For the first time in history, the U.S. government has ordered that flow of Colorado River water from the 50-year-old Glen Canyon Dam be slashed, due to a water crisis brought about by the region’s historic 14-year drought. On Friday, the Federal Bureau of Reclamation–a division of the Department of Interior that manages water and electric power in the West–announced that it would cut water released from Lake Powell’s Glen Canyon Dam by 750,000 acre-feet in 2014. An acre-foot is the amount of water that will cover an acre of land one foot deep; 750,000 acre-feet is enough water to supply at least 750,000 homes for one year. The flow reduction will leave the Colorado River 9% below the 8.23 million acre feet that is supposed to be supplied downstream to Lake Mead for use in California, Nevada, Arizona and Mexico under the Colorado River Compact of 1922 and later agreements. “This is the worst 14-year drought period in the last hundred years,” said Upper Colorado Regional Director Larry Walkoviak in a Bureau of Reclamation press release.

In the winter of 2005, Lake Powell reached its lowest level since filling, an elevation 150′ below full pool. Lake levels recovered some in during 2005 – 2011, but the resurgence of severe to extreme drought conditions have provoked a steep decline in 2012 and 2013, with the lake falling 35′ over the past year. As of August 18, 2013, Lake Powell was 109′ below full pool (45% of capacity), and was falling at a rate of one foot every six days.

Figure 1. Satellite comparisons of water levels in Arizona and Utah’s Lake Powell between 1999 and 2013 show a huge reduction in the amount of water in the lake. Image credit: NASA Earth Observatory.

Figure 2. From October 1, 2012 – July 31, 2013, precipitation over the Colorado River Watershed was about 80% of average. Image credit: Colorado Basin River Forecast Center.

Las Vegas’ Water Supply, Lake Mead, Near a Record Low
Downstream of Lake Powell lies Lake Mead, filled in 1936 when Hoover Dam was completed. Lake Mead supplies Las Vegas with ninety percent of its drinking water, and the water level of Lake Mead is expected to fall by eight feet in 2014 due to the lower water flow levels out of Lake Powell ordered on Friday. Lake Mead has fallen by 100 feet since the current 14-year drought began in 2000, and the higher of the two intake pipes used to supply Las Vegas with water from the lake is in danger of running dry. As a result, a seven-year, $800 million project is underway by the Southern Nevada Water Authority to build a third intake pipe that will tap the deepest part of the reservoir. This so-called “third straw” is scheduled to be available late in 2014, which may be cutting it close, if the Colorado River watershed experiences another year of drought as severe as in 2012 – 2013. Southern Nevada has done well to reduce water usage, though–the region’s annual water consumption decreased by nearly 29 billion gallons between 2002 and 2012, despite a population increase of more than 400,000 during that span.

Figure 3. Lake Mead water levels from 1938 – 2013 in July show a precipitous drop since drought conditions gripped the Western U.S. in 2000. The Lake Mead photo was taken by wunderphotographer LAjoneson June 29, 2007, when the lake had a “bathtub ring” 109′ tall. Water level data from The Bureau of Reclamation.

Figure 4. Workers handle the main drive sections of the tunnel boring machine that is drilling a 3-mile long tunnel through solid rock to supply Las Vegas with water from Lake Mead. The new intake tunnel is designed to maintain the ability to draw upon Colorado River water at lake elevations as low as 1,000 feet above sea level. The lake already has two intake pipes, and the higher of these will go dry when the lake level hits 1050′ – 1075′. As of August 2013, the Lake Mead water level was 1106′ above sea level, which is 114′ below full pool, but 24′ above the record low water level of 1081′ set in November 2010. Image credit: Southern Nevada Water Authority.

Drought conditions worsen over Southwest U.S. in August
According to the U.S. Drought Monitor, the Western U.S. drought peaked in July 2002, when 79% of the West was in at least severe drought, and 45% of the region was in the two highest categories of drought–extreme to exceptional. However, drought conditions have been steadily intensifying this summer. The August 13, 2013 Drought Monitor report showed that drought conditions in the Western U.S. are now the worst since 2004, with 78% of the West in at least severe drought, and 20% in the two highest categories of drought, extreme and exceptional. The latest U.S. Seasonal Drought Outlook, issued on August 15, calls for drought to remain entrenched over the large majority of the Western U.S. through the end of November.

Figure 5. As of August 13, 2013, severe to exceptional drought gripped nearly all of the Colorado RIver’s watershed in Arizona, Utah, New Mexico, Wyoming, California, and Colorado. Image credit: NOAA/NESDIS/NCDC.

