Formulation of SLEMSA Model: 4 Phases | Soil Management

To formulate SLEMSA model, Elwell divided the development programme in following four distinct phases: 1. Physical System 2. Control Variables 3. Sub-Models 4. Main Model (Z).

Phase # 1. Physical System:

The physical system to affect the soil loss in field involves a large number of causative factors of varying importance; they interact in a complex manner.

In this model the physical system which represents the soil erosion environment, is divided into following four sub-systems:

i. Crop

ii. Climate

iii. Soil; and

In model development each system is treated as a separate entity. The factors related to crop, which make influence on soil loss is dealt with the crop system. The climate system considers kinetic-energy of the rainfall, which is responsible to cause the particles detachment from the soil surface.

The natural soil properties, tillage treatments and present and past management practices followed are collectively considered under the soil system. This approach differs in concept from the USLE, in which the tillage and cropping are the part of the crop management factor C. The topography as physical system includes the slope steepness and length of the field affecting the soil loss.

Phase # 2. Control Variables:

The control variables are the major over-riding factors to determine the soil loss within four physical systems. These variables act as ‘brick’ of the model building. The control variables should be easily measurable and rational. In SLEMSA there have been identified five control variables for estimating the sheet erosion losses from arable land. These variables are the –

i. Amount of rainfall energy intercepted by the vegetation (i)

ii. Seasonal rainfall energy (E)

iii. Soil erodibility (F)

iv. Slope steepness (S); and

v. Slope length (L)

All these are described as under:

i. Control Variable, i:

It represents the amount of rainfall energy intercepted by the vegetation. Rainfall energy is the principal causative factor to soil loss. Vegetations intercept a significant percentage of rainfall energy, before reaching the raindrops on soil surface.

The crop type, planting date, plant and management appreciably affect the amount of rainfall energy intercepted by the cropping system. The crop height and rooting characteristics of the plant are considered at secondary importance for this case. In SLEMSA the percent seasonal energy interception by the crop is used for model development.

ii. Control Variable, E:

It refers to the seasonal rainfall energy, is counted for model building. The rainfall energy is one of the rainfall parameters, which denotes the erosivity of rainfall. The rainfall erosivity is a predictor of soil loss. The validity of rainfall erosivity parameter should always be tested. Hudson tested the EI₃₀ method for tropical African regions and found that it was not apparently significant. Later on Rose (1958) reported that the momentum of rainfall is significant for African soil to predict the soil loss.

In SLEMSA the seasonal kinetic energy of rainfall was found to be better correlated with the annual soil loss from the field plots, and is good predictor of annual soil loss from the bare fields.

iii. Control Variable, F:

It describes the soil erodibility factor as an erosion factor. In SLEMSA model, several attempts were made to find a suitable soil erodibility index, but no success could get. Elwell and stocking (1982) reported that, in absence of widely-applicable index of soil erodibility the soils could be rated from 1 to 10 (F) on the basis of field experience. The soil erodibility index (F) have been presented in Table 21.30, which are in current use in Rhodesian design practice.

The basic index values are again modified as per following guidelines:

(A) Subtract the followings from the basic index to get F –

1: For light textured soils containing the sand and silts mainly.

1: For restricted vertical permeability within one meter soil depth from the surface or for severe soil crusting.

1: For ridging up and down the slope.

1: If there is deterioration in soil structure due to excessive soil loss in the previous year (> 20 r/ha.y) or for poor management; and

0.5: For slight to moderate surface crusting or for soil loss from 10 to 20 t/ha. y. in the previous year.

(B) Add the followings to get F –

2: For deep (> 2 m) well-drained and light-textured soils.

1: For tillage operations which encourage maximum retention of water on the soil surface, e.g. ridging on contouring.

1: For tillage operations which encourage high infiltration and maximum water storage in the soil profile, e.g. ripping, wheel track planting etc.

1: For first season of no-tillage; and

1: For subsequent season’s of no-tillage.

iv. Control Variables L and S:

These control variables describe the slope length (L) and slope steepness (S) affecting the soil loss. In this regard, Zing (1940) and Meyer et. al. (1969) have reported to consider the slope length, slope percent and slope shape within the topographical physical system for correlating the soil loss.

The SLEMSA, which was developed for arable land, is protected by ditches constructed across the land slope to break the natural slope of land into small segments. The interval between ridges is counted as ‘L’ and uniform slope of segment length is taken as ‘S’ in the model. In this model, the slope length was considered as 30 m and 4.5% as uniform slope of standard plot.

Phase # 3. Sub-Models:

The sub-models were developed by relating the control variables with the soil loss. There are three sub-models, namely the crop ratio sub-model (C); soil loss from bare soil sub-model (K) and the topographic ratio sub-model (X) in SLEMSA.

These sub-models are described as under:

i. Sub-Model, K:

It is the soil loss sub-model for bare field, which describes the relationship of rainfall erosivity. The relationship is given as under –

Where,

K = sub-model to estimate the soil loss from bare soil

a and b = constants

E = seasonal rainfall energy (j/m²)

To derive this sub-model, the annual soil loss values for six treatments were collected from different slopes; and they were corrected with a standard slope of 4.5% and 30 m as the slope length. These data were plotted on graph paper at arithmetic scale, as shown in Fig. 21.9. The curves obtained are used to develop the relationship between K and E for bare fallow plots.

The values of ‘a’ and ‘b’ of equation 19.53 are also determined from the K, E and F family curves in following forms –

a = 2.884 – 8.1209 F … (21.54 a)

b = 0.4681 + 0.7663 F … (21.54 b)

After substituting ‘a’ and ‘b’ in equation 21.53

K = exp [(0.4681 + 0.7663 F) In E + 2.884 – 8.1209 F] … (21.55)

In which, F is the soil erodibility assumed as an index. In this way, based on the values of F and E of a specific location, the soil loss for a bare fallow land at 4.5% slope and 30 m length can be computed. The relationship between K, E and F is shown in Fig. 21.9.

ii. Sub-Model, C:

It is the crop canopy cover sub-model, denotes the ratio of soil loss from cropped plot to that from the bare fallow land. In SLEMSA the crop canopy cover sub-model (C) is given by the following two equations –

(a) C = exp (–0.06 i) for i < 50% …(21.56)

(b) C = (2.3 – 0.01 i)/30 for i ≥ 50%

In which, ‘i’ is the percentage rainfall energy intercepted by the crop. The relationship between percent energy interception (i) and soil loss ratio (C) is shown in Fig. 21.10.

iii. Sub-Model, X:

It is the topographic sub-model, which is the ratio of soil loss from a plot of length ‘L’ and slope percent ‘S’ to that from the standard plot. This sub-model adjusts the soil loss, accounted for the differences in slope length and slope percent between the standard field plot and the land for which the protection works are being designed. In SLEMSA the value of X was determined by using the slope-length factor of USLE with slight modification.

The modification was in respect of adjusting the slope length and percent slope from the base of 9% slope and 22.1 m length to the plot of 4.5% slope and 30 m length. The equation used to compute the value of X is given as under –

Where,

L = slope length (m)

S = percent slope

Phase # 4. Main Model (Z):

It is the product of sub-models K, C and X, i.e.

Z = K.C.X … (21.58)

In which, Z is the mean annual soil loss (t/ha./year).