SOIL LOSS ESTIMATION MODEL
for SOUTHERN AFRICA(SLEMSA)
N.L Mufute ,
LWRM, MSU
mufutenl@msu.ac.zw /
mufutengoni@gmail.com
INTRODUCTION.
• The model was developed by the Soil and Water
Conservation Research section at The Institute of Agricultural
Engineering, located in Hatcliff in Harare by H.A Elwell and his
team.
• Its purpose is to enable safe rotational systems to be designed
for arable lands that have been protected by contour ridges.
• This is achieved by selecting suitable combinations of farming
practices which reduce soil losses to a pre set target level.
• The main model consists of three sub models. These are:
– Sub model K; for estimating soil loss from bare soil,
– Topographic sub model X; for assessing the effects of changes of
slope steepness and length, and
– Canopy sub model C; to account for crop types and cropping
practices.
The SLEMSA Equation
• Z=K.C.X
• Where;
– Z = predicted mean annual soil loss, t/ha/yr, from the
land under evaluation,
– K= Mean annual soil loss, t/ha/yr; from a standard
tilled plot 30m x 10m at 4,5% slope , for a soil of
known erodibility, F , under a weed free bare fallow,
– C= The ratio of soil loss from a cropped plot, to that
from bare fallow,
– X= The ratio of soil loss from a plot of length L
metres and slope percent S, to that lost from the
standard plot.
The SLEMSA Equation Sub models
• The sub models express the relationships between
soil loss and the important variables which are
referred to as control variables.
• There are five control variables ;
– Rainfall Energy; E,
– Soil Erodibility; F,
– Crop Cover; i,
– Slope; S, and
– Slope length; L.
Sub Model Control Variables
Rainfall Energy E
• The geographic distribution of mean annual rainfall is
shown in fig 1.
• The designer should obtain more detailed information
pertaining to his area from rainfall data provided by the
Department of Meteorological Services.
• Also on this map, is the division between areas which are
classified as guti and non-guti .
• Having classified the design site as guti or non guti and
knowing the mean annual rainfall for the area, the
designer can obtain the value of mean seasonal energy, E,
from fig 2.
• Rainfall energy distribution for Zimbabwe during the wet
season is given in Table2.
Sub Model Control Variables Cont.
Soil erodibility, F
• Soil erodibility is assessed in two parts. Fb reflects
the inherent properties of the soil when
conventionally tilled- i.e. ploughed, disced and rolled
to a fine tilth.
• The value of Fb is selected from table 3 according to
the soil classification, and modified to take account
of other management practices by applying the
correction factors listed in table 4. The resultant
value of Fm is utilised in this model.
Crop Cover i
• Estimated energy interception values, i, for various
crops, planting dates and yield levels are given in
tables 5 to 10.
Sub Model Control Variables Cont.
Topography, S and L
• In the case of crops planted on the flat, S, is the per cent
slope steepness of the land between contour ridges and L
is slope length.
• Where crops are grown on graded ridges , water will flow
in the channels between ridges and S and L are the slope
and length of this channel.
• The design is based upon the steepest slope on the land
(rather than the longest slope) because slope percent has
the greater influence on soil loss.
• When designing a layout for a specific land, the value of
the maximum slope percent (S) and its corresponding
length (L) are measured during the field survey. But for
general planning purposes, extreme values can be selected
for the farm as a whole from pegging sketches or farm
plans.
SLEMSA Field Application
• It can be used to design land protection systems for
individual fields, or to set general safe criteria for land
use planning and legislative purposes.
• Tolerable soil loss levels, Zt, are set as target figures
which the designer achieves by modifying any or all of
the following;
– The percent slope of the land by terracing,
– The effective slope length and percent steepness of the land
by varying the direction of the crop ridges,
– The soil erodibility by means of different tillage techniques
– The degree of vegetation cover protection by adjusting the
crop types, rotational practices, planting densities, planting
dates and management standards.
SLEMSA Field Application Cont.
• Several suitable combinations of tillage , cropping and
mechanical conservation may be tried and the most
practical selected.
• Where target soil loss figures cannot be achieved
however, it may be necessary to accept a soil loss in
excess of the target figure, or to recommend radical
down grading of the land use.
• It should be noted that each individual sub model is of
value in decision making and can solve a variety of
problems.
