Impacts of Surface Water Withdrawals for Irrigation

The storage and redistribution of water by dams, diversions, and canals has been a key element in the development of civilizations. Large-scale water control systems, such as on the Nile in Egypt or the Colorado River in the southwestern U.S. make it possible to support large cities and millions of hectares of agricultural land. As the population grows and water diversions increase, serious questions are being raised about the environmental costs of these large water management systems.

Agricultural water withdrawals are placing significant pressure on water resources in water-scarce regions around the globe (Figure 4.2.2). If more than 20 percent of a region's renewable water resources are withdrawn, the region is in a state of water scarcity and the water resources of the region are under substantial pressure. If the withdrawal rises to 40 percent or more, then the situation is considered critical and evidence of stress on the functions of ecosystems becomes apparent. More than 40% of the world's rural population lives in river basins that are physically water-scarce and some regions, such as parts of the Middle East, Northern Africa, and Central Asia, are already withdrawing water in excess of critical thresholds (FAO 2011).

In order to divert water from rivers, diversion structures or dams are usually constructed and create both positive and negative effects on the diverted river system. Dams can provide a multitude of benefits beyond their contribution to storage and diversion for agricultural uses. Dams can contribute to flood control, produce hydroelectric power, and create recreational opportunities on reservoirs. Negative impacts of dams and agricultural diversions include:

• Habitat fragmentation – blocks fish passage
• Reduction in streamflow downstream, which then results in changes in sediment transport, and in floodplain flooding
• Changes in water temperature downstream from a dam
• Evaporation losses from reservoirs in hot, dry climates
• Dislocation of people
• Sedimentation behind dam fills in reservoirs with sediment and reduces their useful lifespan

Figure 4.2.2. Global Distribution of Physical Water Scarcity by Major River Basin (FAO 2011)

Click for a text description of the global distribution of physical water scarcity image

The image is a world map showing the global distribution of water scarcity risk, categorized into three levels: Low (light yellow), Moderate (orange), and High (dark red). Most regions across North America, South America, Europe, and parts of Asia are shaded in light yellow, indicating low water scarcity risk. However, significant areas in North Africa, the Middle East, South Asia (including India and Pakistan), and parts of East Asia are shaded in dark red, representing high water scarcity risk. Moderate-risk areas, shown in orange, include regions in southern Europe, parts of Africa, and sections of South America and Australia. The map highlights that water scarcity is most severe in arid and semi-arid regions, particularly across the Middle East and North Africa, while temperate and tropical regions generally face lower risk. This visualization underscores the uneven global distribution of water stress, with implications for resource management and sustainability planning.

Credit: © FAO 2011 The State of the World's Land and Water Resources for Food and Agriculture (SOLAW)