Conclusiones del capítulo

Weather modification, including cloud seeding to increase rainfall and suppress hail, has long generated interest among scientists, public officials, and private practitioners in a dozen or more nations. Cloud

seeding has been studied and practiced in the United States for at least five decades. Over this period, research investment by agencies of the federal government has waxed and waned. Early experiments conducted by the U.S. Weather Bureau in the late 1940s showed suf- ficient promise that federally sponsored efforts were scaled up in the 1950s with programs overseen by the Weather Bureau, the U.S. Air Force, and the National Science Foundation, all of which supported cloud seeding research into the 1960s and 1970s. The mid-1970s marked a high point of federal support for cloud seeding, and the Na- tional Weather Modification Act of 1976 spurred federal research efforts and mandated a Department of Commerce Weather Modifica- tion Advisory Committee to coordinate research among federal agen- cies. In this same time frame, assessments were made of scientific progress made over the preceding decade and a half. The assessments include a series of reports from both the National Research Council (NRC) and the National Science Board that concluded that experi- mental evidence for cloud seeding had not yet definitively established its scientific efficacy (NRC, 1964, 1966, 1973; NSB, 1966). The Na- tional Research Council subsequently (in 2003) issued a report on the prospects of cloud seeding and other weather modification tech- niques, concluding that:

There is still no convincing scientific proof of the efficacy of inten- tional weather modification efforts. In some instances there are strong indications of induced changes, but this evidence has not been subjected to tests of significance and reproducibility. This does not challenge the scientific basis of weather modification concepts. Rather, it is the absence of adequate understanding of critical atmospheric processes that, in turn, lead to a failure in pro- ducing predictable, detectable and verifiable results (NRC, 2003). In 2004 the Weather Modification Association (WMA) assessed the NRC report from the perspective of those involved in operational weather modification (Orville et al., 2004). This review supported many of the NRC report’s recommendations but also included some criticisms; specifically, the WMA claimed that the NRC report did not adequately account for recent field applications for precipitation enhancement and hail suppression. Since the NRC and WMA reports were issued, some scientists have sought common ground with opera- tors to develop a cloud seeding program that would include scientifi- cally controlled watershed experiments (Garstang et al., 2004).

Federal support for cloud seeding research has generally declined since the mid-1970s. Nevertheless, several parties and states in the Colorado River basin maintain a strong interest in the prospects of cloud seeding to increase precipitation. For example, in a 2005 letter to the Secretary of the Interior, the Governor’s Representatives on Colorado River Operations sought to work with the Department of the Interior “to implement a precipitation management (cloud seeding) program in the basin (both Upper and Lower)” (Governors, 2005). In light of the stress on federal funding for discretionary expenditures, a renewed large-scale, federally led weather modification initiative does not appear likely (AAAS, 2006). For the foreseeable future, weather modification experiments and operations will depend mainly on funding from state governments, local communities, and private- sector entities (e.g., utility companies).

Six of the seven Colorado River basin states presently support some type of precipitation or snowpack augmentation operations (WMA, 2005). The most prominent cloud seeding project in the ba- sin may be one sponsored by the Wyoming Water Development Commission. This 5-year project is designed to demonstrate if rain- fall and snowpack in the state’s mountainous regions can be enhanced (see http://www.rap.ucar.edu/projects/wyoming/). Cloud seeding op- erations are planned in the Wind River Mountains and the Medicine Bow Range/Sierra Madre Mountains. The program is important be- cause of its potential scientific and operational evaluation for the Colorado River basin states and because the 5-year program is to util- ize a solid scientific base for the experiments. If the Wyoming pilot trials increase snowpack by 10 percent, the additional yield would, on average, be on the order of 130,000 to 260,000 acre-feet of additional runoff each spring (WWDC, 2006), which would represent a notable increase in water supplies. In addition to the Colorado River basin states, entities such as municipalities and the ski industry are inter- ested in the prospects of augmenting water supplies and snowpacks by cloud seeding. Denver Water, for example, commenced cloud seeding again in 2002 after 20 years of putting its program on hold. Denver Water’s cloud seeding program was reinitiated as a response to the 2002 drought and was conducted through March 2003 (see

http://www.denverwater.org/cloud_seeding.html).

In evaluating the success or benefits of cloud seeding operations, the experience of six decades of experiments and applications that failed to produce clear evidence that cloud seeding can reliably en-

hance water supplies on a large scale should be kept in mind. Of course, clear evidence is difficult to produce in cloud seeding experi- ments, as they are not amenable to case-control studies. Furthermore, such experiments are seen by many as being relatively inexpensive even if they do not definitively result in greater precipitation. Given increasing demands for water across the Colorado River basin, cloud seeding is likely to continue to be pursued as a means for augmenting water supply.

DESALINATION

Scientists and engineers, governments, and advocacy groups have long investigated desalination as a means of augmenting freshwater supplies. Most attention has been directed to converting seawater to potable freshwater, while less attention has focused on subterranean and surface brackish water desalination. There have been steady sci- entific and engineering advances in the technologies of salt water conversion, and several desalination facilities have been constructed. Advances in technology have led to cost reductions, improved effi- ciency, and an increase in the numbers of desalination plants world- wide. One recent estimate places the total number of desalting plants at 7,500, capable in total of producing several billion gallons of pota- ble water per day (http://www.waterdesalination.com). Nearly half the world’s desalinated water production today is in the Middle East; about 15 percent of the world’s desalinated water is produced in North America (Wangnick, 2002).