• Sub model K identifies the most and least erodible
soils; the canopy cover model indicates which crops
and grazing practices are especially hazardous ; and the
topographic model identifies the independent
influence of slope characteristics.
Preliminary Survey Data
The following details are required;
• Farm location and mean annual rainfall,
• Soil texture and pedological classification,
• Land slope percent,
• Slope distance between existing contour ridges
and the gradient and length of the steepest
crop ridge
• Crops in rotation, proposed planting dates and
expected yields,
• Tillage, ridging and mulching practices.
Design of Safe Rotational Systems
• The model estimates average annual soil losses from sheet
erosion. It can therefore be applied only to lands which are
protected by contour ridges. The design steps are as follows;
1. Determine K
a. Obtain mean annual rainfall data for the area. Note from fig 1
whether the site is in a guti or non guti area. Then from Fig. 2
read off the mean energy value (E) for the locality,
b. From table 3 assess the basic erodibility value of the soil, Fb,
under conventional tillage,
c. From Fb and Table 4 assess the soil erodibility value, Fm,
applicable to the soil treatment for the first year in the rotation.
Fm values for the subsequent years cannot be assessed until
the soil loss from the previous year in the rotation has been
fully worked out,
d. Enter fig. 3 with Fm and E, and read off the value of K for the
first year in the rotation.
Design of Safe Rotational Systems Cont.
2. Obtain C
• From details of crop yields and planting dates , read off values
of i for each crop in the rotation from table 5 to 10,
• Enter these i values in fig.4 and obtain the values of C for each
year of the rotation.
3. Determine X
X for un ridged lands
• The soil loss value K relates to a 4.5% slope 30 metres long. This
must now be corrected to the percent slope and length of the
land under consideration.
a. As a starting point, adopt the value of the length along the
slope between the existing contours, if any, or the l value
applicable to the contour ridge spacing designed from table 1.
b. S for lands without crop ridges is the steepest percent slope of
the land between contour ridges,
c. Using L and S read off X from Fig.5 or 6 as appropriate.
Design of Safe Rotational Systems Cont.
X for ridged lands
a. Where crops are grown on ridges, the value of
S is the maximum grade expected on crop
ridges in any part of the land, and L is their
length. The value of X is then read direct from
fig. 5 or 6 as before.
Design of Safe Rotational Systems Cont.
4. Determine Z
• Calculate Z for the first year, select Fm for the second year,
calculate Z for the second year , select Fm for the third
year- and so on. Z is calculated from the following formula;
• Z= KCX (t/ha/yr)
• Obtain the mean annual soil loss for the rotation by
summing the soil loss for each year in the rotation and
dividing by the number of years in the rotation (n).
• i.e. Mean annual soil loss for rotation = (Z1 + Z2+ ...+Zn)/n
=
Design of Safe Rotational Systems Cont.
5. Select the Target Level of Soil Loss
• From the soil description, select the target level of soil loss from
table 11.
6. Consider Alternative Systems
• Select several possible alternative rotations and land protection
systems (vary rotation , yields, planting dates , tillage practices ,
ridge grades , or even slope percent if terracing is envisaged)
and recalculate the mean annual soil loss for each system.
7. Evaluate the Alternatives
• Compare the calculated mean annual soil loss from each system
to the target soil loss level,
• Carefully consider the economic implications of each system,
• From the facts given in (a) and (b) above, select in consultation
with the farmer, the most suitable land use system.
Treatment of Wet Season
• A new set of conditions of soil erodibility and cover is imposed
when the land is ploughed or disced during the wet season.
• To deal with this situation the season must be split into two
parts and soil losses calculated separately for both pre- and
post- ploughing periods.
Rainfall Energy
• Rainfall energy for the pre-ploughing period is obtained by
multiplying the total mean seasonal energy E (from fig 2 ) by
the appropriate factor obtained from table 2.
• The amount of energy remaining in the post-ploughing period
is obtained by subtracting the amount in the pre-ploughing
period from the total seasonal energy.
• Alternatively it can be calculated from the product of E and
the proportion of seasonal energy falling in that period,
estimated from table 2.
Design of Safe Rotational Systems Cont.
Soil Erodibility
• Two separate assessments of soil erodibility are
required, one for the pre-ploughing period and
the other for post ploughing.
• Both are assessed from tables 3 and 4, in exactly
the same way as described previously.
Design of Safe Rotational Systems Cont.
3. Crop Cover
• Careful assessments of the energy interception values i must
be made for both parts of the season.
