Agriculture, water, and ecosystems: avoiding the costs of going too far
Interbasin transfers of water, particularly large transfers between major river systems as are being planned in India, for example, are expected to be particularly harmful to downstream ecosystems (Gupta and Deshpande 2004; Alam and Kabir 2004) and to ex- acerbate pressures from hydrological regulation (Snaddon, Davies, and Wishart 1999). Where these are being considered, scientific and transparent assessments of the benefits and problems are strongly encouraged. Junk (2002) has highlighted the similar adverse consequences on water regimes expected from the construction of industrial waterways (hidrovias) through large wetlands, such as the Pantanal of Maso Grosso, Brazil. e na- ture of expected changes depends on the amount and timing of water being transferred and so needs to be assessed case by case.
Shrinking lakes. ere are many instances where consumptive water use and water diversions have contributed to severe degradation of downstream ecosystem services. e degradation of the Aral Sea in Central Asia represents one of the most extreme cases (see box 6.2).
e desiccation of Lake Chad in West Africa is another example. It shrank from
25,000 square kilometers in surface area to one-twentieth that size over a 35-year period. However, there are competing explanations for this reduction. Natural rainfall variability is an important driver. e lake is very shallow, and at various times in its history it has assumed different states, with changes triggered by climate variability (Lemoalle 2003). It is unclear what role human-induced change has played, but different drivers include the withdrawal of irrigation water, land-use changes reducing precipitation through changes in albedo (the energy that is reflected by the earth and that varies with land surface charac- teristics), and reduced moisture recycling (Coe and Foley 2001).
Lake Chapala, the world’s largest shallow lake, situated in the Lerma-Chapala Basin in central Mexico, is another example of consumptive water use upstream affecting the size of a lake. During 1979–2001 water volume in the lake dropped substantially to about 20% of capacity due to excessive water extraction for agricultural and municipal needs. Average annual rainfall from 1993 to 2003 was only 5% below the historical average and efforts were made to reduce water use in irrigation, but still the amount of surface and groundwater used in the basin exceeded supply by 9% on average (Wester, Scott, and Burton 2005). Above average rains in 2003 and 2004 increased the water volume to about 6,000 million cubic meters. ere is still intense competition over water allocation, and environmental water requirements have yet to be determined, leaving the future of the lake and the allocation of water for urban and agricultural purposes under threat.
e high variability in lake volume in both Lake Chad and Lake Chapala means that
the people depending on ecosystem services from these basins need to have a high adaptive capacity to cope with the rapidly changing circumstances, whether induced by people or nature.
Shrinking rivers. Consumptive use and interbasin transfers have transformed sev- eral of the world’s largest rivers into highly stabilized and, in some cases, seasonally non- discharging, channels (Meybeck and Ragu 1997; Snaddon, Davies, and Wishart 1999; Cohen 2002). Streamflow depletion is a widespread phenomenon in tropical and sub- tropical regions in rivers with large-scale irrigation, including the Pangani (IUCN 2003),
Worldwide, large artificial impoundments hold vast quantities of water and cause significant distortion of flow regimes, often with harmful effects on human health