Causes of the great Western U.S. drought
It is well-known that natural variations in sea surface temperature patterns, such as seen from the El Niño/La Niña oscillation, can influence storm tracks and can cause prolonged periods of drought. These natural variations likely had a hand in causing the great 2000 – 2013 Western U.S. drought. However, changes in the amount of sea ice covering the Arctic can also have a major impact on Northern Hemisphere atmospheric circulation patterns. We must consider if global warming, which has led to a 50% decline in summer Arctic sea ice extent since 1979, may be altering storm tracks and contributing to drought. In 2004, Lisa Sloan, professor of Earth sciences at UC Santa Cruz, and her graduate student Jacob Sewall published an article in Geophysical Research Letters, Disappearing Arctic sea ice reduces available water in the American west. An accompanying news release explained that their climate models found “a significant reduction in rain and snowfall in the American West” as a result of Arctic sea ice loss:

What they found was a change in atmospheric circulation patterns that caused a small northward shift in the paths of winter storms over western North America. This shift in winter storm tracks resulted in significantly reduced winter precipitation from southern British Columbia to the Gulf of California. In some areas, average annual precipitation dropped by as much as 30 percent. The reductions were greatest along the West Coast, with lesser changes further inland. But even as far inland as the Rocky Mountains, winter precipitation fell by 17 percent.

The sea ice acts like a lid over the ocean surface during the winter, blocking the transfer of heat from the ocean to the atmosphere, Sewall explained. Where the sea ice is reduced, heat transfer from the ocean warms the atmosphere, resulting in a rising column of relatively warm air. The shift in storm tracks over North America was linked to the formation of these columns of warmer air over areas of reduced sea ice in the Greenland Sea and a few other locations.

A follow-up paper by Dr. Sewall in 2005, “Precipitation Shifts over Western North America as a Result of Declining Arctic Sea Ice Cover: The Coupled System Response”, used a more sophisticated modeling technique but confirmed the results of the 2004 paper. In a June 2013 interview with climateprogress.org, Dr. Sewall commented:

“I think the hypothesis from 2004 and 2005 is being borne out by current changes. The only real difference is that reality is moving faster then we though/hoped it would almost a decade ago.”

Figure 6. The area of the Western U.S. in drought peaked during 2002 – 2004, but during 2013 has been approaching levels not seen since 2004. Image credit: U.S. Drought Portal.

Western North America drought of 2000 – 2004 the worst in over 800 years
The Colorado River’s water woes are due to an extraordinary 14-year drought that began in 2000, which peaked during 2000 – 2004. A 2012 study titled, Reduction in carbon uptake during turn of the century drought in western North America, found that the 2000 – 2004 drought was the most severe Western North America event of its kind since the last mega drought over 800 years ago, during the years 1146 – 1151. The paper analyzed the latest generation of climate models used for the 2013 IPCC report, which project that the weather conditions that spawned the 2000 – 2004 drought will be the new normal in the Western U.S. by 2030, and will be considered extremely wet by the year 2100. If these dire predictions of a coming “megadrought” are anywhere close to correct, it will be extremely challenging for the Southwest U.S. to support a growing population in the coming decades.

Figure 7. Normalized precipitation over Western North America (five-year mean) from 22 climate models used to formulate the 2013 IPCC report, as summarized by Schwalm et al., 2012, Reduction in carbon uptake during turn of the century drought in western North America. The horizontal line marks the precipitation level of the 2000 – 2004 drought, the worst of the past 800 years. Droughts of this intensity are predicted to be the new normal by 2030, and will be considered an outlier of extreme wetness by 2100. The paper states: “This impending drydown of western North America is consistent with present trends in snowpack decline as well as expected in-creases in aridity and extreme climate events,including drought, and is driven by anthropogenically forced increases in temperature with coincident increases in evapotranspiration and decreases in soil moisture. Although regional precipitation patterns are difficult to forecast, climate models tend to underestimate the extent and severity of drought relative to available observations. As such, actual reductions in precipitation may be greater than shown. Forecasted precipitation patterns are consistent with a probable twenty-first century megadrought.” Image credit: Schwalm et al., 2012, Reduction in carbon uptake during turn of the century drought in western North America, Nature Geoscience 5, 551-555, Published online 29 JULY 2012, DOI: 10.1038/NGEO1529, www.nature.com/naturegeoscience.

from:    http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2495