• All the values in Tables 5 and 10 are given as percentages of
the total seasonal rainfall.
• These must be corrected where part seasons are involved.
The following example illustrates how this is done.
• Mid- season ploughing most commonly follows the tobacco
crop; a grass ley is often planted immediately or a weed
fallow may be encouraged.
• For example, an irrigated tobacco crop, planted on the 1st
September is reaped by Christmas.
• The land is then ploughed in mid January and planted to
grass.
Design of Safe Rotational Systems Cont.
3. Crop Cover cont.
• Table 2 shows that 49% of the seasonal rainfall energy occurs before
ploughing and 51% falls during the remainder of the season.
• If the tobacco crop intercepts 20% of the seasonal rainfall energy ( as
obtained from table 5), it will intercept 20/0.49 % of the energy falling prior
to ploughing.
• Thus the new i value for the pre ploughing period is 41%. Similarly, a well
managed pasture planted on the first 1st of February has an effective
seasonal cover of 16% . The corrected i value for the post ploughing period
is 16/0.51 =31%.
• The case of a second or third year of ploughed in grass in somewhat
different.
• Because the amount of cover can be regarded as constant up to the date of
ploughing, the percentage energy intercepted up to this point is the same
irrespective of ploughing date.
• In other words, if the grass ley intercepts 90% of the seasonal energy (table
7) it will also intercept 90% of the energy falling in the partial season.
Design of Safe Rotational Systems Cont.
4. Topography
• Any slope length and percent slope changes
made because of changes in ridging practices
etc., must be taken into account in the
appropriate part of the year.
5. Seasonal Soil Loss
• The soil loss from each part of the year is
estimated from Z = KCX in exactly the same way
as before. Seasonal soil loss is simply the total
soil loss from the sum of the pre-and postploughing periods.
Design Example: Tobacco Rotation
1. Survey data
A. Location: Mvurwi
B. Well-drained sands (A texture); 5G
C. Maximum slope 6%
D. 21m between existing contour ridges on 6% slope. If the land is
ridged on contour , maximum gradient would be 1 in 200 and
crops ridges 200m long.
E. First year: Tobacco planted on the 1st November, yielding
1700kg/ha, with moderate losses arising from reaping, curing and
grading. Ridged up and down the slope on large ridges more than
200mm high when consolidated. Ridges maintained throughout
the rains.
– Second year: Maize planted on the flat emerging on 15th November,
yielding 6000kg/ha under conventional tillage.
– Third to fifth year: Katambora Rhodes grass emerging mid-November
with moderate residual fertility following the maize crop. Lightly grazed
in summer and winter in the second and third year. Third-year grass
ploughed-in during winter.
Design Example Cont.
2. Determine K
a. Rainfall; Mean annual rainfall 850mm,
• Non guti area (map fig 1)
• E = 16 000J/m2 (fig 2)
b. Fb= 4 (Table 3)
c. Table 4;
• First year, large crop ridges up and down slope (-1).
•
Fm = 4 -1 = 3.
•
• NB: Fm for any year cannot be assessed until the soil loss for
the previous year has been worked out. This is because of the
correction factor for soil loss, table 4.Where the rotation ends
in a grass ley, however , soil losses in the previous year are
generally below 10t/ha/yr.
d. Fig 3
• First year. Fm =3 . E = 16000, K = 200t/ha/yr.
Design Example Cont.
C. Obtain C
a. Table 5, Table 7 and Fig. 4
• Potential tobacco yield = 1700x 1.60= 2720kg/ha
•
•
•
•
•
First year
Second year
Third year
Fourth year
Fifth year
i = 40%
i = 45%
i = 53%
i = 87%
i = 72%
C = 0.090 (C1 curve)
C = 0.066 (C1 curve)
C = 0.059 (C1 curve)
C = 0.006 (C2 curve)
C = 0.014 (C2 curve)
Design Example Cont.
4. Obtain X
• Obtain X for un ridged lands
• Fig 5
S = 6%;
, L = 21m; X = 1.20
•
• Obtain X for ridged lands
• Lands are ridged up and down the slope for tobacco,
hence S , L and X are unchanged (X=1.20),
• If crop ridges are constructed parallel to the contour
ridges , however, X is calculated from the maximum
length and grade of the crop ridges. Say L = 200m and
S = 1 in 200.
