Why and when will lake Mead go dry?

Posted on June 29th, 2012 in Environment,Health by Robert Miller

Lake Mead water Levels at Hoover Dam. White mineral deposits around the lake indicate where water levels were when the lake was full

THE BEGINNING: In 2008, Tim Barnett and David Pierce, two climate researchers from the Scripps Institute of Oceanography, printed a provocative paper entitled “When will Lake Mead go dry?” which appeared in Water Resources Research (Vol 44). This was the first study that applied models of global climate change to address the current and future state of the public water supply provided  by the Lower Colorado River Basin, whose runoff storage is in Lake Mead, a lake created by the Hoover Dam construction and located near Las Vegas, Nevada. At the present time, the Lower Colorado River Basin provides water for the 30 million people living in the Southwest and at the present rate of growth, this system will have to provide water for 38 million inhabitants by the year 2020, a date that is just around the corner.   Major cities served by the Lower Basin runoff, include Los Angeles, Las Vegas, Phoenix, Tucson, San Diego and their surrounding regions. Mexico also gets water from this system.

MISMEASUREMENT OF RIVER FLOW:  Before one considers the impact of global climate change on the Colorado River and its contributions to the future of Lake Mead (major dam sites on the Colorado include Hoover Dam and its reservoir Lake Mead and Glenn Canyon Dam which created Lake Powell, most of which is in Utah, though the dam itself is in Arizona),  one needs to carry out some simple arithmetic on the water flowing now compared to that needed for the explosive population growth anticipated within a decade. In the 1920s, when measurements were first made on the volume of water flowing in the Colorado River Basin it was assumed to be a relatively constant value with small differences in the year to year variance. We now know that these early measurements took place during one of the highest runoff periods in the documented history of the Basin. Reassessments show that the water system for the southwest, is already over-subscribed; the contemporary illusion that plenty of water is still available from the Colorado River system has evaporated as the water levels in both Lake Powell and Lake Mead have seriously eroded in less than a decade. Knowledge about historical runoff volumes for the Colorado have been obtained from tree ring analysis derived from several different tree sources in the region. Tree ring analysis is regarded as an accurate representation of the historic, year by year changes in the relative  abundance of water and such records show that in the past 1200 years, the Southwest has experienced significant droughts together with intermittent periods of big runoff seasons. When moisture is abundant, annual tree growth is expansive and when droughts occur, trees cut back on growth to conserve water. Thus, the tree rings laid down provide a log of the water resource history of the region. Droughts in the Southwest have been very severe at times and may have accounted for the dismantling of the impressive early settlements at Mesa Verde, Chaco Canyon and other regions of the Southwest.

CORRECTING HISTORY: Early twentieth century measurements suggested that the Colorado River supplied about 17 million acre feet per year (maf; an acre foot is one foot of water depth covering an acre and is approximately equal to 326,000 gallons; this is roughly enough water to provide for four families for one year). But the new reality of the Colorado River, now that we are in the beginning of a drier, hotter period (part of which has been created by anthropogenic forces), makes it apparent that, on average,  the Colorado River runoff is much less than the 17 maf originally assumed to exist in perpetuity; it is more realistically in the range of 14-15 maf/yr or less and global warming will severely cut into that value. This estimate of water runoff for the Colorado needs to be compared to current water commitments which are 16.5 maf/yr.  Because the Colorado River is not the only source of water for Lake Mead,  inflows from other rivers below Lake Powell contribute about 1.56 maf/yr and that additional water makes the system  look like it’s almost in balance. But subtract  from that nearly balanced value, the water costs of maintaining reservoirs with losses from evaporation and water consumption by Lower Basin vegetation and the total water expenditures come to 19 maf/yr: thus the current ledger on water in the Lower Basin is in red ink before we even get to global climate change issues. Today, there is a potential deficit in water, just to meet today’s needs, of nearly 3 maf/year which will very likely to lead to serious water shortages in the near future. Now imagine adding 8 million more water consumers by 2020 and you begin to see emergency lights flashing. This water shortage can only be met by reducing water availability to all water consumers and plans have been discussed to implement such cutbacks. In California 79 percent of the diverted surface water and pumped ground water goes into support crops and livestock and most other states devote a similar fraction of their water towards agriculture. To conserve on water, Southwest states will have to take a hard look at agriculture, and in doing so, they must carefully weigh the possibility of food shortages as a component of forced water shortage. But indications are such that cuts in water could be more drastic than imagined.  As the population in the area goes through further expansion, needs for additional electrical power will likely be required and power generators are one of the biggest consumers of water. About half of all the water removed from rivers and lakes in the United States goes into the production of thermoelectric power generation to support the cooling requirements of those massive systems. Here wind and solar energy needs to be a serious component of the new power requirements for the Southwest. The water needs for power are lower in the West, where hydroelectric power contributes substantially to the grid, though the capacity of Lakes Mead and Powell to continue generating power over the long haul is now in question. But 10 to 20% of the electricity produced in this country is used to move water around and the need for this use of electricity will only expand in the Southwest as population expansion creeps into new areas of development. Part of the power generated at Hoover Dam is used to pump the water over the mountains into southern California (via the All-American Canal).

