Soil & Conservation Report 1

NAME: Thabo chuchu
STUDENT ID: 201700080
PROGRAMME: BSc SWCE
Report 1 :
determining rainfall erosivity of an
individual storm
INTRODUCTION
Bofu (2008) states that, soil degradation results from erosion by a storm i.e. (soil becomes
exposed to the direct action of the two most potent causes of erosion, water and wind).
However water is perceived as one of the main degradation of soil because of the vast
environmental effects. According to (Zorn & Komac (2013)) one of the most important
factors in water erosion is the erosive potential of raindrop intensity which is particularly
important as a parameter of erosivity because it is the only feature of rainfall which, in
addition to amount, is frequently recorded at conventional meteorological stations. The
rainfall erosivity factor (R) in the Universal Soil Loss Equation(RUSLE) is generally
recognized as one of the best parameters for the prediction of the erosive potential of
raindrop impact (Wischmeier and Smith (1978)).
The subject of erosivity has been studied worldwide and various properties of raindrops such
as intensity and kinetic energy are among the frequently used parameters to develop erosivity
indices. There exists a general relationship between kinetic energy and rainfall intensity.
Based on the work of Laws and Parsons (1943), Wischmeier and Smith (1958) obtained the
equations:
KE=11. 9+8 . 7 Log10 I

Where; I = the rainfall intensity (mm/hr)
KE = the kinetic energy (J/m2/mm).
In addition Hudson (1981) gives the equation below for tropical rainfall:
127 . 5
KE=29 . 8−
I
Furthermore EI30 (rainfall energy *maximum 30-min intensity) and KE>25(total kinetic
energy of all of the rain falling at more than 25mm /h) indices are the most important rainfall
erosivity indices.
METHODOLOGY
Rainfall Information collected at Sir Seretse Khama International Airport was used to
determine rainfall erosivity. The artifact of total rainfall energy of a storm and its supreme 30
minutes rainfall amount is used to calculate the best single rainfall related to the soil loss. The
last outcome of the analysis would be the EI 30 and KE>25 rainfall erosivity values which
signifies amount of soil loss likely to be caused by the rainfall effect on the surface of the
soil.
RESULTS
Table 1: Showing The Calculation of EI30

XTime interval (min) rainfall (mm) intensity (mm/hr) KE (J/m^2/mm total KE (J/m^2)
0-30 4,5 9 20,20 90,91
30-60 21,0 42 26,02 546,47
60-90 5,5 11 20,96 115,28
90-120 5,5 11 20,96 115,28
120-150 4,5 9 20,20 90,91
150-180 5,5 11 20,96 115,28
180-210 4,5 9 20,20 90,91
210-240 7,0 14 21,87 153,10
240-270 3,0 6 18,67 56,01
300-330 2,0 4 17,14 34,28
330-360 3,0 6 18,67 56,01
360-390 2,0 4 17,14 34,28
390-420 0,5 1 11,90 5,95
420-450 2,5 5 17,98 44,95
450-480 0,8 1,5 13,43 10,07
TOTAL 1559,68
Calculation of EI30
KE =11.9+8.7log10I
EI30 = (Total kinetic energy*maximum rainfall intensity)
EI30 = 1559.68 * 30
= 46 790.4 J/m2.mm/hr
Table 2: Showing The Calculation of KE>25
KE (J/m2/mm) = 29.8 - 127.5
I
Time interval (min) rainfall (mm) intensity (mm/hr) KE (J/m^2/mm total KE (J/m^2)
0-30 4,5 9 15,63 70,35
30-60 21,0 42 26,76 562,05
60-90 5,5 11 18,21 100,15
90-120 5,5 11 18,21 100,15
120-150 4,5 9 15,63 70,35
150-180 5,5 11 18,21 100,15
180-210 4,5 9 15,63 70,35
210-240 7,0 14 20,69 144,85
240-270 3,0 6 8,55 25,65
300-330 2,0 4 -2,08 -4,15
330-360 3,0 6 8,55 25,65
360-390 2,0 4 -2,08 -4,15
390-420 0,5 1 -97,70 -48,85
420-450 2,5 5 4,30 10,75
450-480 0,8 1,5 -55,20 -41,40
TOTAL 1181,90
KE ≥ 25 = 26.76*21
=561.96 J/m2
DISCUSSION
High variability in the erosive storm brings about water erosion in semi arid areas as
compared to when the erosive factors are constant. To explore the relationship between the
rainfall intensity based indices and soil loss amounts, more linear progressions can be
calculated after rainfall events to conclude the average amount of soil losses from plots
against different rainfall erosivity indices.
CONCLUSION
Erosion occurs with light and high intensity rain.
REFERENCES
Bofu, Y., 2008. Erosion and Precipitation. In Trimble, S. W. (ed.), Encyclopedia of Water

Science. Boca Raton: CRC Press, pp. 214–217.
Gaboutloeloe, G., Monageng, P., Dikinya, O., Parida B. P., 2006. The evaluationof temporal and
spatial distribution of rainfall erosivity in Botswana, 29(1), 1-6
McGraw-Hill Book Co.New York,USA (1939) . 958 PP.
Wischmeier, W. H., and Smith, D. D., 1978. Predicting rainfall erosion losses–A guide to
conservation planning. Agricultural Handbook No. 537. Washington DC: U.S. Department of
Agriculture.
Zorn M., Komac B.(2013)Erosivity.In : Bobrowsky P.T.(eds) Encyclopedia of Natural

Hazards.Encyclopedia of Earth Sciences Series. Springer , Dordrent

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NAME: Thabo chuchu

STUDENT ID: 201700080

PROGRAMME: BSc SWCE

KE=11. 9+8 . 7 Log10 I

KE = the kinetic energy (J/m2/mm).

Table 1: Showing The Calculation of EI30

EI30 = (Total kinetic energy*maximum rainfall intensity)

Table 2: Showing The Calculation of KE>25

KE (J/m2/mm) = 29.8 - 127.5

Bofu, Y., 2008. Erosion and Precipitation. In Trimble, S. W. (ed.), Encyclopedia of Water

McGraw-Hill Book Co.New York,USA (1939) . 958 PP.

Zorn M., Komac B.(2013)Erosivity.In : Bobrowsky P.T.(eds) Encyclopedia of Natural

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