• Fig 6
S= 0.50%;
, L = 200m; X =
0.525
Design Example Cont.
5. Determine Z
• Calculate Z for the first year. Fm can now be
selected for the second year and Z can be worked
out for that year. Hence Fm can be selected for
the third year, and so on.
• The calculations are given in tabular form.
Crop
Fm
K
X
i
C
Z
Tobacco
Maize
Grass
Grass
Grass
3
3
3.5
4.5
6
200
200
150
80
25
1.20
1.20
1.20
1.20
1.20
40
45
53
87
72
0.09
0.066
0.059
0.006
0.014
21.6
15.8
10.6
0.6
0.4
Total
49.0t/ha
Design Example Cont.
• Fm for tobacco
= 4 -1 for crop ridge grades
• Fm for maize =4-1 for soil losses greater than
20t/ha in the previous year.
• Fm for first year grass = 4 – 0.5 for soil losses of
10-20t/ha in the previous year.
• Fm for second year grass = 4 +1 for second year
fallow – 0.5 for soil losses.
• Fm for fourth year grass = 4+2 for the third year
fallow.
Design Example Cont.
• Mean annual soil loss ;
•
• = 49,0/5 = 9.8t/ha/yr
• Select Target Level;
• Table 11 Sand. Zt = 5t/ha/yr for extension
purpose
Design Example Cont.
Select Target Level;
• Table 11 Sand. Zt = 5t/ha/yr for extension purpose
Consider alternative systems;
• The mean annual soil loss for the existing farming
system is clearly excessive and exceeds the target
soil-loss figure.
• The designer’s objective now is to reduce soil losses
to acceptable levels by modifying the cropping
practices.
• The most obvious practices to change in this case
are the crop ridge grades, the crops in the rotation
and the number of years of grass ley.
Design Example Cont.
• Alternative 1
TMGGGG
• An extra year of grass added to the rotation (i= 72%; C =
0.014)
• Alternative 2
TGGGG
• Maize removed and an extra year of grass added. The first
year grass can now be planted in April (i= 87%; C = 0.006;
Fm = 3)
• Second year of grass Fm = 5 ; C= 0.006.
• Alternative 3
TMGGG
• Tobacco grown on big ridges on contour (Fm =5). Maize Fm
= 4.
• Alternative 4
TTGGG
• Tobacco grown on big ridges on contour with an extra
tobacco (Fm =5) replacing the maize. The first year grass is
planted in April. (Fm = 4; C = 0.006)
• The results are given below.
Design Example Cont.
Alternatives T
M
T
G
G
G
G
Mean Extension
Decision
As farmed
21.6
15.8 -
10.6 0.6
0.4 -
9.8
Not acceptable
1
21.6
15.8
10.6 0.6
0.4 0.4 8.2
Not acceptable
2
21.6
-
-
1.4
0.4
0.4 0.4 4.8
Acceptable
3
2.4
8.7
-
7.8
0.4
0.4 -
3.9
Acceptable
4
2.4
-
2.4 0.8
0.4
0.4 -
1.3
Acceptable
Design Example Cont.
• Soil loss from the first year of grass is critical. Losses
are high when the grass follows maize because it is
not established before the start of the rains and is
grown under conditions of moderate fertility.
• Soil losses from alternative 2 can be reduced further
by running the tobacco ridges across the slope at an
angle to the contour ridge. Contour farming is
however a more acceptable practice.
• The cover between 30 -40% is critical. If the potential
tobacco yield drops from 2720kg/ha, the i values will
fall from 40% to 30% and the soil loss will almost
double -21.6 to 40,8t.
What to be noted about the use of SLEMSA
a. The model estimates soil losses from sheet erosion only.
• It is intended primarily for the design of safe rotational
systems for lands which are already adequately protected
from rill and gully erosion for through the use of well
designed contour ridges.
b. Where mechanical works exist, the model will allow the
conservation merits of the existing farming systems and
possible alternatives to be evaluated, leading to selection of
the best all- round farming enterprise.
c. Factors to take into account when selecting the best farming
systems are;
• The predicted rate of soil loss relative to the acceptable target level,
• The practicalities of the system with respect to equipment and
management practices and preferences,
• The economic implications
What to be noted about the use of SLEMSA Cont.
d. Where there is a need to adjust an existing system
because of high soil losses, the designer should
begin by modifying those practices which offer the
greatest opportunity for improvement.
e. When the land is ploughed during the rainy
season, separate calculations are necessary for
pre- and post- ploughing periods.
f. Bare eroded patches must not be permitted to
develop within lands. The steepest slopes should
therefore be selected for design purposes.