GLOBAL CLIMATE CHANGE: If the red ink for the current water picture in the Southwest isn’t bad enough, we now have to add the additional reductions predicted from global climate change models, which are estimated to be in the range of a 10-30 per cent reduction in river runoff during this century; these numbers are highly variable and relate directly to our success in combatting greenhouse gas reductions and in this domain, we are not off to a good start. With an expanding population and a diminishing water supply in the Southwest, you could easily wind up with an astonishing dust bowl picture likely to occur in some regions and we have already seen monstrous dust storms in the Phoenix area. The Lower Basin water must provide for the urban needs of cities as well as the needs of farmers, who at least in California, have first rights on the water (first come, first served policy, but that rule does not apply to other states in the region). Add to that sobering picture the fact that we have treaty obligations with Mexico to provide them with 1.5 maf/yr and you can easily imagine that all the ingredients are in place for an international water war at some time in the near future.  Based on climate modeling studies, Barnett and Pierce suggested that there was a 50-50 chance that Lake Mead could run dry by 2021! At the time their paper appeared in 2008, both lake Mead and Lake Powell stood at less than half of their capacity, both of which receded from full capacity in less than a decade. You can use Google earth to visualize the Hoover Dam and see the white rings of mineral deposits where the water used to be when the lake was full. You can also see how far the down the lake is below the intake for the power generators.  Officials of the Bureau of Reclamation, who are responsible for managing the lakes, know that the water decline in the lakes is serious, but the Barnett and Pierce paper gave a new urgency to the situation by providing a more longitudinal view of what could be expected in the future; it was not a pretty picture and this new reality was not that far away.

WHAT CAUSES DESERTS?: As the temperature in the atmosphere rises, the air can hold more moisture which, somewhat counter-intuitively, can translate into a drier climate for the Southwest and more precipitation in regions that are already getting rain and snowfall. The so-called Hadley cell explains why regions near the equator can readily become deserts. Warm moist air heated at the equator rises into the atmosphere and cools, loosing it’s moisture in the form of clouds and rain. This upwelling of the moist warm air comes down as very dry air while moving out to regions north and south of the equator. This is one of the major mechanisms of temperature equilibration for our lovely blue planet. Because the dry air can hold more moisture, more so when it is warmed by energy reflected back to earth through greenhouse gases, it has a drying effect on the American Southwest and other great deserts north and south of the equator. The Hadley cell mechanism is responsible for many of the great deserts of the world: south of the equator the Atacama Desert in Chile, the Kalahari in southern Africa and the outback of Australia all share approximately the same latitude. The Hadley deserts of the Northern Hemisphere include the Sahara, the deserts of Arabia and the North American deserts which include the Sonoran, Chihuahuan, Mohave and the Great Basin. The dry air that helps maintain them also provides the cloudless blue skies which serve as a constant reminder that the clear beauty of the sky is created by moistureless air. Florida would very likely have been a desert except for the earth’s rotation, which allows the downward dry air to become moist again over the Atlantic Ocean before moving diagonally towards the west to engage Florida. The Southwest on the other hand does not enjoy the luxury of having their dry Hadley air premoisturized by flirtation with an ocean.

CORRECTED APOCALYPTIC INTERPRETATION:  When the Barnett and Pierce paper came out, you can imagine the shock and criticism that greeted the apocalyptic view derived from their analysis. Reports coming out of Las Vegas were especially critical of their interpretation and, if for no other reason than to avoid a mass exodus from the gambling capital of America, the paper was declared irrelevant for the hydrology future of Las Vegas. The Las Vegas Review Journal wanted to challenge the authors with a bet saying “are they willing to put their money where their mouths are.” In the meantime, anticipating water shortages, the Bureau of Reclamation emphasized that they had already negotiated a water-shortage agreement, so they were on top of the situation. Barnett and Pierce responded by saying it was that precise water agreement that initiated their own study and pointed out that the cutbacks on water distribution they negotiated (6.6% cut in the Lower Basin) were too small, that it would be too little too late and that the real cuts had to be at least twice that volume to prevent a severe water shortage associated with a more draconian emergency cutback. But there were some legitimate criticisms of the Barnett and Pierce paper—they had not addressed the fact that there were other water sources for Lake Mead besides the Colorado and they had not computed the reality that as Lake Mead’s water level declined, so too would the magnitude of lake evaporation. The authors complied and redid their simulations and published the results in the Proceedings of the National Academy of Sciences (Vol 107, 21271, 2010). In this most recent paper the authors acknowledged that new modeling studies had pushed back the 50-50 odds of Lake Mead going dry by 4 to 10 years. Whew!  Crisis over! Thank God we don’t have to worry about that anymore! In the meantime, Las Vegas is investing nearly  $ 1 billion to build a three-mile tunnel under the bottom of Lake Mead to install a drain plug, giving it access to drain the last drop of water from the lake when water intake port # 2 (at  1,000 ft above sea level) can no longer access Lake Mead’s water, created by further drops in the lake level. At full capacity, Lake Mead is 1,219.6 ft above sea level; in January 2000 the water level was at 1,214.26 ft and at the end of September 2010, the water level was 1, 083.81 ft above sea level. When Lake Mead is full, it has more than three years of storage capacity to meet its obligations to the Lower Basin. The Hoover Dam is also the source of power for a million people and provides support for our power grid. In recent years the water level has been too low for the power turbines to be driven by water. One of the problems in dealing with serious water shortages such as those facing the water future of the Southwest United States, is the age-old, time-worn problem of the American people being disengaged, complacent and too inclined to believe in Ronald Reagan bedtime stories (you remember the shining city on the hill?). The land of sky blue waters still has the sky, but has less water to reflect and this is the country that seems willing to convert public functions into private ones. Good luck to the Southwest when your water bills come from Bechtel!

A NOTE ON SOURCES: Material for this article was based on publications by Barnett, Pierce and colleagues, referred to above, but also on an excellent book written by William Debuys “A Great Aridness: Climate Change and the Future of the American Southwest,” Oxford University Press, 2011.

RFM

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