In California there are currently 16 coastal operating or planned desalination facilities (http://www.coastal.ca.gov/desalrpt/dsynops.

htm). The San Diego County Water Authority is committed to de-

salination, and by 2020 expects 15 percent of its supply to come from desalination (http://www.sdcwa.org/manage/sources-desalination.

phtml). In addition to interests of municipalities and utilities for

coastal desalination facilities, energy companies are operating small desalination plants on offshore oil and gas exploration and production rigs; there are nine rigs with desalination facilities off the coast of California (California Coastal Commission and State Lands Commis- sion, 1999). Not all desalination initiatives have proven fully suc- cessful, however. For example, in 1999 water authorities jointly sponsored a privately financed desalination plant at Tampa Bay, Flor-

ida, to supplement freshwater supplies for their 1.8 million customers. As of May 2006, the plant was not in operation, being plagued by management and technical problems (Cooley et al., 2006). The ex- perience of the City of Santa Barbara, California represents another prominent example of the challenges associated with large-scale de- salination (see Box 4-2).

Recent improvements in desalination technology have led to en- ergy cost reductions per unit of water produced. There is, for exam- ple, a variety of membrane technologies such as reverse osmosis, nanofiltration, and ultrafiltration. These all remove salts, dissolved organics, bacteria, and other seawater constituents from salt water (Pankratz and Tonner, 2003). There is also a range of thermal tech- nologies that boil or freeze water, then capture the purified water while the contaminants remain behind.

Energy requirements and costs are important considerations in desalination projects and greatly affect construction plans and deci- sions in the United States (especially as compared to areas such as the Middle East, where oil and natural gas costs are heavily subsidized). Energy costs notwithstanding, relative production costs have fallen since the early 1990s and the capacity of facilities has risen (AMTA, 2005). In the United States there is some interest in coupling future desalination plants with new power plant production for co- generation to reduce energy cost in desalination; rising energy costs, however, make it unclear if this trend will continue (Cooley et al., 2006). On the other hand, technical advances may continue and in- crease desalination efficiency even if energy costs rise. For example, a team led by scientists from the Lawrence Livermore National Labo- ratory estimates that a membrane system using carbon nanotube- based membranes may be able to reduce future costs of desalination by 75 percent compared to current reverse osmosis membrane tech- nology (Holt et al., 2006).

Longstanding federal research and development programs for de- salination have been advanced by a series of congressional authoriza- tions, such as the Water Purification and Desalination Act of 1996 (P.L. 104-298). An NRC report reviewed the Bureau of Reclama- tion’s desalination and water purification program and offered rec- ommendations for program improvement (NRC, 2004). State gov- ernments and municipal water districts are also investing in desalina- tion research, development, and demonstration facilities. The Bureau

BOX 4-2

Desalination in Santa Barbara

The City of Santa Barbara, California, relies heavily on rainfall and local groundwater to meet its water supply needs. These sources were impacted by severe drought conditions between 1987 and 1992, which caused sharp declines in local reservoir levels. The water shortage led city officials to consider a new source(s) of water supply, and Santa Bar- bara residents approved construction of a desalination plant to augment the city’s water supplies (they also approved a piped connection to Cali- fornia’s State Water Project).

Construction of a reverse osmosis facility began in 1991 and was completed in 1992. The plant successfully produced water during its testing phases, but soon after plant completion, drought conditions in the region subsided. The plant was placed on active standby mode be- cause of the high costs of producing water during nondrought periods. At the same time, the higher costs of water driven by the desalination plant and the connection to the State Water Project contributed to de- clining water demands. Conservation measures enacted during the 1987-1992 drought, such as low-flow toilets and xerophytic landscaping, contributed to water savings, and per capita demands never rebounded to predrought levels.

The desalination plant today is decommissioned, with a large por- tion of the plant’s infrastructure having been sold to a company in Saudi Arabia. Today, the plant serves as an “insurance policy, allowing the City to use its other supplies more fully” (http://www.santabarbaraca. gov/Government/Departments/PW/SupplySources.htm?js=false).

Although the plant is not currently operational, its future will bear watching as California’s population continues to grow, as the City of Santa Barbara continues to strive for urban water efficiencies, and as the economics of energy and water production continue to shift.

of Reclamation has recently focused its desalination research and development strategies in three areas: grants to university scientists, studying the feasibility of reopening the Yuma desalination plant, and constructing a test facility at Alamogordo, New Mexico to explore the feasibility of desalting brackish groundwater.

Beyond energy costs, desalination entails several environmental implications. A key barrier to economically viable desalination is disposal of the briny water that is a byproduct of the process. This is especially a problem in areas that do not have access to the ocean, but it can also be problematic for coastal locales. For example, native species in bays and estuaries are impacted by large seawater intake and by discharge of briny concentrates that are byproducts of desali- nation processes. Drawdown of brackish water in subterranean reser- voirs can lead to ground subsidence and/or a lowering of the water table. Regulations and technologies to mitigate adverse possible en- vironmental effects associated with desalination have been and will continue to be implemented by municipalities, states, and the federal government.

Technical, economic, and environmental issues notwithstanding, desalination offers the Colorado River basin states an option for actu- ally increasing water supplies. This option is limited primarily to ar- eas with access to water derived from the Pacific Ocean, although there may be other, select Colorado River basin sites at which desali- nation facilities may be feasible (e.g., Yuma, Arizona). With increas- ing regional water demands, and with increases in technical efficien- cies, desalination is likely to be perceived as an increasingly attractive option for augmenting supply, which will be especially true for wealthier communities. Prospective desalination projects will have to address and overcome the barriers of its energy requirements and ac- ceptable means of disposing desalination’s highly saline byproduct. Because of both its prospects and its potential limitations, desalina- tion will also continue to be an important topic of research (see, e.g.

http://watercampws.uiuc.edu/, which is a National Science Founda-

tion-sponsored center for the study of materials and systems for safely and economically purifying water for human use).

REMOVING WATER-CONSUMING