What to be noted about the use of SLEMSA Cont.
g. Models do not think! They provide solutions appropriate to an
idealised set of conditions presented to them by the designer.
He must therefore make the final decision so as to whether the
answer can be applied in practice.
For instance, he might ask the model to give him an estimate of the
length of crop ridge which will limit soil losses from a specified
set of circumstances to 4 tons/ha/yr.
Let us assume the model gives an answer of 2000 metres! at a 1 in
400 grade.
Clearly it is highly unlikely that a continuous grade of this length
could be achieved in practice. In these circumstances, the
designer would limit the crop ridges to a length he felt practical.
This example illustrates that the soil loss estimation model is an aid
to the decision making process.
The answers it yields must be weighed against the designer’s
experience and not accepted blindly.
GENERAL PRACTICES THAT RESULT IN REDUCED
SOIL LOSS
• In the interest of preventing soil loss and contribute towards
natural resource sustainable utilisation, all lands should be
adequately protected from soil erosion.
• Where land slopes exceed two percent, complete soil protection is
provide by correctly designed safe rotational systems.
• Full contour layouts may not be necessary on land slope below 2%
grade, protection from headland runoff should always be provided
and designed safe rotational systems provided.
• Information on the way the land is cropped, tilled, etc. is necessary
to limit soil losses from sheet erosion to acceptable levels.
• Methods or designing contour layouts have been in use by
Extension Staff for many years and the facility to design safe
rotational systems in detail is provided by SLEMSA.
• Guidelines which will lead to a reduction in soil losses can also be
given to farmers and other concerned stakeholders. These must be
balanced against the needs of the particular crop and land
concerned.
GENERAL PRACTICES THAT RESULT IN
REDUCED SOIL LOSS
General
• Lands should be contour ridged to standards where necessary and
recommended crop rotations should be practiced.
Land preparations
• Overworking the soil is uneconomic and leads to a finely powdered,
highly erodic soil.
• A rough cloddy surface resists soil erosion.
• Treatments which store water on soil surface reduce soil loss.
• Ploughing and planting on contour is considerably safer than ploughing
and planting up and down or at angles to the slope of the land.
• The flatter the grade of the crop ridges and the larger the ridges, the
better.
• Crop residues left on the surface as mulches, improve soil moisture, soil
structure and reduce soil erosion.
• Plough-in fallows and leys as near to the end of the rains as practicable.
GENERAL PRACTICES THAT RESULT IN REDUCED
SOIL LOSS
Cropping
• Bare soils constitute an erosion hazard.
• The better the yield, the lower the soil loss therefore follows
the recommended plant spacing and fertiliser requirements.
• Late planting reduces yields and increases soil loss.
• Some very early planted crops leave the soil unprotected in the
later half of the rainy season. This applies especially to tobacco
and sunflowers and consideration must be given to protecting
the land later in the rainy season.
• Where low yields are commonly experienced, lands need
additional all year round protection. This could be in the form
of crop ridges on contour or by making full use of crop residues
as mulches.
• These systems have the potential to reduce soil losses to
acceptable levels if implemented properly.
References
• Elwell. (1982) Developing a Simple Yet Practical Method of Soil
Loss Estimation. Tropical Agriculture (Trinadard) Vol. 59, No. 1
January 1982.
• Elwell (1980). Design of Safe Rotational Systems. February
1980.
• Hudson N. (1981) Soil Conservation. Batsford.
• Morgan R.P.C (1986) Soil Erosion and Conservation. Longman
• Toy, T.J and Foster, G.R (Co-editors), 1998. Guidelines for the
Use of the Revised Universal Soil Loss Equation (RUSLE) Version
1.06 on Mined ands, Construction Sites, and Reclaimed Lands.
Publ., The Office of Technology Transfer Western Regional
Coordinating Center, Office of Surface Mining, Denver.
• Wischmeier, W.H., and Smith,DD. 1978. Predicting Rainfall
Erosion Losses-A Guide to Conservation Planning. U.S.
Department of Agriculture, Agriculture Handbook No. 537.
• USLE:
http://www.gov.on.ca.OMAFRA/english/engineer/facts/00001.htm