LAS VEGAS — Concerned about ebbing water levels at nearby Lake Mead and the future of the Colorado River that fills it, southern Nevada's water management agency aims to curb water use in the Las Vegas metro area by 20 percent.
Even if successful, however, this conservation plan alone will fall short of meeting the Las Vegas Valley's water needs in 2035, according to research presented at the 94th annual meeting of the AAAS Pacific Division.
"We will not be able to meet our future demands with our current growth trends and supply options," said Sajjad Ahmad of the University of Nevada, Las Vegas' Department of Civil and Environmental Engineering. "Either more aggressive conservation coupled with slower population growth, or an additional water supply will be needed."
The population of the Las Vegas Valley has nearly tripled in two decades, although this growth has slowed recently due to the recession. The area isn't alone in its thirst. Of the seven states that draw their water from the Colorado River, five including Nevada are among the fastest growing states in the country.
Ahmad and others presented results from computer modeling studies aimed at helping resource managers in the arid West, where population growth and climate change are putting increasing pressure on the region's water supply. "We are taking outputs from global climate models and hydrological models and bridging the gap between science and what the decision-maker community needs," Ahmad said.
The meeting, held 16-19 June, drew hundreds of scientists, students, professionals and the public to the University of Nevada Las Vegas and was loosely organized around the theme "desert science." Over 75 students, including some from as far away as Nigeria, presented their work in oral and poster sessions. The Pacific Division is the oldest of the four AAAS regional divisions, which bring together scientists and non-scientific communities to discuss issues with immediate local impacts.
No Simple Answers
Sajjad Ahmad [AAAS]
Colorado River basin. (Large version in slideshow.) [Bureau of Reclamation]
Ahmad's analysis of the area's water conservation options revealed a variety of tradeoffs. For example, outdoor water conservation offers the most potential savings, because water used indoors can be returned to Lake Mead for credits that allow the city to draw yet more water. But, returning this water increases the salinity of the lake.
The water can be treated first to remove these salts and make it clean enough to drink, but then managers must decide whether to send it back to the lake or re-use it directly. Direct potable water reuse could save approximately $35 million per year in energy costs, Ahmad has calculated, but he cautioned that making this option appealing to the general public may be a challenge.
Even if the Southern Nevada Water Authority meets its 20 percent conservation target by 2035, in that time the demand for water will still grow to the point where there will be essentially no surplus available in case of drought or other spikes in water use, Ahmad reported. Combining conservation with a slowdown in population growth could provide a healthier surplus, as long as the Las Vegas Valley's population does not exceed 2.6 million. (Currently at 2 million, the area's population is on course to reach 3.2 million by 2035.)
Curbing population expansion may not be a politically popular strategy, given Nevada's pro-growth business model, but it is a "political choice," said Ahmad, which "could be revisited to help us with our water management issues."
Finding an alternative water source may be equally controversial, but there are two options on the table. State and federal agencies have approved the building of a pipeline that would supply Las Vegas with groundwater pumped from northern Nevada. This plan has been hotly contested by a variety of groups however and is currently being challenged in Nevada state court.
Alternatively, Nevada might potentially build a desalination plant near the coast of California or Mexico. Authorities could then arrange a "paper trade" that would exchange desalinated seawater for an additional share of Lake Mead's water. Ahmad compared these two alternatives considering energy use and construction, operation, and maintenance costs. On a unit-cost basis alone, the desalination option is cheaper compared to pipeline plan, he reported, but it would use twice as much energy and its carbon footprint would be 48 percent higher.
A Silver Lining to Sprawl
Water is not the only environmental concern in the arid West's expanding cities. Hoping to temper an urban heat-island effect, the city of Las Vegas has set its own goal for 2035: doubling the amount of tree canopy by 2035.
To determine temperature changes over time and help city landscapers target the right areas, Haroon Stephen, also of the Department of Civil and Environmental Engineering at UNLV, began with ground temperature measurements from weather stations operated by volunteers across the city. He analyzed these along with satellite data showing the various types of landcover since 1990.
His results revealed a general warming trend of approximately 0.1 to 0.3 degrees Celsius per year in Las Vegas' older urban neighborhoods and in outlying rural areas. The newer areas – often the targets of criticism about urban sprawl – actually showed a cooling trend, likely because they add at least some vegetation to a desert environment. Overall, however, the city has seen relatively little increase in tree cover or open green space since 1990, compared to the increase in buildup and paved surfaces.
Haroon Stephen [AAAS]
Long-term temperature trends in Las Vegas from 1986 to 2010, based on thermal infrared images from the LandSat 5 Thematic Mapper. [Courtesy of Haroon Stephen]
Changes in sea surface temperatures in these red and blue regions can be used to forecast streamflow changes in the Colorado River. [Courtesy of Ajay Kalra]
A third study looked at the Colorado River Basin more broadly and offered a new approach to forecasting spring-summer "streamflow," or the volume of water flowing past a fixed point in the river. This approach should be useful to water managers who must decide how much water to release for agricultural use or to conserve behind dams, especially as climate change is expected to bring about more frequent and extreme floods and droughts.
Many streamflow forecasting methods, including those used by the National Oceanic and Atmospheric Administration and the National Weather Service, predict the likelihood of certain-sized flood events occurring within various windows of time.
"But water managers are more interested in the quantity of the forecast, so that they can plan ahead in events of climate extremes. This research is focused more on addressing the quantity of streamflow," said Ajay Kalra of the Desert Research Institute in Las Vegas.
Kalra has identified several regions of the Pacific Ocean where changes in sea surface temperature appear to be statistically linked to the Colorado River's streamflow. The changes in these regions can account for about 90 percent of the observed variation in the river's streamflow over the last century with one to four months lead-time. Unlike conventional physical streamflow forecasting models, this approach uses a statistical, data-driven modeling approach coupled with climate information and is thus relatively simple and parsimonious, according to Kalra.
The changing sea surface temperatures in these regions — including a significant area east of Japan known as "Hondo" and another off the coast of the Pacific Northwest — are part of local oscillations in ocean temperature, which shift between warming and cooling phases. Kalra's model does not address the underlying physical process by which these local ocean temperature shifts would affect streamflow in North America, but presumably they are part of a climate system that causes snow to buildup in the Rocky Mountains and then run off into the river come springtime.