Unit 8: Water Resources // Section 6: Depletion of Freshwater Resources
In many parts of the world people are extracting water from aquifers more quickly than the aquifers are replenished by recharge. In addition to draining aquifers, excessive groundwater pumping changes groundwater flow patterns around wells and can drain nearby rivers and streams. This happens because pumping changes the natural equilibrium that exists in an undeveloped aquifer with discharge balancing recharge.
When pumping starts, groundwater stores are depleted in the vicinity of the well, creating a cone of depression in the hydraulic head. If a new water source such as a river or stream is available close by, the well may capture (draw water from) that source and increase its recharge rate (Fig. 10) until this inflow matches the pumping rate. If no such source is available and pumping draws the water table down far enough, it will dry up the aquifer or deplete it so far that is it not physically possible or affordable to pump out the last stores of water.
Figure 10. Effects of groundwater pumping
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Source: © United States Geological Survey.
Pumping quickly lowers the pressure within confined aquifers so that water no longer rises to the surface naturally. Fifty years ago artesian aquifers were common, but today they have become rare because of widespread groundwater withdrawals. In unconfined aquifers, air fills pores above the water table, so the water table falls much more slowly than in confined aquifers.
As aquifers are depleted, water has to be lifted from much greater depths. In some parts of the world, the energy costs of lifting groundwater from deep beneath the surface have become prohibitive. Overuse of groundwater can also reduce the quality of the remaining water if wells draw from contaminated surface sources or if water tables near the coast drop below sea level, causing salt water to flow into aquifers.
Serious groundwater depletion has occurred in major parts of North Africa, the Middle East, South and Central Asia, North China, North America, and Australia, along with other localized areas worldwide (footnote 10). In some cases, such as the Ogallala aquifer in the central United States, water tables are falling so low that wells can no longer produce water. In a draft plan issued in mid-2006, the Texas Water Development Board projected that the state's water supplies would fall by about 18 percent between 2010 and 2060, "primarily due to the accumulation of sediments in reservoirs and the depletion of aquifers," and that at the same time the state's population would more than double. If Texas did not implement the water management plan, the board estimated, water shortages could cost the state nearly $100 billion by 2060 (footnote 11).
Many rivers around the globe have also been depleted by increasing water withdrawals. Some, such as the Colorado and Rio Grande, no longer reach the sea during much of the year because their flow levels have been reduced so drastically by dams and water diversion (Fig. 11). This overuse destroys estuaries at river mouths, which are important habitats and breeding grounds for fish and birds.
Figure 11. Dams and diversions along the Rio Grande
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Source: © United States Fish and Wildlife Service.
Under normal conditions, most rivers are gaining rivers: groundwater flows into the rivers because the local water table sits at a higher elevation than the river water. However, with excessive groundwater pumping, water tables slowly decline and natural discharge to the rivers is reduced, so river flow declines. Over the long term, groundwater extraction may greatly reduce river flows in many regions. This connection between water levels in aquifers and river flows complicates the process of estimating sustainable yield from aquifers. If users pump more water from an aquifer than the natural rate of recharge, the aquifer may draw water from adjoining rivers and increase its rate of recharge. However, by doing so it will reduce surface water flows.
Almost every country in the world that uses groundwater as a resource is having troubles with it affecting surface water systems.
Tom Maddock, University of Arizona
By regulating river flows to reduce floods and increase flows during dry periods, dams have major impacts on river ecosystems. Like forest fires, river floods play important ecological roles that we have only begun to appreciate and foster in recent decades. Among other services, floods scour out channels, deposit nutrient-rich sediments on flood plains, and help to replenish groundwater.
In regions where rivers have been channeled between levees to prevent flooding, they no longer deposit sediments and nutrients on surrounding lands. Scientists widely agree that damage from Hurricane Katrina in August 2005 was magnified because levees and canals around New Orleans had directed the Mississippi River's flow straight into the Gulf of Mexico for decades. Without fresh water and sediment from the Mississippi, southern Louisiana's wetlands degraded and subsided, reducing their ability to buffer the region against storms and flooding.