Geoscience Reference
In-Depth Information
shows these thresholds and yield declines for some com-
mon crops.
Salinisation is one of the most clear-cut ways in which
mismanagement can lead to desertification, an issue that
is otherwise surrounded by controversy (see below). Irri-
gation schemes in some of the dryland countries of Asia
are among the most seriously affected. In Pakistan, where
irrigated land supplies more than 90 % of agricultural
production, salinity problems affect about a quarter of the
irrigated area or approximately 11 % of the country's land
area (Middleton and van Lynden, 2000). Most of the af-
fected soils are part of the Indus irrigation system, the
largest single irrigation system in the world. Salinity and
the associated problems of waterlogging are estimated to
cost farmers in Pakistan about 25 % of their production
potential for major crops (Qureshi
et al
., 2008).
susceptibility to erosive forces. Similar outcomes have
also been noted in areas where cultivation has expanded
into new zones that are marginal for agricultural use be-
cause they are more prone to drought or are made up of
steeper slopes that are more susceptible to erosion.
Probably the most infamous case of wind erosion came
after widespread conversion of grasslands to cereal cul-
tivation in the US Great Plains, which helped to create
the notorious Dust Bowl during a period of drought in
the 1930s (Worster, 1979). The most severe dust storms
(so-called 'black blizzards') occurred between 1933 and
1938, with maximum wind erosion taking place during the
spring of these years. At Amarillo, Texas, at the height of
the period, one month had 23 days with at least ten hours
of airborne dust and one in five storms had zero visibility
(Choun, 1936). In 1937 the US Soil Conservation Service
estimated that 43 % of a 6.5 million ha area in the heart of
the Dust Bowl had been seriously damaged by wind ero-
sion. Similar widespread deflation of soils resulted from
analogous expansion of cultivation into grasslands in the
Argentine Pampas during the 1930s and 1940s (Viglizzo
and Frank, 2006) and after the 1950s Virgin Lands Scheme
in the former USSR.
Increased wind erosion has also frequently been an
off-site result of human activities, particularly those as-
sociated with drainage or water diversion that has led to
the desiccation of water bodies. Dried lakebeds have, in
consequence, become major new sources of airborne dust
in several parts of the world (Table 22.3). A review of
anthropogenically desiccated playas and their associated
erosion is provided by Gill (1996). One of the best-known
examples has occurred in Central Asia over 50 years or
so, a period during which off-takes from two major rivers,
the Amu Darya and Syr Darya, to irrigate plantations,
predominantly growing cotton, have had dramatic con-
sequences for the Aral Sea (Middleton, 2002; Micklin,
2007). Expansion of the irrigated area in the former So-
viet region of Central Asia, from 2.9 million hectares in
1950 to about 7.2 million hectares by the late 1980s, re-
sulted in the annual inflow to the Aral from the two rivers,
the source of 90 % of its water, declining by an order of
magnitude between the 1960s (about 55 m
3
/yr) and the
1980s (about 5 m
3
/yr).
In 1960, the Aral Sea was the fourth largest lake in the
world, but since that time its surface area has more than
halved, it has lost two-thirds of its volume and its water
level has dropped by more than 20 m. The average wa-
ter level in the Aral Sea in 1960 was about 53 m above
sea level. By early 2003, it had receded to about 30 m
a.s.l., a level last seen during the fourteenth-fifteenth cen-
turies, but in those days for predominantly natural reasons
22.2.3
Accelerated erosion
Human-induced changes in erosion rates are well docu-
mented from drylands all over the world and can be at-
tributed to a wide variety of activities. In Australian range-
lands, for example, the introduction of both domestic and
feral herbivores by Europeans in the nineteenth century
is believed to have widely increased rates of soil erosion
(Wasson and Galloway, 1986). The landscape of arid west-
ern New South Wales is not atypical, having experienced
accelerated erosion by wind and water though sheetwash,
rilling, gullying and aeolian deflation (Fanning, 1999).
Large-scale military movements have been the cause
of enhanced deflation due to the disruption of desert sur-
faces in several cases, not least during the North African
campaign in the early 1940s (Oliver, 1945). Wind erosion
dramatically increased during the height of the fighting in
North Africa, but deflation had returned to pre-war lev-
els within a few years of the end of the campaign. The
effects are longer lasting in desert areas used for train-
ing, where large military vehicles continually compact
soil, crush and shear vegetation, and alter the structure of
plant communities. The US National Training Centre in
the Mojave Desert is a case in point (Caldwell, McDonald
and Young, 2006).
Agricultural activities on dryland soils have often re-
sulted in increased soil erosion. Any farmer who culti-
vates a crop that leaves substantial areas of soil uncovered
can expect rapid soil erosion during intense rain storms
or strong wind storms. Examples include woody dryland
crops such as almonds, olives and grapes. In other circum-
stances, erosion has been exacerbated by the introduction
of mechanised agriculture with its deep ploughing and
