1) The study evaluated the impact of rainfall erosivity, soil cover, and organic carbon on soil loss and runoff under conventional tillage and conservation agriculture in Zimbabwe. 2) It found that soil cover was the most important factor in reducing soil loss and runoff, particularly under conservation agriculture where mulching dissipated rainfall energy. 3) Under conventional tillage, soil moisture and organic carbon also influenced erosion, but these factors were less important under conservation agriculture where mulching provided good soil cover.

1. Contribution of Rainfall Erosivity, Soil Cover and
Organic Carbon to Soil loss and Run-off
under Conservation Agriculture
in Zimbabwe
Isaiah Nyagumbo
CIMMYT: Conservation Agriculture Programme
Southern Africa Regional Office, Harare
i.nyagumbo@cgiar.org
World Congress On Conservation Agriculture
Brisbane, Australia
26-29th September 2011

2. 1. Introduction
● Soil Erosion is generally known to be a function of 4
physical factors:
● Vegetation (cover amount and type)
● Soil (erodibility itself a function of texture, depth,
carbon, infiltration etc.)
● Topography (slope and slope length)
● Climate (Rainfall erosivity)
So
Soil loss = f (Soil, Cover,Topography, Erosivity)

3. Effects of soil carbon on soil loss and run-off using laboratory
rainfall simulation (Elwell, 1986)

4. Introduction cont’d....
● Due to the high cost and large time scales associated
with establishing the importance of these factors soil
loss models are often used such as the USLE
(Wischmeier, 1976) and in Southern Africa the Soil
loss estimation Model for Southern Africa (SLEMSA )
is used
Z= KCX
Where K is the soil erodibility factor, C is the crop cover sub-model, X is the
topographic factor
● Soil loss targets for Zim: 3-5t/ha/yr
● Unfortunately few studies in Southern Africa have
attempted to quantify the influence of these erosion
factors in a CA system

5. Objective
● To evaluate the relative importance of rainfall energy
(erosivity), soil cover and soil organic carbon on
annual soil loss and run-off under conventional
mouldboard ploughing and Conservation Agriculture
systems on a Fersiallitic Red clay soil in Zimbabwe

6. 2. Materials and Methods
● Studies carried out at the Institute of Agric Eng, Hatcliffe,
Harare over 3 seasons on rectangular plots 30x 10 m at 4.5%
slope
● Soils: Deep well drained red clay Fersiallitic soils
● Rainfall erosivity determined from rainfall intensities
(autographic raingauge) using Hudson Index
● KE = 29.8 – 127.5/I
 I= 10-min rain intensity and KE> 25 (only storms with intensities greater
than 25 mm/hr considered)
 Total storm KE= KE* Rainfall amount
● Soil loss and run-off measured
from conical tanks and collection
troughs at the lower end of each plot

7. Tillage Systems tested
1. Conventional mouldboard ploughing
Only involved reduced soil
disturbance and permanent soil
2.Conservation Agriculture ( Mulch Ripping) cover. Rotation not practised
7

8. Methods cont’d
● 2 treatments implemented in a completely randomized
design with 3 replicates
● Measurements conducted over 3 consecutive seasons
93/4 to 95/6.
● Data analysed using ANOVA and multiple regression
statistical tools

9. 3. Results and Discussions

10. Soil Cover changes over time by tillage system over
3 season at Hatcliffe, Harare
CA provided much higher soil cover compared to CMP throughout the cropping
seasons with a significantly different (p<0.001) seasonal mean of 57
compared to 41 %
Note:
Error bars
denote +/-
SE of
mean

11. Rainfall energy per unit amount of cover was highest under under CMP
at the start of each season thereby providing an opportunity for
generation of high levels of soil loss and runoff in CMP. The residue
cover in CA dissipated most of this energy

12. Cumulative soil loss (kg/ha) comparing CA and conventional
mouldboard ploughing over 3 seasons at Hatcliffe, Harare
Av CMP-cum.soil loss (kg/ha)
MR-avg-cum.soil loss (kg/ha)
120 6000
Total rainfall = 481 mm Total rainfall = 957 mm
Total rainfall = 774 mm
Total Erosivity = 9694 J/m2 Total Erosivity = 13 919 J/m2
Total Erosivity = 9647 J/m2
100 5000
80 4000
Daily rainfall (mm)
soil loss target= 3.5t/ha/yr
60 3000
40 2000
20 1000
0 0
10-Oct-93 10-Jan-94 10-Apr-94 10-Jul-94 10-Oct-94 10-Jan-95 10-Apr-95 10-Jul-95 10-Oct-95 10-Jan-96 10-Apr-96
Date

13. Effects of Rainfall erosivity on Soil loss (kg/ha)at Hatcliffe, Harare
under CA and CMP systems over 3 cropping seasons
NB: Relationships significantly different (p<0.001)
1200
CMP-Soil loss kg/ha MR-Soil loss kg/ha
1000
800 Z (CMP)= 0.5691x - 21.007
Soil loss (kg/ha)
N= 567; R² = 0.7988 ****
600
400
200
Z (MR) = 0.0512x - 2.9053
N=567; R² = 0.5961 ***
0
0 200 400 600 800 1000 1200 1400 1600
Rainfall erosivity (J/m2)

