Large-Scale Management: Strategies and Water Transfer

Making more water available where and when it is needed

Increasing water supply does not always mean finding a new source. Water may already be available, but in the wrong place or at the wrong time. A water surplus means that supply exceeds demand; a water deficit means that demand exceeds supply. Management can store water for later, move it to another area, or turn seawater into usable freshwater.

These approaches often work together. A reservoir might store river water, while a canal carries that water to a distant city.

Diverting supplies and increasing storage

Diversion changes the route taken by water. Some river water can be directed into canals or storage areas instead of continuing along its original course. This makes it available elsewhere or keeps it for a later period of shortage.

Storage can be above ground in reservoirs or underground in permeable rocks that hold water. Underground storage reduces losses through evaporation because the water is not exposed at the surface. However, constructing diversion and storage infrastructure can be expensive.

The important distinction is that diversion changes where water goes, while storage changes when it can be used.

Dams and reservoirs: saving wet-season water

A dam is a barrier across a river. Water builds up behind it to form a reservoir. During wetter periods, the reservoir collects water; during drier periods, stored water can be released for homes, industry and irrigation—the watering of crops.

Controlled releases can also reduce downstream flooding, and some dams generate hydroelectric power. However, building a large dam is costly. Land flooded by the reservoir may include homes, farmland and wildlife habitats, so people may have to relocate. Holding back water changes the flow downstream, while evaporation from the reservoir reduces the amount available for use.

Storage therefore improves reliability, but does not create an unlimited supply: it still depends on water entering the reservoir.

Water transfers: moving surplus water to deficit areas

A water transfer scheme moves water through canals and pipelines between places, often between river basins. This allows an area with high demand or limited rainfall to use water from a wetter region.

Transfers can support drinking-water supplies, farming and industry. Their main limitation is that the supplying region must have enough water to spare. Removing water can reduce supplies for its own people and ecosystems. Large networks are also expensive to build, and greater supply in the receiving area may encourage wasteful use.

Desalination: making freshwater from seawater

Desalination removes salt from seawater. One method is reverse osmosis: seawater is first filtered to remove particles, then pushed under high pressure through very fine membranes. Water passes through, while much of the salt is left behind. The freshwater is prepared for use, stored and distributed; the concentrated salty waste is returned to the sea.

Desalination draws on a plentiful source and can increase supply in dry coastal regions. It is used extensively in the Middle East. However, plants are expensive and require large amounts of energy. Their carbon footprint depends on how that energy is generated, and transporting water inland adds costs. Concentrated salt waste can also damage marine ecosystems. Desalination therefore brings benefits but requires careful management of energy use and waste disposal.

Example: China's South–North Water Transfer Project

China's South–North Water Transfer Project moves water from the wetter south towards the drier north, including the region around Beijing and Tianjin. Population growth, manufacturing and irrigated farming have increased demand in this northern region. Heavy reliance on groundwater has led to over-abstraction and ground subsidence in parts of Beijing.

The project uses major water-transfer infrastructure, including canals and pipelines. Its central section was completed in 2014, but this did not mean that the entire project was finished.

The scheme supplies industries and large-scale farms, helping economic activity and food production. It also reduces water insecurity and dependence on groundwater. Lower groundwater abstraction helps protect underground water stores, while improved water quality is another benefit.

These gains come with substantial costs. By 2014, US$79 billion had been spent. People have been relocated to make way for dams, reservoirs, pipes and canals. In some southern areas, local water use has been restricted to leave enough water for transfer. Droughts in the south have also reduced the amount available, showing that a supplying region's surplus is not guaranteed.

Weighing up the scheme

The project addresses a major mismatch between water supply and demand and brings important benefits to northern cities, farms and industries. However, those benefits are unevenly shared: people displaced by construction and communities facing restrictions in the south bear some of the costs.

Overall, the scheme can substantially improve northern water security, but it cannot guarantee water during drought everywhere. Its success depends on maintaining adequate supplies in the south as well as meeting demand in the north.

Supply strategies

• Diversion: redirects water to another place or storage area.
• Storage: saves water for later; underground storage reduces evaporation.
• Dams and reservoirs: store wet-period water for dry periods. Benefits include controlled flow and possible hydroelectric power; costs include flooding land, relocation, habitat loss and reduced downstream flow.
• Transfers: move water from surplus to deficit areas through canals and pipelines. Expensive; may reduce supplies in the donor region.
• Desalination: removes salt from seawater. Increases freshwater supply, but requires energy and produces potentially harmful salty waste.

China: South–North Water Transfer Project

Purpose: move water from the wetter south to the drier north, including Beijing and Tianjin. Growing demand and groundwater over-abstraction created a need for additional supplies.

Development: central section completed in 2014; US$79 billion spent by 2014.

Advantages: supplies industry and irrigated farming; reduces water insecurity and groundwater abstraction; improves water quality.

Disadvantages: high cost; relocation; restrictions on some southern communities' water use; southern droughts reduce transfer supplies.

Judgement: major benefits for northern users, but social costs and dependence on southern supplies limit its overall success.