circumference at any radius. It may be noted that R is independent of the speed of rotation of the wheel.

Therefore, the Coshocton type runoff sampler can also be used to compute the total runoff from the sampler ratio provided q can be recorded. Because of the difficulty in recording q, these samplers are used only for sediment sampling and Q is measured using some rate measuring flumes.

Since the runoff and the average silt concentration are known, the total soil loss during the particular storm can be computed. The H-Flume and the Coshocton sampler in combination are used for runoff and sediment estimation from small agricultural watersheds.

Universal Soil Loss Equation:

Attempts have been made for years to quantify the erosion effects of rainfall, land factors and crop factors in order to predict erosion under a given set of conditions. Such a formulation will not only be useful in estimating the soil loss that could occur in a given set of conditions but also to choose the conservation practices in reducing the soil loss.

Wischmeier in 1959 presented the universal soil loss equation, which has an adaptability to a wide range of conditions. The factors involsectionved in the equation and its applicability to some situations in India have been discussed by Tejwani et al. (1975).

The universal soil loss equation (USLE) is given by –

The different factors in the above equation are to be selected to suit the units under considerations. Extensive experimental evidence is needed to determine these factors.

An explanatory note about each of the factors involved in the above equation is as follows:

1. Soil Loss (A):

The factor A represents soil loss per unit area per unit time. Because L, S, C, and P are dimensionless, units for A result from the multiplication of R and K in the solution of the USLE.

Units may be chosen for R and K to give units for A in metric tons per hectare. The time unit of A depends upon the time period of R, which is usually average annual for a calendar year.

2. Erosivity (R):

The R factor is the sum of individual storm erosivity values, EI, for qualifying storms over a time period, usually average annual or perhaps an average crop stage. Storms of less than 0.5 inch (13 mm) and separated from other rain periods by more than 6 hours are not included in the computation unless as much as 0.25 inch (6 mm) of rainfall occurs in 15 minutes. The factor E is the total energy for a storm and I is the storm’s maximum 30-minute intensity. Mathematically, R is given as –

Where n is the number of storms in the series. Implicitly, a time dimension is associated with R, although the dimension is seldom shown. The variable EI is the product of the total energy for a storm and the storm’s maximum 30-minute intensity.

Example of calculating EI values is given in Sec. 20.6. Estimates of R have been made for many locations in the world. These are usually published as isoerodent maps, the lines indicating equal values of R on an annual basis.

3. Soil Erodibility (K):

The soil erodibility factor, K, is the rate of soil loss per unit of R or EI for a specified soil as measured on a unit plot, which is a 72.6-foot (22.1 m) length of uniform 9 per cent slope continuously in clean-tilled fallow. Therefore, K has a unit of mass per area per erosivity unit.

In the SI system, one set of units (metric ton. hectare, hour/hectare, megajoule. milli-metre) can be abbreviated as (t.ha.h/ha.MJ.mm).

If values for K are to be determined from measured data, units for K depend upon those chosen for soil loss and storm erosivity. Soil loss, A_O, measured on the given field plots, is adjusted to estimated soil loss for unit plot conditions by –

Therefore, if A_s has units of t/ha and EI has units of MJ. mm/ha.h, K has units of t.ha. h/ha.MJ.mm in Eq. 20.9. Obviously, ha cancels in the numerator and denominator, but they are left again to emphasize that K is soil loss (mass per unit area) per unit of EI.

The standard plot size considered in the above discussion was result of the sizes that were adopted for runoff plot studies. Runoff plots were earlier made to be of 1/100 acre, i.e., plots of 6 ft. (nearly 1.8 m) wide and 72.6 ft. (nearly 22.1 m) long. Thus the standard conditions assumed have no special significance but a historical accident.

A nomograph and estimate K for a given soil is given in Fig. 20.9. It can also be calculated from the regression equation.

4. The Slope Length Factor (L) and the Slope Steepness Factor (S):

The effects of slope length and gradient are represented in the USLE and L and S, respectively. However, they are often evaluated as a single topographic factor, L.S. Slope length is defined as the distance from the point of origin of overland flow to the point where the slope decreases sufficiently for deposition to occur or to the point where runoff enters a defined channel. Slope gradient is the field or segment slope, usually expressed as percentage. The slope length factor is defined as-

Soil loss increases much more rapidly than runoff as slope increases. The combined LS factor is given by –

The value of 100 sin θ for per cent slope, which is 100 tan θ, is sometimes used upto 20 per cent slope. Eq. 20.12 is to be used for single uniform slopes.

5. Crop Management Factor (C):

This is defined as the ratio of soil loss from land cropped under specified conditions to corresponding soil loss from continuous fallow on identical soil, slope and rainfall conditions. Soil loss from a field is influenced by density, kind of crop cover, root growth, water use by crops etc.

These conditions differ significantly during the crop growth period from planting to harvest of crops. Wischmeier and Smith (1978) approximated the erosion control effectiveness of each crop on the basis of five crop stage periods.

