wetland facts

Indirect Use Values

Flood control

Floods are a regular feature of Mediterranean rivers and major protection works have cost billions of dollars. Natural wetlans can play a role in reducing damage from floods. Wetlands of various types can contribute to the reduction of floods. Lakes, marshes and swamps can act as reservoirs that fill up with excess water. Floodplains are the natural extensions of rivers when high discharges occur, acting as temporary rivers that conduct the excess water.

The effect is a lowering of the peak water levels, which is especially useful when surrounding areas are sensitive to flooding and are heavily utilised. Without wetlands run off water is not checked and can lead to flooding.

Sebkhat el Kelbia is a wetland area situated in a vast depression at the heart of Tunisia. It receives the waters of three of the biggest gorges in Central Tunisia and acts as a sponge, retaining flood waters until they slowly seep underground or evaporate, thus providing protection from floods. Soil transported by surface run-off water and deposited into the wetland reservoir however limited its capacity to store water. The 1960s and early 1970s saw very heavy floods that swept the Kairouan area causing loss of life and severe damages.

Groundwater recharge

Where the soil and geology of a wetland system is appropriate, water may be able to filter down into the local or regional groundwater system. This may then be available for exploitation by local or regional populations for drinking water or irrigation.

This can be a very significant wetland value, particularly in semi-arid to arid areas where water resources are scarce.

However, there are few case studies that have quantified it, due to the large research effort that is required.

In addition, there are a number of hydrological reasons why the relationship between wetlands and groundwater recharge may not always be as evident as it seems.

Prevention of saline water intrusion

Coastal freshwater wetlands can help to maintain supplies of drinking, washing and irrigation water to local communities and prevent salinization of the soil. In low-lying coastal wetlands where the underlying substrate is permeable, a wedge of freshwater frequently overlies deeper saline water, preventing it from moving to the surface.

Removal or reduction of this freshwater wedge through degradation (e.g. through groundwater abstraction) or removal of a wetland can allow the deeper saline water to penetrate the land surface, causing water quality problems.

Shoreline stabilisation and storm protection

Coastal wetlands can help prevent or reduce erosion of coastlines by acting as a physical barrier to seawater intrusion. Plant roots can bind and stabilise soil/sediment and vegetative matter, reducing erosion. In addition, wetland vegetation can shield from damage caused to farmland or buildings by strong salt-laden wind. This can be a sustainable and highly cost effective method of sea defense.

Deltas are formed from sediment brought down and deposited by the river. As sediment arrives the delta expands naturally outwards into the sea.

However deltas depend on sediment deposition to exist. The Nile delta in Egypt, has seen a reduction in silt load as a result of sediment trapped upstream in the Aswan damn. this has contributed to a retreat of the delta by two kilometers between 1971 and 1988.

Water quality improvement

Wetlands can act as filters trapping pollutants found in the water in more or less permanent ways. This means that upto a certain extent wetlands have the ability to clean themselves of pollutants introduced into the waters as well as filter out pollutants before water seeps into underground aquifers. Pollutants can be permanently removed or changed into a more benevolent form, stored within plants or chemically attached to sediment.

Rice plants in the delta area of Camargue, in France, purify waters from nitrate used by farmers to fertilize the crops. High moisture levels combined with low oxygen conditions are key to this process that involves plants capturing nitrates and transforming them into a gas which is then circulated back into the atmosphere.

Above a certain threshold self-cleaning will not be possible which and the wetland functions will be disrupted. For instance if a wetland receives too many nutrients the food chain will change with possible negative impacts for the wetland's plants and animals.

Some pollutants can be chemically stored for long periods of time only when specific soil, chemical and hydrological circumstances are met. In such cases, using a wetland for water purification comes with a price; it is essential that the consequences are carefully evaluated.

Carbon sequestration

Certain wetland types accumulate large amounts of carbon in the form of undecomposed organic material.

Peatlands are particularly significant in this respect storing over 16-24% of all carbon in soils whilst covering only 3% of all global land area.

There is great concern over increases in carbon dioxide in the atmosphere since this contributes to global warming. Destruction of wetlands, especially peatlands, will contribute to this effect.

Climate change mitigation

Wetlands may affect the microclimate in the area of the wetland itself and its immediate surroundings. Evaporation from wetlands can maintain local humidity and rainfall levels. As yet, little research has been performed on these processes.