Environmental Engineering Reference
In-Depth Information
2.6
Global water cycle with annual flows. According to Oki
(1999).
energy of a raindrop is curvilinear; the efficiency of im-
pact energy in removing target material declines as the
energy increases (Ghadiri 2004).
Even ordinary velocities of 6 m/s convey kinetic en-
ergy of 180 J/m
2
for every centimeter of rain; should
the runoff equal one-third of all precipitation, its kinetic
energy, with an average speed of 1 m/s, would be 1.65
J/m
2
, a rate 2 OM smaller than the rain's kinetic energy
density. Not surprisingly, field studies found the stron-
gest correlation between soil erosion and the kinetic en-
ergy of the rain and its intensity. The importance of plant
cover in controlling erosion is obvious: relative soil losses
are at least 100 times higher in fields planted to row
crops than in forests. With extensive row cropping (corn,
soybeans), these losses are a worrisome threat to long-
term crop productivity in many regions (Larson, Pierce,
and Dowdy 1983; Smil 2000b).
Hailstones are much more damaging. Even the largest
raindrops (diameter 5 mm), hitting with a velocity of
about 9 m/s, have just 2.6 mJ of kinetic energy. In
contrast, hailstones with diameters of 2 cm, not uncom-
mon in heavy storms, will have kinetic energy about 75
times higher (200 mJ). Northern Colorado, the area of
heaviest damage in the United States, averages about
2,200 hailstones/m
2
with mean diameter of about 1.25
cm (Frazier 1979). The kinetic energy of such an event
adds up to about 500 J/m
2
, and the resulting power
densities of 10-20 W/m
2
are 100-200 times those of
a heavy rainfall. At the point of impact, kinetic power
releases are easily equivalent to 10
3
-10
4
W/m
2
, enough