The crop stage periods suggested by them are:

Period F – Rough fallow—Summer ploughing or seed bed preparation to sowing,

Period 1 – Seed bed—Seeding to one month thereafter,

Period 2 – Establishment—From one to two months after seeding,

Period 3 – Growing period—From period 2 to crop harvest, and

Period 4 – Residue or stubble—From crop harvest to ploughing or new seed bed preparation works.

They computed the ratios of soil losses from cropped plots to corresponding losses from continuous fallow from available basic data separately for each five crop stages along with various combinations of crop sequence and productivity level.

Table 20.4 indicates an example of similar information for conditions at Dehradun given by Tejwani et al. (1971). For using the universal soil loss equation similar information for the region has to be compiled.

In Table 20.4 during 1/10-15/10 as there was no rainfall, no soil loss was considered.

6. Conservation Practice Factor (P):

This is the ratio of soil loss for a given practice to that for up and down the slope farming. Table 20.5 presents the values given by Wischmeier and Smith (1978) and Table 20.6 presents the values reported by Tejwani et al. (1971) for Dehradun conditions.

By evaluating the factors of the soil loss equation, the soil loss from a field under a given set of conditions can be determined. If the soil loss is higher than the soil loss permissible for maintaining productivity, suitable changes in the crop management and conservation practices should be made to reduce the expected soil loss.

Example:

In an area subjected to soil erosion, the following information is available:

What will be estimated annual loss? Explain how this soil loss will decrease by adopting conservation practices.

Solution:

Applications of USLE:

The USLE is used broadly for the following purposes:

1. Predict average annual soil loss from a field with specific land use conditions.

2. Guide the selection of cropping and management system, and conservation practices for specific soils and slopes.

3. Predict the change in soil loss that would result from a change in cropping or conservation practices on a specific field.

4. Estimate soil losses from land use areas other than agricultural lands, and

5. Provide soil loss estimates for conservationists to use for determining conservation needs.

In addition to the above, USLE could be used as a first approximation for estimating the sediment yield of watersheds.

In this approach, the approximation used is –

Y = E (DR) … (20.13)

Where,

Y = Sediment yield

E = Gross erosion, and

DR = Sediment delivery ratio

In this approach, the heterogeneous watershed area is divided into subareas for which representative soil type, slope length, gradient, cover, and erosion, control practice factors can be defined.

The USLE is then used for each of the subareas. These values are multiplied with DR values for the region in order to obtain approximate sediment yield values.

Modifications of USLE:

Since its initial development, the USLE has been widely used for estimating soil erosion from individual plots. Extensive data need to be generated (usually from research stations) for use of the USLE for a given situation. There have been some changes in the original USLE equation. Renard et al. (1991) outline some significant changes in the USLE.

These are:

1. Correction in the R factor for flat slopes to adjust for splash erosion associated with raindrops falling on ponded water.

2. Development of a seasonally variable soil erodibility term,

3. Calculation of LS factor for slopes of varying shape, and

4. Improvements in the values of C and P taking existing physical conditions.

Models for Erosion Processes and Sediment Yields:

In spite of the wide applications of the USLE, it has some empirical components and does not fully describe the erosion process and sediment yield. A better understanding of the erosion process and sediment yield from individual plots as well as watersheds can be obtained using mathematical models.

The different erosion processes are described using mathematical approaches and they are put together to obtain the sediment yield from a given area. Such procedures require the use of digital computers. Several approaches have been developed for the purpose and some of them are outlined in Haan et al. (1982).

Assessment of Erosion Hazard:

Assessment of erosion hazard for given area is required for planning soil conservation measures as well as for evaluating land resources. The assessment aims at dividing a given land area into smaller units which are similar in degree and kind of erosion. While soil surveys collect most of the information, soil erosion surveys could consist of reconnaissance surveys followed by detailed data collection.

For assessing erosion hazard due to rainfall, the terms rainfall aggressiveness and rainfall erosion index could be used. Rainfall aggressiveness is defined as the ratio P²/p, where P is the highest mean monthly precipitation and p is the mean annual-precipitation. It is an index of the concentration of precipitation into single month.

It is stated that this index is well correlated with the sediment yield in rivers. Based on rainfall erosion index, isoerodent maps are prepared and these maps give a broad idea of the erosion hazard.

Factorial scoring is another procedure used for erosion hazard assessment. The area under consideration is divided into square grids of suitable size. Each unit is rated on a scale from 1 to 5 in respect of erosivity, erodibility, slope, ground cover and population. The scoring is arranged so that 1 is associated with a low risk of erosion and 5 with high risk. The five factor scores are added and the total is used for a classification of low moderate and high erosion risk areas. The scores could be used on a map to delineate areas of similar risk.

In assigning the number from 1 to 5 each of the factors like erosivity, erodibility etc., are grouped into 5 classes using a parameter describing each of the factors.

The following table indicates such an example-

Consider three areas 1,2 and 3 with physical characteristics and assign scores as in table 20.8.

Using the factorial approach, it can be concluded that Area 3 has the maximum erosion hazard followed by Areas 1 and 2.