When there is no easily accessible runoff, people drill wells and tap into the groundwater. Approximately one-fourth of Earth’s population relies on groundwater, which accounts for approximately 99% of liquid freshwater on Earth (Figure 6.13). In parts of North Africa, groundwater is being pumped at twice the rate it is being replenished by recharge, resulting in rapidly falling water tables. As a consequence, groundwater-fed oases in the Sahara Desert that people have relied on for centuries are drying out. In 2000 the depth to groundwater (i.e., the water table) in parts of the North China plain was dropping by as many as 4 meters (13 feet) per year; in southern India, it was dropping by 2 to 3 meters (6.5 to 10 feet) per year. In the central Great Plains of the United States, groundwater levels in some areas have fallen more than 70 meters (230 feet) since the 1940s, an average of about 1 meter per year.

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The problem is that while groundwater deposits are vast, approximately three-fourths of present-day groundwater was deposited long ago and is renewed on timescales ranging from hundreds to thousands of years. This “fossil” water, once used, will not be replaced for a very long time (Figure 6.14). Pumping and utilization of these nonrenewable supplies of groundwater represent the mining of a limited resource similar to the extraction of fossil fuels such as petroleum and coal.

In the United States, some of the most productive farmland in the High Plains depends on a massive groundwater resource, the Ogallala Aquifer. This aquifer underlies approximately 450,000 km² (174,000 mi²) of the High Plains, more than the combined areas of Nebraska and Kansas. Pumping of water from the Ogallala Aquifer began mainly after the 1940s, when farmers began to drill deep wells into the aquifer, with the number of wells increasing rapidly to approximately 170,000 by 1978. From 1949 to 1978, the yearly amount of water pumped from the Ogallala increased from 4.9 km³ to 28.4 km³. That’s nearly twice the average yearly flow of the Colorado River during the 20th century.

Unfortunately, the rate of withdrawal is approximately 2.5 times greater than the annual recharge rate (see Figure 6.14). As a result, the water table across the Ogallala Aquifer has dropped an average of 4.3 meters (14 feet). In some areas, such as Texas and Kansas, the water table has dropped more than 30 meters (100 feet). As a result of the imbalance between rates of groundwater pumping and recharge, the Ogallala aquifer may run out of water within a century (Figure 6.15). We discuss changes in agricultural practice that could greatly increase the expected life of the Ogallala Aquifer in Chapter 7 (see page 220).

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One common physical consequence of excessive groundwater withdrawal is subsidence of the overlying land surface into the spaces left as water is pumped out. Such subsidence can substantially reduce the storage capacity within the aquifer and damage surface structures in rural and urban areas alike. The U.S. Geological Survey estimates that an area in the United States roughly the size of New Hampshire and Vermont has been impacted by subsidence. Particularly dramatic and sudden subsidence occurs in Florida, with the appearance of house-swallowing sinkholes. In some cases, hundreds of sinkholes have formed following the installation and pumping by a single irrigation well.

The highest rates of groundwater depletion and land subsidence in the United States in recent years have occurred in California’s San Joaquin Valley. Faced with an epic drought, farmers in the San Joaquin Valley were pumping massive amounts of groundwater to compensate for the lack of water in storage reservoirs. In response, water tables in some parts of the valley fell 60 meters (200 feet) in just two years, and land was subsiding at the unprecedented rate of 30.5 centimeters (1 foot) per year. This rate of subsidence was greater than even the historic extent of land sinking in the San Joaquin Valley (Figure 6.16). An unsustainable reliance on groundwater is a global-scale problem. A 2010 analysis of the global extent of groundwater depletion, the amount of groundwater pumped in excess of recharge, found that yearly depletion of groundwater more than doubled from 126 km³ in 1960 to 283 km³ in 2000.