Agriculture Reference
In-Depth Information
Box 7.7
Calculating the Kinetic Energy of Rain
The
KE
of rain is given by the equation
1
KE
Raindrop mass
(Velocity)
2
(B7.7.1)
2
Raindrop mass is directly related to drop size. In any one storm, raindrop
sizes vary, but the median diameter is 2.5-3 mm for rainfall in temperate regions.
The velocity in equation B7.7.1 is the terminal velocity, which increases to a
maximum of about 9 m/s for drops with a diameter of 5 mm. Primarily because of
the interactive effects of drop size and velocity, the
KE
of rain per mm of rain is
found to vary with the rainfall intensity. Although this relationship changes with
latitude (temperate versus tropical regions) and with the duration of rain events,
the general result is that
KE
/mm increases for rainfall intensities up to ca. 75
mm/hr. The frequency of high-intensity storms (
25 mm/hr) is only ca. 5% for
rain in temperate regions, compared to 40% in tropical regions, so temperate
rainfall is much less erosive than tropical rainfall.
cles by raindrop impact, and the transport of soil particles by raindrop splash and
running water.
The energy of falling rain is much greater than the energy of surface runoff
generated by that rain. The
KE
of rain depends on its physical characteristics, par-
ticularly the rainfall intensity, as discussed in box 7.7.
Indices of rainfall erosivity have been developed on the basis of rainfall
KE
and intensity. One widely used in temperate regions is the
EI index
, calculated as
the product of the
KE
of a storm and its maximum 30-minute intensity. The lat-
ter is the maximum rainfall during any 30-minute period, expressed in mm/hr.
The
EI index
was developed for the United States. Values can be summed over
periods of time to give weekly, monthly, or annual values for erosivity. The
EI
index
is used in the
Universal Soil Loss Equation
(
USLE
) (see box 7.8).
Soil Erodibility
Erodibility is the reciprocal of the resistance the soil offers to erosion. It is deter-
mined by three broad factors that interact.
Soil Physical, Chemical, and Biological Properties that Affect Aggregation and
Aggregate Stability
. Properties such the particle size distribution and type of clays,
exchangeable cations and organic matter content, and roots and fungi as stabiliz-
ing agents mainly affect the detachment process of erosion. Aggregate size and
surface roughness operate more on the transport process. For example, if aggre-
gates at the surface slake under raindrop impact, and runoff occurs, the soil par-
ticles can then move; some may block pores, reducing the infiltration rate, whereas
others may move in the runoff water, possibly initiating saltation (section 1.3.1.2).
If aggregates slake
and
disperse (as in sodic soils), the blocking of pores is more
severe, and the smaller clay particles are available to move in runoff. The soil sur-
face becomes flatter and smoother, so there is less resistance to runoff.
Topography
. The effect of erosive forces (raindrop splash and transport) is
greater on sloping land. The effect of slope steepness is confounded with the length
7.5.2.2
