Mr. Anthony J. Parsons, University of Sheffield, UK. Global Symposium on Soil Erosion (GSER19), 15 - 17 May 2019 at FAO HQ.

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2. How reliable are our methods for
estimating soil erosion by water?
Anthony Parsons - University of Sheffield, UK
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3. Conclusion
• Existing methods for assessing soil erosion by water
are flawed.
• There is an urgent need to develop approaches to
estimating rates of soil erosion that are consistent
with current understanding of erosion processes.
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4. Methods to estimate soil erosion
by water
• Plot Studies
• Monitoring
• Modelling
• Use of Radionuclides
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5. Plot Studies
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6. Plot Studies
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“The most absurd pretentions, that half a kilo of soil
from two or three square metres can give estimates
of loss rates from hill slopes have been boosted by ‘experts’.”
Pereira (cited by Hudson 1993)

7. Rainfall
simulations
Runoff
coefficient (%)
Sediment
g/m2/min
Total dissolved
N (mg/l rain)
Grassland 24 c.0.5 4.3
Creosotebush 46 c.1.5 1.67
Ratio 0.52 0.33 2.57
Natural
Events
Runoff
coefficient (%)
Kg/m2/yr Kg/ha/yr
Grassland 3.6 0.192 0.15
Creosotebush 18.3 0.525 0.33
Ratio 0.20 0.37 0.45
Plot Studies

8. Plot Studies
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In a laboratory study of simulated storms on three soil
types, using storms delivering the same rainfall kinetic
energy those of variable intensity showed as much as a
33% increase in erosion over constant intensity rain.
Parsons & Stone (2006)

9. Monitoring
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10. Monitoring
“splash and sheetwash are of minor importance in
redistributing soil within a field other than over a
distance of a few metres…[and] it is rills and gullies
that redistribute soil within a field or a landscape.”
Evans (1990)
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11. Monitoring
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In an experiment comprising four simulated storms
of durations between 10 and 30 minutes, only between
0% and 56% of the sediment exported by the rills derived
from within the rills themselves.
Luk et al. (1993)
Monitoring a field for 13 months following tilling,
erosion of rills accounted for only about 40% of the
total soil erosion over the monitoring period.
Govers and Poesen (1988)

12. Monitoring
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13. Modelling
USLE still widely used.
Would you trust your health to a doctor who knew
nothing of medical advances of the past 50 years?
All models depend on the empirical foundation on
which they rest. The extent to which this empirical
foundation represents soil erosion processes (or at
least their outcomes) determines how good models
can be.
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14. Modelling
The limitations to our understanding of soil-erosion
processes that are incorporated into process-based, soil-
erosion models limit their predictive capability.
Aside from the greater complexity of process-based
models compared to the USLE, the methodology used to
predict rates of soil erosion is identical.
Because all regression equations explain only part of the
variance in the predicted quantity, the greater the
number of equations in any model the greater is the
likely uncertainty in the predicted rate of soil erosion.
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15. Use of Radionuclides
Commonest is 137Cs
Rests on 4 assumptions (+ process assumptions)
- Fallout is locally, spatially uniform
- Fallout is rapidly and irreversibly fixed onto soil particles
- Redistribution of fallout is due to the movement of soil particles
- Soil erosion can be derived from measurements of 137Cs
inventories
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16. Use of Radionuclides
Fallout is locally, spatially uniform
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17. Use of Radionuclides
Fallout is rapidly and irreversibly fixed onto soil particles
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18. Use of Radionuclides
Redistribution of fallout is due to the movement of soil
particles
Provided all 137Cs fallout is adsorbed irreversibly onto in situ soil
particles, the only way the inventory of 137Cs at a particular location
can change is if there is net movement of soil particles to or from
that location. Insofar as this is not wholly the case, the capacity of
137Cs to be used to assess soil erosion is compromised.
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19. Use of Radionuclides
Soil erosion can be derived from measurements of 137Cs
inventories
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20. Conclusion
• Existing methods for assessing soil erosion by water
are flawed.
• There is an urgent need to develop approaches to
estimating rates of soil erosion that are consistent
with current understanding of erosion processes.
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