land area contribute 60% of the total sediment discharge (Milliman 1991). e high sedi- ment loads carried by Asian rivers are a consequence of land-use practices, particularly land-clearing practices for agriculture that lead to erosion, a situation likely to continue as a consequence of the expansion of agriculture in Africa, Asia, and Latin America (Hall 2002). A notable outcome of the supply of sediment and associated nutrients to the oceans is the increased frequency and intensity of anoxic conditions in recent years (Hall 2002).
ere are also situations where river regulation has caused a decline in silt transport to
downstream habitats, with reduced siltation along floodplains and in deltas and other down- stream ecosystems. is has occurred in the Mesopotamian Marshes, where large-scale drain- age is a bigger problem than silt-related changes in the downstream ecosystems (box 6.8).
Terrestrial ecosystems Hydrological changes that occur as a result of agricultural expansion, particularly into forests, are seldom thought of in terms of water management in agriculture, although such changes are of at least the same magnitude as those resulting from irrigation (Gordon and others 2005). is is an area in need of further research, especially as biofuels and tree
Desiccation of the Mesopotamian wetlands
The Mesopotamian wetlands, one of the cradles of civilization and a biodiversity center of global importance, used to cover more than 15,000 square kilometers in the lower Euphrates and Tigris Basins. Agricultural development and other drainage activities over the past 30 years have reduced them to 14% of their original size, and vast areas have been turned into bare land and salt crusts (Richardson and others 2005). The ecological implications have been severe, with drastic land deg- radation and impacts on wildlife, including bird migration and the extinction of endemic species, and on the ecology of the downstream Shatt el Arab and coastal sheries in the Persian Gulf. The local population of half a million Marsh Arabs have become environmental refugees.
The causes of this severe ecological degradation are complex. Some of the causes were inten- tional, the results of drainage efforts to reclaim marshland, deal with soil salinization, improve agri- cultural productivity, and strengthen military security in southern Iraq in the 1980s and 1990s. Other causes were unintentional and included both the large-scale consumptive water use in irrigation systems and the return of saline drainage, agricultural and industrial chemical pollution, and the loss of ood ow, with its load of silt and nutrients, linked to recent large-scale streamow regulation in
With the extent of existing regulation and degradation, the proposed rehabilitation of 30% of the Central Marshes upstream of the conuence of the Euphrates and Tigris could generate its own adverse impacts on aquatic ecosystems further downstream. The additional evaporation from just 1,000 square kilometers of restored open-water surfaces would consume an average ow of 67 cubic meters per second, or 25% of the original (pre-regulation) dry season ow, and reduce downstream streamow even further. Without an increase in the amount of water available, simply returning the water to upstream areas may not be enough to restore the marshes and could further reduce the ow of water to downstream areas.
Source: Partow 2001; Italy, Ministry for the Environment and Territory, and Free Iraq Foundation 2004.