The Urban Water Crisis
In the late afternoon of November 5, 1913, a crowd of ﬂag-waving Angelenos gathered to watch the christening of the Los Angeles Aqueduct, a network of canals, pipes and tunnels that would quench the thirst of their parched and growing city. The chief engineer of the aqueduct—a mustachioed man named William Mulholland—stood in the glaring California sun and gave a historic address. “This rude platform is an altar,” he began, “and on it we are here consecrating this water supply and dedicating the Aqueduct to you and your children and your children’s children—for all time.” Then, he leaned over the railing and gestured to the wall of water cascading down the concrete culvert. “There it is,” he said. “Take it.”
The crystalline waters before him had traveled some 230 miles from the eastern slopes of the Sierra Nevada mountains. Built over the previous ﬁ ve years, the aqueduct would carry 260 million gallons of water a day from the Owens River across the Mojave Desert and through the San Gabriel Mountains, where it entered the city’s water system.
Today, the city draws more water from outside its natural watershed than any other in the world. To safeguard its water, Los Angeles owns about 315,000 acres in the Sierra watershed, yet with warmer temperatures, more frequent forest ﬁ res and decreased snowfall, the supply is in jeopardy. The region is in a state of “water stress”: It uses more than 40 percent of the water available to it.
Los Angeles represents an extreme case of how fresh water forms a bond between the concrete jungle and the natural world. When we turn taps on, we take for granted that clean water will ﬂow out. But accomplishing this feat takes a lot of infrastructure: Cities move about 130 billion gallons of water per day a distance of nearly 17,000 miles. About 41 percent of Earth’s land area acts as a funnel for urban regions, gathering rainfall and directing it toward the 3.9 billion people who live in cities. These lifelines will be under intense pressure. The United Nations Population Division predicts that the world’s urban population will expand by 2.5 billion by 2050. Much of this growth will occur in cash-starved developing countries in Africa and Asia that can’t just build their way out of scarcity, as Los Angeles did.
“It’s been pretty clear for the last decade that there is a water crisis emerging. Rather than just painting the map red, we wanted to paint it green to identify the places where nature has the best chance of helping.”
Recognizing the need for a new path, Rob McDonald, The Nature Conservancy’s senior scientist for urban sustainability, and Daniel Shemie, the director of water funds, led a study in which more than 30 coauthors identiﬁ ed where the world’s 534 largest cities obtain their water and what conservation strategies can safeguard these sources.
Large cities are facing both a rising demand for water in a time of scarcity and threats to the quality of the water supply. “It’s been pretty clear for the last decade that there is a water crisis emerging,” says McDonald, a landscape ecologist. “Rather than just painting the map red, we wanted to paint it green to identify the places where nature has the best chance of helping.”
Improving the way we manage watersheds would have an immediate effect on water quality and, in the longer term, water quantity. Urban dwellers around the world face two common threats to water-supply quality. The ﬁrst threat comes from erosion and sedimentation, which can reduce the storage capacity of reservoirs and increase the cost of ﬁltration and pumping for water utilities. Erosion typically occurs at a higher rate on barren land, which lacks rooty vegetation to keep the soil from washing away. McDonald and Shemie say that land conservation, including protecting existing forests and reforesting cleared areas, is the best way to prevent erosion. In addition, uncontrolled forest ﬁ res can leave an area denuded of vegetation, so proper management of forests through controlled burns and thinning are also important strategies.
A second threat to water quality comes from farms and ranches, which release nutrient pollution from livestock waste and fertilizer runoff. When these nutrients enter the water supply, they can cause toxic algal blooms and increase water treatment costs. Last year, for example, an algal bloom in Lake Erie forced Toledo to truck in drinking water for two days. Cities can avoid nutrient pollution in their watersheds by encouraging better farming practices, such as limiting fertilizer use and terracing sloping land to prevent runoff.
Keeping watersheds clean is also a ﬁnancial issue. McDonald and Shemie estimate that on average, a 10 percent reduction in both sediment and nutrient loads can provide a 5 percent reduction in treatment costs for the world’s largest cities. Their global survey found that one in four cities would proﬁ t by pursuing one or more conservation strategies. Worldwide, water utilities could save up to $890 million a year.
Their research also shows that working closely with ranchers and farmers has the potential to be more cost-effective than buying and protecting land. For instance, pursuing better farming practices on just 0.2 percent of farmlands in urban watersheds could improve water quality for 600 million people.
The study has already caught city managers’ attention. After it came out last fall, the mayor of a heavily industrialized Chinese city of 15 million asked the Conservancy for an in-depth analysis of how it could protect its water supply.
As for Los Angeles, McDonald and Shemie suggest that a relatively small investment in forest fuel reduction could improve water-source quality. Such a result would certainly be cause for celebration, even without ﬂags and fanfare.
Conservation Strategies in Five Cities