Geoscience Reference
In-Depth Information
The two critical mechanisms of water transport are
overland flow and infiltration. Infiltration affects the
development of vegetation on the pavement, whereas
overland flow transfers water to plants adjacent to the
pavement. In addition, water flow redistributes sediment,
organic litter and seeds. Many desert pavements are
largely devoid of vegetation owing to the absence of soil
moisture (Musick, 1975) caused by the reduced infiltra-
tion capacity of the soil and salt accumulation at shallow
depths. Where plants occur, they are smaller in stature
and lower in density than on nearby nonpavement areas
(McAuliffe and McDonald, 2006). The low infiltration ca-
pacity of the pavement enhances runoff, channelling water
to adjacent wash zones or dune areas where plants grow
vigorously (Figures 9.11 and 9.12). The redistribution of
runoff and retardation of plant growth is a function of
evolutionary changes to the hydrologic properties of the
pavement, discussed below.
a favourable soil moisture environment in the adjacent
channels.
Drainage density on pavements may increase as a func-
tion of decreasing infiltration capacity over time. On Cima
volcanic field pavements, drainage density (km/km 2 )in-
creases with age: a 0.14 my flow has a density of 0.6,
whereas a 0.99 my old flow has a density of 14.9 (Wells
et al. , 1985). Runoff increases as the soil becomes plugged
with carbonates and clay and distinct soil horizons develop
(Amit and Gerson, 1986), eventually eroding away pave-
ments. At Cima, fluvial stripping has reduced the mantle
thickness of the older flow surfaces (Wells et al. , 1985)
and in the Nahal Zeelim sequence of alluvial fans in Israel
gullying is widespread (Amit and Gerson, 1986).
Where pavements are extensive, their low permeability
gives rise to rapid rates of runoff and the potential for
flash flooding. The spatial distribution of different surface
classes can be assessed using satellite data and the poten-
tial for flooding determined by hydrological modelling
(Foody, Ghoneim and Arnell, 2004).
9.9.1 Infiltration in pavements
and runoff potential
9.9.2 Ecohydrologic relationships and vegetation
associations
As pavements and soils evolve, their hydrologic properties
change. In the primary stages of soil evolution, porosity
(30-40 %) and permeability (equivalent to 60-80 mm/h
of precipitation) are high, and runoff is negligible (Amit
and Gerson, 1986). These properties diminish with time.
The introduction of fines and salts into the soil profile
retards the infiltration of water (Sena et al. , 1994; Young
et al. , 2004). As a result, stone pavement surfaces with
mature soils, embedded fragments and a well-knit surface
texture tend to have very low infiltration rates, with little
absorption of rain (Wells et al. , 1985; Amit and Gerson,
1986; Valentin, 1994; Wood, Graham and Wells, 2005).
The infiltration rates decrease with increasing clast cover
(Abrahams and Parsons, 1991) and change with soil age
(Young et al. , 2004). Thus, runoff is relatively high from
well-developed pavement soils.
Sheet flow from pavement funnels water into nearby
drainage courses (Symmons and Hemming, 1968). In the
Eastern Desert of Egypt, desert pavement had the lowest
infiltration rates of the surface types studied by Foody,
Ghoneim and Arnell (2004) (0.7 mm/h), whereas uncon-
solidated wadi deposits had the highest (140.1 mm/h) .The
depth of infiltration is marked by the location of the wet-
ting front. Following 52 mm of precipitation in the Sono-
ran Desert, the wetting front in the central area of a well-
developed varnished pavement was 139 mm, increasing to
271 mm just outside the pavement margin (McAuliffe and
McDonald, 2006). The runoff from the pavements creates
There are two basic plant associations with accretionary
pavements. The first is vegetated ephemeral washes that
lie adjacent to and receive runoff from the pavements and
the second is 'islands' of desert scrub development within
the pavement itself.
On dissected alluvial fans (Chapter 14), pavements
often form elongate interfluves. The relatively imper-
meable pavements shed flow to adjacent ephemeral
washes (Wood, Graham and Wells, 2005; McAuliffe and
McDonald, 2006). In the channels, water infiltrates
more deeply, leaching salts (Wood, Graham and Wells,
2005) and providing a favourable environment for plants
(Figures 9.11 and 9.12).
On the surface of the pavement itself, vegetation is
largely lacking owing to the saline nature of the soil and
its limited infiltration capacity. However, the presence of
plant scar mounds and depressions indicates that long-
lived perennial plants were more common in the past
(McAuliffe and McDonald, 2006). In the Sonoran Desert
of Arizona, modern vegetation is largely limited to stream
channels, but prominent, light-coloured plant scar mounds
(up to 25 cm in elevation and 200-600 cm in diameter) are
widespread on old (probably Pleistocene age), varnished
pavements of alluvial fans. The light surface colour is
a result of the lack of varnish on surface clasts, coat-
ings of pedogenic carbonate on noncalcareous clasts and
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