Desertification in Africa, Asia and Australia: Causes, consequences, solutions
by
Williams, Martin
Geographical & Environmental Studies, University of Adelaide, Adelaide, South Australia 5005, Australia.

Introduction: Desertification is land degradation in arid, semi-arid and dry sub-humid areas resulting from climatic variations and human activities. Consequences include · accelerated soil erosion by wind and water · salt accumulation in the surface horizons of dryland soils · a decline in soil structural stability with an attendant increase in surface crusting and surface runoff and a concomitant reduction in soil infiltration capacity and soil moisture storage · replacement of forest or woodland by secondary savannah grassland or scrub · an increase in the flow variability of dryland rivers and streams · an increase in the salt content of previously freshwater lakes, wetlands and rivers · a reduction in species diversity and plant biomass in dryland ecosystems.

Three case studies, drawn from three continents, demonstrate the importance of maintaining the native vegetation cover and provide the basis for deducing three principles of sustainable land use.

Deforestation and accelerated soil loss in Ethiopia Ethiopia has the potential to be one of the most successful agricultural nations in Africa, being richly endowed with deep, fertile volcanic soils, a range of microclimates and a great diversity of natural ecosystems. However, rapid rates of forest clearing this century in areas of steep slopes and seasonally torrential rains have resulted in accelerated loss of soil from many upland regions.

Traditional farming methods recognized that soil losses during cultivation were high and so allowed long years of fallow for the soils to recuperate. Mean annual rates of soil loss amount to about 40 t/ha (2 mm/a) on mountain slopes, but attain rates of over 300 t/ha (15 mm/a) during cultivation years, or some 5-10 times more than in non-mountainous areas. The increasing demand for land has meant a reduction in fallow to virtually zero and an expansion of the area under cultivation. In one region in Gojjam Province the area cultivated rose from 40

Removal of forest in Ethiopia and other tropical uplands can alter the local hydrological balance, increasing runoff and soil erosion, and reducing infiltration and the perennial maintenance of springs and stream headwaters. The downstream effects are not always confined to the country of origin. Within ten years of its completion, The Roseires dam was built on the Blue Nile near the Ethiopia-Sudan border to provide irrigation water for the central Sudan. By 1996, the capacity of the Roseires reservoir had been reduced by almost 60

In order to achieve a more sustainable form of agriculture, the local farmers need to be actively involved in soil and water conservation measures and a programme of long-term re-afforestation of steeplands and interfluves initiated as a matter of urgency.

Overgrazing and desertification in northern China The Alashan region of Inner Mongolia in northern China is one of the driest regions in China and covers an area of about 270, 000 km2. Rainfall declines from about 300 mm in the east to less than 50 mm in the west. Mountains occupy roughly 10, 000 km2 of the area and are flanked by gently sloping sand and gravel alluvial plains. These cover about 91, 000 km2. There are three major active dunefields that cover a total area of about 81, 000 km2. Fixed and semi-active dunefields cover about 90, 000 km2 and are the areas most vulnerable to desertification. Until the 1950s many of these low dunefields and sandsheets were covered in a relatively dense cover of shrubs, trees and grasses. Since that time the human population has doubled and livestock numbers have tripled. In addition, there have been a number of severe droughts, including the exceptionally severe 1989 drought.

The combination of greatly increased stock numbers, the influx of immigrants from the south, and the occurrence of sporadic but severe droughts over large tracts of Alashan have resulted in widespread and locally severe desertification. Official local estimates suggest that some 30, 000 km2 of land are now severely degraded and that the rate of desertification is increasing by about 1, 000 km2 each year.

There is widespread concern over the perceived increase in sand and dust storms and the decline in quality of pasture. Accelerated soil erosion by wind and water has increased since the 1950s. Former agricultural settlements immediately west of the Helan Shan ranges have been abandoned and the fine-grained alluvial soils are extensively gullied. In places, a single tree is all that remains of the 1950s riparian woodland. In the sand deserts north and west of these mountains, removal of the plant cover of previously vegetated and stable dunes through overgrazing by goats and sheep has reactivated many of the dunes. Monitored rates of dune advance range from more than 10 m/a near the Yellow River to less than 1 m/a further inland. The dunes along an 80 km long stretch of the left bank of the Yellow River opposite the industrial city of Wuhai are currently advancing from the northwest at rates of up to 10 m/a. An estimated 80 million m3 of sand is being blown into the river each year.

Control of sand movement into the Yellow River is an important element of China’s national plans to combat desertification. Major re-afforestation of the active dunes in this sector has now begun. The key issue is to minimise grazing pressure and not to exceed the carrying capacity of the vegetated dunefields. Dune stabilisation using the straw mulch chequer-board technique combined with forest shelter-belt planting is both feasible and desirable in areas of potentially high agricultural and economic productivity or major strategic and environmental importance such as close to the Yellow River and along the main railway line.

Tree clearing and dryland salinity in southern Australia Clearing of the deep-rooted eucalyptus trees from extensive regions of southern Australia since the onset of European settlement some 200 years ago has caused local and regional water-tables to rise, bringing dissolved salts to the surface, resulting in widespread dryland salinization. Half the original woodland and forest that grew 200 years ago has now gone from Australia. In terms of land degradation, the repercussions have been equally dramatic, and the process continues. In the ten years before 1993, Australia cleared an average of 500 000 ha of woodland and scrub, equivalent to two football pitches a minute. In 1990 Australia cleared 650 000 ha (more than half the area cleared in the Amazon Basin).

One insidious consequence of this tree clearing only became apparent many years later. The deep-rooted eucalyptus trees acted as natural groundwater pumps and kept the local water table well below the rooting depths of wheat, barley, improved pasture and other crops widely grown in the southern third of the continent. Groundwater recharge under native vegetation is 1-2 mm/a. Under wheat cultivation it is 40-120 mm/a. Once cleared, the groundwater levels rose, slowly but inexorably, bringing dissolved salts to the surface.

This process of dryland salinization has resulted in massive loss of productive agricultural land in many parts of southern Australia. More than 2.5 million ha of former agricultural land in Australia are now unusable because of dryland salinity. The cost to the Australian economy is nearly

Date received: January 27, 2004

Copyright © 2004 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # camu-33.