Geography Reference
In-Depth Information
conservation and land management infrastructure
(Plate 13.2). An interesting example of the effects
of high population growth rates, combined with
the effects of outmigration, can be found inYemen
(Box 13.1).
Changes in the political control of the resource
base is a further factor that affects both land
management practices and population dynamics
and, as a consequence, desertification. These
changes may be brought about by economic
polices, such as the adoption of neo-liberal
economic models and GATT, or through
fundamental political change. Important
contemporary examples of the breakdown in the
control of land management in dryland areas due
to a shift in the political systems can be found
throughout Central Asia in the post-Soviet era
(Box 13.2).
The consequences of demographic and policy
triggers often initially manifest themselves in terms
of vegetation destruction—due to overgrazing and
overbrowsing, vegetation clearance for cultivation,
or biofuel collection. The subsequent breakdown
of the complex vegetation-soil-water relationships
found in drylands means that vegetation's many
environmental benefits are diminished or lost
altogether. The resulting soil degradation and
reduced soil water availability lead to a reduction
in soil fertility (used here as a qualitative term
covering a wide range of specific soil characteristics,
but which provides a useful guide to land
productivity in terms of potential crop or forage
production). The reduction in potential land
productivity has clear implications for household
livelihoods and national food security.
Climatic forcing and desertification
The second scenario occurs when climatic forcing
(in terms of droughts, floods and longer-term
desiccation) affects the resource base. While
climate has direct affects on the soil-water-
vegetation environment (e.g. in terms of increased
evapotranspiration or vegetation destruction by
flooding), the greatest impact of climatic forcing
factors is probably indirectly through their impacts
upon dryland populations and their economic
activities.
There is general agreement that desiccation of
most of the world's drylands has been occurring
since the middle of the last century (Goudie 1990).
This is important for understanding contemporary
dryland problems in two ways. First, it provides an
important underpinning to current distributions
of population, agricultural activities and urban
areas. Second, it can provide indications of the
likely impacts of future climate change in drylands.
Parry (1990), in reviewing the evidence for the
Plate 13.2
Ridge-top villages,
such as the one in this
photograph, are common
throughout northern Yemen.
They are typically surrounded
by flights of agricultural
terraces. A decline in rural
labour has meant that some
terraces have not been
maintained (see immediately
below the village). This causes
the terrace walls to disintegrate
and water runoff and topsoil
erosion to increase, resulting in
a decline in land productivity in
the uplands and increased
flooding in the
Tihama
(the Red
Sea lowlands).
