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Land use: a critical factor
Box 2: Land cover – a key to water quality

There appears to be a clear link between the presence of forests and the quality of water coming out of acatchment. A large bottler of mineral water in Europe,draws its main water supplies from heavily-farmed watersheds, where the run-off of nutrients and pesticides threatens the aquifers upon which the company depends. In response, the company has found that reforesting sensitive infiltration zones and switching to organic farming practices is proving to be cheaper than building water filtration plants.

Some water authorities also have made the link between protecting for water supply and protecting for nature. The city of New York applies a catchment protection approach to maintaining its high quality drinking water.
A land and forest resource protection strategy is being implemented that will result in substantial savings for New York City, as compared to putting in a new water treatment plant. The start-up cost for this strategy is estimated to be US$1 000–1 500 million over 10 years, as compared to the cost of US$6 000–8 000 million, plus US$300–500 million annual operating costs, otherwise required for a treatment plant. In Ecuador, about 80 per cent of Quito’s drinking water comes from two protected areas, the Cayambe Coca Ecological Reserve and the Antisana Ecological Reserve. A nominal water use fee on citizens of Quito, together with one per cent of revenues of hydroelectric companies, are used to finance conservation of the reserves.
On a global scale, about one-third of the largest cities obtain a significant proportion of their drinking water directly from protected areas.

Source: Dudley and Stolton 2003, Echavarria 2002

Land cover and land use changes have great influence on freshwater resources around the world. The protection of catchment areas is extremely important in maintaining high freshwater quality (Box 2). Forest cover is particularly beneficial. Recent literature shows a clear link between forests and the quality of water from a catchment, a more sporadic link between forests and the quantity of water available, and a variable link between forests and constancy of flow (Dudley and Stolton 2003). Well managed natural forests almost always provide higher quality water, with less sediment and fewer pollutants, than water from other catchments. Forests therefore, often provide the basis for integrated management of water resources. Removal of forest can adversely affect water supplies, putting people at risk and damaging the environment (FAO 2003a).

Both aquatic and terrestrial ecosystems play important roles in regulating freshwater flows. Wetlands, for instance, buffer flood flows and filter incoming water, among other benefits (Table 1). This natural capacity however, has been much reduced due to human activities. About half of the world’s wetlands were lost during the 20th century, primarily through conversion for agriculture (Finlayson and Davidson 1999). As we understand more about aquatic ecosystem dynamics, technologies are being developed that mimic their functions, such as the construction of artificial wetlands for the purification of water (Box 3).

 

 

 


Table 1: Benefits of wetlands
Ecological functions
Products

Water storage and purification

Flood control

Shoreline stabilization

Storm protection

Biomass export


Maintenance of ecosystem stability

Maintenance of biodiversity

Groundwater recharge and discharge

Water transport

Sediment retention and erosion control

Nutrient storage and recycling

Micro climate stabilisation

Maintenance of integrity of other ecosystems

Forest resources

Fisheries

Agricultural resources

Medicinal resources

Raw materials for building, construction, and industrial use

Wildlife resources

Forage resources

Water supply

Genetic resources

Tourism and recreation opportunities

Energy supply

Sources: adapted from Dugan 1990 and Schuyt and Brander 2004

 

Box 3: Technology at work – biological water purification
Water purification is based on physical, chemical and biological processes. While we tend to rely on human engineered systems, technologies based on natural processes can also be used to purify water and treat wastewater. These include phytotechnology based on wetlands, lagoons, grass-filtration, soil purification, and soil aquifer storage and treatment. These technologies, shown in the graphics below, link water and food production, and are generally suited to developing communities.
Natural purification systems designed, constructed, operated and maintained in the same way as engineered systems perform just as well and usually cost less to construct and operate. The lower cost compared to mechanical treatment, which occupies less land area, depends on the sufficient availability of free or cheap land.
Constructed wetlands or phytoremediation:
This is a technology for treating stormwater or wastewater. A constructed wetlandconsists of a gravel bed on which suitable wetland plants are grown. As water passes through the substrate, it is purified through the activity of bacteria attached to the gravel, plant roots, soil and other particles.
Soil and grass filtration for purification of wastewater:
Soil filtration relies on filtration by soil particles and bacteria growing on the surfaces of the soil particles and plants (usually grasses) which take up water and nutrients (nitrogen and phosphorus), reducing the concentration of these nutrients. Soil filtration is used during the dry season. Grass filtration is used in the rainy season when the soil is saturated by water. Grasses can be grazed by livestock.

 

 

Soil aquifer treatment for purification and storage of treated wastewater:
Soil aquifer treatment relies on filtration through the soil and purifying action of bacteria attached to soil particles. Purified water is stored in the groundwater aquifer and can be drawn for
agricultural purposes, for example.
Symbiotic activity between algae and bacteria in a wastewater lagoon to purify sewage:
As wastewater flows through a lagoon, bacteria consume the organic carbon in the waste. In assimilating the organic carbon, bacterial respiration produces carbon dioxide (CO2) and takes up oxygen (O2). Algae utilize the carbon dioxide for photosynthesis and in turn produce oxygen required by the bacteria, hence the symbiotic activity. Solids settle to the bottom of the lagoon, and pathogenic bacteria die-off in competition with other bacteria in the lagoon.
Several lagoons in series are more effective and the last lagoon can be used for aquaculture.
Source: UNEP 2002a adapted from Shiklomanov 1999

 


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