(John Kemp is a Reuters columnist. The views expressed are his own)
LONDON, Feb 10 (Reuters) - Water supplies across the western and southwestern United States are becoming severely strained as more people move into areas with limited rainfall and shrinking groundwater.
Since the turn of the century, the 15 states that the federal government includes in its Pacific, Mountain and West South Central census divisions have seen their population rise by almost 17 million.
These three census divisions accounted for half the total population growth in the United States between 2000 and 2013. The western and southwestern divisions include most of the fastest-growing urban areas in the country, according to the U.S. Census Bureau.
Living without water chartbook:
Population in the western and southwestern divisions grew twice as fast (1.3 percent a year) as the rest of the country (0.7 percent). Nevada (2.5 percent), Utah (2.0 percent), Arizona (1.9 percent), Texas (1.8 percent) and Colorado (1.5 percent) had some of the fastest compound annual growth rates.
But the west and southwest are also the driest regions in the country. Most areas receive less than 15 inches (38 cm) of rain per year, and in some cases less than 10. Rainfall is less than half that in the rest of the country. In effect, population is migrating to parts of the country that lack the water resources to support the increase.
Water has long been the most valuable and fought-over commodity in the west.
By far the most important source in the region is the Colorado River. Nearly 40 million people in the basin states of Arizona, California, Nevada, Colorado, New Mexico, Utah and Wyoming rely on the river and its tributaries for some or all of their supply, according to the U.S. Bureau of Reclamation, the federal government's water management agency.
Use of Colorado water is apportioned among the seven basin states according to a complex set of interstate compacts, federal laws and Supreme Court decisions collectively known as the "Law of the River".
The Law of the River sets out how much water each state can divert for its own use to supply homes, businesses and farms.
But as regional demands for irrigation and municipal water supplies increase in line with economic development, pressures on the water supply are intensifying.
To make matters worse, much of the region is suffering a long-term drought, with below-average rainfall since 1999. That has sharply curtailed the volume of water in the Colorado and other rivers, and trapped behind the region's giant dams.
Between 2010 and 2012, the Bureau of Reclamation and basin states conducted a voluminous "Colorado Basin Water Supply and Demand Study" to assess likely imbalances over the half century to 2060.
The aim was to provide a technical foundation for what are likely to be difficult negotiations among the basin states, federal agencies and the U.S. Congress over how to update past agreements and share the river water sustainably in future.
STATE OF EMERGENCY
Long-term drought across much of the west is being compounded by a short-term downturn in rainfall in California and a number of other states, leaving them acutely short of water.
On Jan. 17, California's governor declared a state of emergency and directed officials to take all necessary measures to cut water consumption and boost supplies. Local water providers were ordered to activate contingency plans to try to forestall outright restrictions that could still be necessary later in the year.
California's rainfall is highly variable. Droughts are common. The worst one in recent times occurred between 1928 and 1934, which saw the lowest rainfall since the mid-1500s, and provided the backdrop to John Steinbeck's novel "East of Eden".
Other significant droughts occurred in 1976-77, 1987-1992 and 2007-2009, according to the state Department of Water Resources.
California receives about three quarters of its yearly rainfall between November and March in the form of a small number of storms.
"A few storms more or less during the winter season can determine if the year will be wet or dry," the department explains on its website. "If a persistent Pacific high pressure zone remains over California in mid-winter, there is a tendency for the year to be dry."
The winters of 2011/12 and 2012/13 saw very low rainfall. Now the state is experiencing its third dry winter in a row, pushing water resources to critical levels.
Just 3.3 inches of rain and snow fell across the Northern Sierra region between October and December, less than 20 percent of the normal amount.
On Dec. 31, the state's key reservoirs contained only 65 percent of their normal amount of water. Four of the 12 main reservoir systems had under 50 percent of their normal water. By Feb. 7, total storage had fallen to 58 percent of the norm.
California depends on winter rainfall, as well as snow melt from mountains, for its water supplies as well as the hydroelectric power which provides a large share of the state's electricity.
But with reservoirs already critically low, the state will have to rely heavily on its allocation of water from the Colorado River as well as groundwater supplies from the giant aquifers beneath the Central Valley.
The problem is that aquifers across California and much of the rest of the west and southwest United States have been depleting as homes, businesses and especially farmers extract water from them faster than it can be replenished.
Between 1900 and 2008, the 40 most important aquifers in the United States lost almost 1,000 cubic kilometres of water as a result of depletion, according to the U.S. Geological Survey ("Groundwater Depletion in the United States", 2013).
Nearly all the losses occurred in the west and southwest. California's Central Valley aquifers have lost almost 145 cubic kilometres of water since 1900. Other western aquifers lost 177 cubic kilometres.
Aquifers along the Gulf Coast (stretching from Florida to Texas) lost 266 cubic kilometres. The High Plains or Ogallala aquifer (which runs from South Dakota down to Texas) lost a staggering 340 cubic kilometres. In the worst-affected parts of the Ogallala, the water table has dropped more than 150 feet.
The rate of depletion appears to be accelerating. The High Plains aquifer lost an average of 10 cubic kilometres a year between 2000 and 2008, while Central Valley lost an average of 4 cubic kilometres of water, according to the USGS.
The Central Valley is one of the biggest agricultural areas in the United States. The underlying aquifer depends on run-off from the Sierra Nevada to its east and the Klamath Mountains in the north to recharge. But groundwater supplies have been under pressure for some time. Falls in the water table were recorded as long ago as the 1930s.
By the 1980s, there were 100,000 high-capacity groundwater wells for irrigation or municipal supply, according to USGS. During the 1976-77 drought, they pumped a staggering 18.5 cubic kilometres of water out of the aquifer. By 1980, the water level in some wells had declined by 200 feet. There has been widespread and severe subsidence.
Water use in California as well as much of the west and southwest is clearly unsustainable. But proposals to bring large volumes of water in from rivers in the Pacific Northwest, Canada or even Alaska, through giant canals or even pipelines, are probably impractical.
There is a pressing need to curb regional water consumption. However, effective water management has eluded the region since it opened up for homesteaders in the 19th century.
State and local governments are no closer to solving the problem, so water shortages seem set to worsen in the next few years unless the west and southwest get luckier with rainfall.
(Editing by Dale Hudson)