Geoscience Reference
In-Depth Information
disturbed plot with an initial stone cover of 41 % increased
to a 71 % cover following the experiment - close to the
77 % cover of the pre-disturbance pavement. About 10
runoff events are annually recorded at the site of the study,
suggesting that pavements would be able to regenerate on
an annual cycle following disturbance.
where recent high rates of population growth have put
pressure on available land, stones have been, and continue
to be, removed to permit cultivation. In Jordan, access to
groundwater has allowed increasingly intensive agricul-
ture and caused extensive clearance of pavements to pro-
vide new land. Following irrigation, there are chemical
and physical changes to the soil, including development
of a surface crust (Allison
et al.
, 2000).
There is also documentation of pavement stones being
cleared for agriculture in the past. The ancient settlers of
the Negev removed pavement stones more than a thousand
years ago (Sharon, 1962). The renewal of the pavement
since that time is evidence of the dynamic nature of the
surface.
9.8.4
Earthquakes
In seismically active areas, earthquakes displace surface
clasts, thereby influencing pavement development. Haff
(2005) noted a number of disturbances to desert pave-
ments generated by the October 1999 Hector mine earth-
quake in the Mojave Desert (with a moment magnitude,
M
W
, of 7.1). These included zones of wholesale gravel
displacement, rotated and displaced cobbles, and moat
formations around loosened boulders. Displaced clasts
tended to move downslope. The effects of seismic shak-
ing are similar to animal displacement, rainbeat and wash
processes.
9.9
Ecohydrology of pavement surfaces
In most deserts, rainfall infiltrates only to shallow depths
and soil moisture values are low. As a consequence, there
is little leaching of soils and dissolved materials are de-
posited in the upper horizons. Spatially, there is consider-
able variability in infiltration, as water seeps into porous
surfaces, including sandy soils, soils around plants, and
stream beds or other alluvial surfaces, but runs off from
less permeable covers such as pavements (Cornet, Del-
houme and Montana, 1988; Abrahams and Parsons, 1991;
Dunkerley, 2002; Belnap, 2003).
Ecohydrologic studies examine the relationship be-
tween plant cover, soils and surface characteristics. In
deserts, the striking differences in soil properties that
commonly occur over short distances may create a mo-
saic landscape. A binary or two-phase system of soils of-
ten develops, with one component supporting a relatively
dense vascular plant cover (patches, groves or bands) and
the other component consisting primarily of unvegetated
ground. The bare ground is relatively impermeable and
sheds water downslope to vegetated bands. In the plant
patches, the soils are soft and friable, with high infiltration
capacities. As a result of the differences in soil properties,
the vegetated zones may receive water amounts close to
twice the climatological rainfall (Cornet, Delhoume and
Montana, 1988), whereas the bare patches are more arid
than rainfall totals suggest. The result is a highly heteroge-
neous landscape (Cornet, Delhoume and Montana, 1988;
Seghieri and Galle, 1999; Dunkerley and Brown, 1997).
The patchiness of the vegetation may help to conserve
scarce water and nutrients within part of the landscape.
Areas of desert pavement in the American southwest ap-
pear to be part of this type of ecohydrologic system.
9.8.5
Off-road vehicle disturbance
Owing to their trafficable nature, pavements are com-
monly traversed by off-road vehicles. Despite their strong
appearance, however, the pavements are easily disrupted
by traffic. Tracks from World War II military exercises
(1942-1944) are still discernable in the Mojave Desert,
suggesting that the time for full recovery may be decades
(Belnap and Warren, 2002). Off-road vehicles produce
both compression and shear forces, and penetration resis-
tance is significantly higher inside than outside the tracks.
The recovery of soils in hot deserts is slow, as clay mineral
expansion during wetting and drying cycles, freeze-thaw
heaving and biological activity are limited. Vehicle activ-
ity affects the distribution of clast sizes, pushing the desert
pavement downwards into the soil and allowing the rut to
infill with smaller particles (Belnap and Warren, 2002).
Desert pavements have been studied as part of military
characterizations of trafficable surfaces. In the Sonoran
Desert, vehicles were tested over an endurance course in
which the degree of desert pavement development was
qualitatively ranked from 'none' to 'strongly developed'
(Bacon
et al.
, 2008).
9.8.6
Removal of stones for agriculture
Desert pavement surfaces often occur in regions that are
too arid to permit widespread agriculture. Nonetheless,