14. Soil loss multiple regression summary
Soil loss
Conventional
Estimated Mouldboard Conservation Is CA factor significantly diff
Parameters ploughing Agriculture from CMP?
Est. t- Est. t-
Coeff value t-prob Coeff value tprob
Rainfall
0.576 36.91 <0.001 0.055 22.92 <0.001 <0.001 ***
Energy
Soil Cover % -1.10 -7.62 <0.001 -0.12 -6.33 <0.001 <0.001 ***
Soil carbon
0.5 0.01 0.99 1.50 0.13 0.893 0.987 n.s
Top soil moist
1.63 2.92 0.004 -0.04 -0.42 0.675 0.003 **
Overall Multiple regression: Significant at p<0.001; R2 = 0.798
Comments:
(1) Soil loss mostly influenced by Rainfall Energy and soil cover for both CMP and CA
(2) Top Soil Moisture content significant for CMP but not for CA and a signifcant
difference in behaviour between the two is observed. For CMP soil loss increased
with increase in moisture but not for CA.
(3) In both cases Soil Organic Carbon (ranged 9.9 to 12mg-C/g soil) did not

15. Effects of CA and Conventional Mouldboard Ploughing on cumulative runoff in 3
consecutive seasons at Hatcliffe
120 Total rainfall 774 mm Total rainfall 481 mm Total rainfall 956 mm
Rainfall (mm)
Cummulative run-off or Daily Rainfall (mm)
100 Av CMP-cum.runoff(mm)
Av-MR-cum.runoff(mm)
80
60
40
20
0
Error bars denote +/- SE of mean
Total seasonal run-off amounted to 7.4,16 and 8.1 % of seasonal rainfall for CMP
compared to 0.5, 0.8 and 0.6 % for CA over the 3 seasons

16. Runoff multiple regression summary
Run-off
Estimated Conventional Mouldboard Is CA factor significantly diff
Parameters ploughing Conservation Agriculture from CMP?
Est. Coeff t-value t-prob Est. Coeff t-value tprob
Constant 0.07 0.16 0.871 -0.027 -0.22 0.828 0.916 n.S
Rainfall Energy 0.009 65.87 <0.001 0.0006 27.25 <0.001 <0.001 ***
Soil Cover % -0.0075 -5.83 <0.001 -0.001 -7.29 <0.001 <0.001 ***
Soil org. carbon
0.266 0.67 0.501 0.047 0.47 0.64 0.779 n.s
Top soil moisture
content -0.0196 -3.96 <0.001 0.0004 0.50 0.619 0.004 **
Overall Multiple regression: Significant at p<0.001; R2 = 0.862
Comments:
(1) Runoff followed same trend as soil loss being mostly influenced by Rainfall Energy
and soil cover for both CMP and CA
(2) Top Soil Moisture content also significant for CMP and not significant for CA
(3) In both systems Soil Organic carbon did not significantly influence runoff.

17. Discussion
● Dissipation of rainfall energy due mulching in CA
systems contributes to taming the erosive power of
tropical rainstorms
● In the presence of a good residue cover, soil moisture
status and organic carbon become irrelevant or
unimportant drivers for soil loss and runoff in CA but
remain important determinants for run-off and soil loss
in Conventional ploughing.
● Although few studies compare the relative importance
of these factors, similar reductions in soil loss and
runoff from CA are widespread in literature
(Contill, 1998) and more recently from Ethiopia
(Araya, et al, 2011)

18. Discussion cont’d.....
● This suggests that with application of a good residue
cover, degradation on soils with low organic carbon
may be effectively controlled although SOC remains
important in other soil quality attributes such as soil
water storage (Nyagumbo,2002).
● From a conservation point of view the results obtained
also emphasize the importance of residue cover even
in clay soils in contrast to Chivenge, et al 2007 who
suggest that optimization of SOC and sustainability
can be focussed more on reduced soil disturbance and
less on C inputs in clay soils!

19. 4. Conclusion & Recommendations
● The most important soil loss and run-off factors were
rainfall erosivity, soil cover and antecedent soil
moisture. In CA this erosivity is easily dissipated by
the mulch cover.
● The presence of a good mulch cover in CA overrides
and nullifies the contribution of other erosion factors
such as SOC and moisture to soil loss and run-off.
● In contrast to CA, significant amounts of sheet erosion
in conventional systems occur at the start of the
season and still progressively continue with time even
if the crop builds up appreciable canopy cover.

20. Conclusion & Recommendations cont’d
● Results obtained here re-emphasize the need for
developing innovative ways for soil cover provision in
CA systems under these tropical conditions given the
challenges of livestock competition for residues and
low biomass production

21. 5. Acknowledgements
● GTZ for the financial support to this research
work
● Government of Zimbabwe for the research sites
and personnel manning the station throughout
the study period
● Victor Mateveke (a University of Zimbabwe
student) who assisted in the initial data capturing
and compilations.

22. Thank you!