By Brendan Borrell
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
Better Farming Practices Improve Water Quality
Rice is a staple of many of China’s culinary traditions, but ﬂooed paddies pose a threat to the water that ﬂ ows to Beijing, the country’s second-largest city. Rice paddies tend to make heavy use of the Chao River and are often located on steep slopes, which send fertilizer and sedi-ment runoff into the crucial Miyun reservoir. To protect the reservoir, the Paddy Land-to-Dry Land program now pays farmers in the headwaters to switch from rice to corn—a crop that requires less water and produces less runoff. Stanford University sci-entists estimate that the program produces more than $800 per acre in beneﬁ ts through increased water yield and quality, but costs only about $530 per acre of farmland.
© 145/Stuart Dee/Ocean/Corbis
São Paulo, Brazil
Paying Farmers, Reforesting Watersheds
Since last year, the 20 million residents of São Paulo’s metropolitan area have had unwelcome reminders of how critical their water supply is: Their taps periodically run dry. In 2014, during the driest summer in 84 years, the city’s reservoirs were at 10 percent their normal level, and emergency measures had to be implemented. Part of the challenge is that the watersheds supplying São Paulo have lost 70 percent of their forest cover, and the resulting sedimentation has reduced the capacity of the region’s reservoirs. To reverse the trend, the Conservancy helps manage water funds: Water users pay farmers and ranchers to reforest or terrace their ﬁelds to reduce erosion. To date, the program covers about 8,650 acres, and the Conservancy has proposed restoring an additional 35,000 acres.
© Twist and Shout/Westend61/Corbis
Preventing Forest Fires Protects Water Supplies
On the morning of May 5, 2000, ﬂames roared down the ﬂanks of New Mexico’s Cerro Grande and into the heart of the drought-afﬂicted town of Los Alamos. Nearly 280 homes were destroyed and about 48,000 acres burned. Without trees to prevent erosion, the town’s main reservoir became clouded with polluting sediments. The cleanup cost $17 million. In the aftermath, the nearby city of Santa Fe partnered with the Conservancy to develop a plan to prevent such a costly disaster there. The city decided to establish a water fund, in which the beneﬁciaries of a clean watershed invest in its protection. For the next 20 years, a small fee on Sante Fe water users will raise roughly $250,000 a year, which the city will use to thin and restore forests in its water-shed with the aim of preventing out-of-control wildﬁres.
© Matt Mawson/Getty Images
Riverside Buffers Reduce Erosion
Ten miles northeast of downtown Manila lies the La Mesa reservoir, which supplies water to the booming metropolitan area of nearly 12 million people. Illegal logging and settlers cleared the critical forest watershed surrounding the reservoir, causing heavy erosion and polluting the water supply. But the city launched a remarkable reforestation effort in 1999, planting nearly 700,000 trees on 3,700 acres. The program focused on restoring forests along the banks of streams and rivers to help prevent sediment from ﬂ owing directly into the water supply. The reforested area was turned into La Mesa Ecopark, whose $1 entry fee helps fund further restoration work and includes access to a swimming pool, a zip line and a climbing wall.
Protecting Water by Controlling Invasive Plants
Lying in the shadow of the lush, mist-covered Table Mountain, the cosmopolitan city of Cape Town does not lack a healthy water supply. However, invasive plants, including acacia and eucalyptus, suck up nearly one-third of the water that rains down on the mountain. In 1995, the national Working for Water program began employing people to clear invasive plants from areas near reservoirs and rivers around the country. The removal of invasive plants from more than 32,000 acres has increased stream ﬂows by 1.8 million to 2.6 million cubic meters, the equivalent of about 700 to 1,000 Olympic swimming pools.