Those areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under normal conditions do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs and similar areas.
The relationship among irrigated agriculture and it’s on effects on wetland an ecosystem has often been portrayed as one of a direct tradeoff between the human need for food versus nature. The reality, as revealed by this paper, is much more complex, as both systems to human and nature to may be adaptive. Where nature might adapt automatically, such as a waterfowl adapting to paddy rice as a replacement for natural wetland habitat, humans too adapt consciously. For example, as humans have learned about the valuable services wetlands provide, the response has been to find ways to preserve and restore wetlands. This is relatively achievable in the developed countries, which has access to funds and the institutional and legal capacity to impose no loss of wetlands, but it is much more difficult in the 3rd world countries where there are pressing needs for increased food production with the limited funds available.
Background on Wetlands and Irrigated Agriculture
There is a long history of development leading to the complete destruction of massive areas of wetlands particularly in the developed countries. In a generalized overview, the Organization for Economic Co-operation and Development (OECD 1996) stated:
Some estimates show that the world may have lost 50 percent of the wetlands that existed since 1900; whilst much of this occurred in the northern countries during the first 50 years of the century; increasing pressure for conversion to alternative land use has been put on tropical and sub-tropical wetlands since the 1950s.
By 1985 it was estimated that 56-65% of the available wetland had been drained for intensive agriculture in Europe and North America; the figures for tropical and subtropical regions were 27% for Asia, 6% for South America and 2% for Africa, making a total of 26% worldwide. Future predictions show the pressure to drain land for agriculture intensifying in these regions.
The global wetland area is generally estimated to be 7 to 9 million km2 (4-6 percent of the land surface of the Earth) (Mitsch and Gosselink 2000). “Wetland” many would agree that these are areas with high water tables contributing to a specific ecology. The most broadly accepted definition of wetlands is: Areas of marsh, fen, peat land or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt including areas of marine water, the depth of which at low tide does not exceed 6 meters´(Ramsar Convention 1971; Articles 1.1 and 2.1).
The services Wetlands provide
Food production, Groundwater recharge, Shoreline stabilization and storm protection, Water purification,Biodiversity,Water storage, including mitigating the effects of floods and droughts, Nutrient cycling and Sediment retention and export, Education and research; and Aesthetic and cultural value, Recreation and tourism and Climate change mitigation, Habitat for aquatic birds, other animals and plants, fish and shell fish production and timber production.
Irrigation has been practiced for at least 4,000 years, primarily because it allows for increased productivity through more optimal timing of water application. More recently, irrigated agriculture in combination with improved crop varieties and chemical inputs has led to 24 percent more food per person between 1961 and 1997, despite the population increase (Pilot Analysis of Global Ecosystems 2000). In addition to increase food security, there is a net increase in economic gains to farmers.
It is estimated that around 5 percent of agricultural land globally (264 million ha) is irrigated, with South Asia (35%), Southeast Asia (15%) and East Asia (7%) showing a high dependency on irrigation. China and India represent 39 percent of the global irrigated area and Western Europe and United States have 13 percent, while sub-Saharan Africa and Oceania have less than 1 percent of their agricultural land irrigated (Page and Wood 2000).
The Effects of Agriculture on Wetlands
1. Direct loss of wetlands due to draining and conversion to agricultural land.
2. Indirect loss of wetlands area due to water withdrawal from rivers and streams for irrigation.
3. Loss of wetland area and function due to damming for water storage.
4. Loss of seasonal wetlands due to changed hydrologic cycle from water storage.
5. Losses of wetland function due to salinization, sediment deposition, erosion, and eutrophication.
6. Pollution from use of pesticides and other chemicals; and Creation of wetlands.
Acreman, M.C.; Hollis, G.E.1996.Water Management and Wetlands in sub-Saharan Africa.
Glanz, Switzerland: IUCN-World conservation Union.
FAO (Food and Agriculture Organization).1995.Irrigation in Africa in Figures. Report No.7. Italy:
Hollis,G.E.;S.J.Penson,J.R. Thompson and A. R. Sule.1993a. Hydrology of the river basin.
In: The Hadeijia-Nguru Wetlands: environment, economy and sustainable
Development of a sahelian floodplain wetland.G.E. Hollis. ; W.M. Adams; M.Aminu-
Kano (eds).Glanz, Switzerland:IUCN.
Mitsch, W.J.; Gosselink., J.G.2000.The value of Wetlands: Impacts of scale and landscape
Setting. Ecological Economics, 35(1)25-33.
Morimoto, Y.; Morimura, A.; Ogar, N.1997. Several landscape concepts on the Aral sea crisis
Revealed by remote sensing. CERES International Symposium on Role of Remote
Sensing for Environment Issues (obtained from www.http://rosa.eni.osakafu-u.ac.jp/aral.html).
NBI (2005). National Nile Basin Water Quality Monitoring Report for Rwanda. Nile
Transboundary Environmental Action Project, Nile Basin Initiative (NBI), Kigali.
Ramsar.1991.Ramsar advisory missions: Report No.26, Egypt. Lakes Bardawil and Burullus.