Environmental Engineering Reference
In-Depth Information
rates are no more than 10 B, in closed canopy forests
they may be merely 4-5 B, but in badly damaged crop-
lands they may reach nearly 2 kB. Actual field measure-
ments of various landscape denudation rates have been
infrequent (Rapp 1960; Caine 1976; Clayton 1997;
Bierman and Nichols 2004). Their results indicate that
most geomorphic processes proceed at power densities
of just 10
7
to 10
6
W/m
2
. Maximum reported rates
per square meter are 200 nW for surface wash, 500 nW
for snow avalanche debris and rockfalls, 2 mW for soil
creep and solifluction, 20 mW for solute transport, and
50 mW for earth slides and mudflows.
Given this variability, it is not surprising that the total
detrital sediment flux from continents to oceans remains
uncertain (W. W. Hay 1998). A widely quoted rate of 20
Gt/a (Milliman and Syvitski 1992) would prorate (with
average density of 2.5 g/cm
3
) to about 65 B for ice-free
land (glacial denudation rates are extremely difficult
to estimate). This rate implies erosion of some 160 t/
km
2
a and (assuming, once again, the mean continental
elevation of 840 m) is equivalent to a change of roughly
165 PJ of potential energy, or about 5.2 GW, a mere 35
mW/m
2
of continental surfaces. (If recent erosion rates
were roughly equal to the increase in potential energy of
continents, then the sum of 5.2 GW also represents the
power going into the uplift of terrestrial formations.)
This is a minuscule fraction of potential energy loss in
runoff or geothermal flux, and an even smaller fraction
of solar heat absorbed by soils. Geomorphic power den-
sities are tiny, but their durations are immense. The Ro-
mans knew it well: gutta cavat lapidem, non vi, sed saepe
cadendo (the drop hollows out the stone by frequent
dropping, not by force).
Finally, a few facts about lightning, perhaps the most
spectacular atmospheric energy conversion. Cloud con-
vection and a proper mix of water and ice particles deter-
mine the electrification of clouds (Black and Hallett
1998). High current strokes (10-100 kA) originate
mostly in cumulonimbus clouds, their power going up
with the fifth power of the cloud size (doubling the
cloud increases the total power 30-fold). More than half
of all discharges are within these clouds. Cloud-to-
ground strikes are a major cause of forest fires, and elec-
tricity outages and return strokes are their main visible
event. The average cumulonimbus contains about 4 GJ
of energy that can be dissipated in lightning. Cloud-to-
ground discharges dissipate 10
9
-10
10
J, and estimates
of input energy in return strokes range from less than
5 kJ/m to more than 100 kJ/m (Rakov and Uman
2003). The entire flash lasts usually no more than 200
ms, but the bulk of energy is dissipated by a return stroke
within less than 100 ms, resulting in enormous power of
10
9
-10
12
W.
Globally lightning dissipates merely about 4
10
7
of the atmosphere's convective energy, and spectral
measurements of visible stroke light indicate typical
releases of 200-2000 J/m, or less than 5% of all flash
energy and (with a 5-km stroke) total optical power of
up to 1 TW. Most of the discharge goes into heating
the atmosphere (peak temperatures are @30,000 K,
more than five times the Sun's photosphere) and into
producing the acoustic energy of thunder, which is typi-
cally three times greater for ground flashes than for
in-cloud discharges. Data from the Optical Transient
Detector (MicroLab-1 Satellite) indicate 1.4 billion
flashes worldwide per year (44/s), with the highest fre-
quency in the Congo Basin, ten times more activity over
land than over sea, and nearly 80% of all discharges
taking place between 30
S and 30
N (Christian et al.
2003).
