Wetlands form transitional lands between fully terrestrial and aquatic systems. They are defined as areas of marsh, fen, peatland or water; whether natural or artificial; permanent or temporary; with water that is static or flowing, fresh, brackish or saline, including areas of marine water.

Wetlands have significant influence on the hydrological cycle (see box below). Hydrological processes are influenced by the storage capacity for water, the transmission loss from a wetland to an aquifer or the recharge capacity of the groundwater. Flood attenuation is one of the most important wetland functions. This flood attenuation service occurs primarily on large flood plains in the lower reaches of river basins where over bank flood water is stored in large hollows and depressions. Additionally, the flood wave is slowed and reduced by resistance caused by the roughness of the wetland vegetation, thus delaying and reducing floods downstream.

Clearly, flood attenuation only takes place if the storage is not full at the time the flood impinges. In wetlands that are saturated, little storage may be available for subsequent floods.

A study on the water quantity functions of wetlands based on 169 studies worldwide concluded:

  • Wetlands are significant in altering the water cycle;

  • Most wetlands in flood plains reduce or delay floods, but not always. In fact, more than one-third of headwater wetlands increase flood peaks;
  • Wetlands evaporate more water than other types of land and therefore in two-thirds of cases, they reduce average annual river flows;
  • In two-thirds of the wetlands, the flow of water in downstream rivers is reduced during dry periods;
  • Some floodplain wetlands on sandy soils recharge groundwater when flooded. But most wetlands exist because they overlie impermeable soils or rocks and there is little interaction with groundwater.

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Bullock and Acreman (2003)