As the Earth’s surface warms, climate models predict that the amount of fresh water for human consumption will likely decrease in parts of the globe. While that prospect looms for many cities around the world, a new study finds a more imminent threat to water supplies of cities in the tropical Andes, such as Lima, Peru and Quito, Ecuador.

“Despite all the uncertainty of the future impact of climate change, the impact of population growth is much bigger,” said Wouter Buytaert of Imperial College London, an environmental engineer and lead author of the study. This could mean harsher times ahead for millions including the 7.6 and 2.2 million inhabitants of the fast growing cities of Lima and Quito.

Some parts of the tropical Andes, a region along the northwestern coast of South America, already lack sufficient water to meet demand. To help policy makers combat this water scarcity, Buytaert and his colleague, Bert De Bièvre of the Consortium for the Sustainable Development of the Andean Ecoregion in Quito, Ecuador, compared the two main drivers of water depletion in that region – climate change and population growth.

The scientists used 19 climate models to project how climate change may affect urban water resources of the tropical Andes over the next 60 years. While the most pessimistic findings from models projected an average water depletion of up to 10 percent of current values, some optimistic outlooks estimated a 10 percent increase in water availability.

When the researchers separately modeled the impact of population growth, they found a drop of 38 to 62 percent in the amount of water available for each person. In their projections, the demand for more water as populations increase surpasses the amount of water lost through evaporation from warmer temperatures due to climate change.

Combining the two effects into a more realistic scenario of climate change and population growth happening simultaneously, the team saw the downward trend take over.

“Under whatever climate scenario, if you combine it with the impact of population growth it’s nearly certain that the impact will be negative,” Buytaert said. “So, it’s very, very unlikely that there will be more water available in the future [for this region].”

The study has been accepted for publication in Water Resources Research, a journal of the American Geophysical Union.

Buytaert and De Bièvre, a civil engineer with specialization in irrigation, focused their models on four cities in the tropical Andes: Lima; Quito; Bogotá, Colombia; and La Paz, Bolivia. These four cities are all increasing in population at a faster rate than the average population growth rate of South America as a whole.

While the scientists’ methods and models could be applied to different regions worldwide, Buytaert said, their results don’t necessarily translate to other highly populated cities around the world.

This is because the extent to which population growth and climate change impact a city’s water resources depends significantly on each city’s location and surrounding environment. Cities at different elevations obtain their water from different sources, which are variably affected by climate change.

For instance, cities at low elevations such as Lima obtain their water from streams flowing down the Andes. But cities at high elevations, such as Bogotá, Quito, and La Paz, are more restricted in how they obtain water and often collect it from nearby lakes or ponds, since it would be too costly to pump up from lower sources.

“The way you look at the issue of water scarcity in the future is very different for a city like Lima than for a city like Quito, and that is a main point we want to highlight,” Buytaert said.

Further, Lima ranks as the second largest desert city in the world while Quito, for instance, has a relatively humid climate. These differences in elevation and climate make it difficult to generalize future water availability for a broad region, he said.

Previous studies have approached issues of future water scarcity, but on global scales. To really inform policy-making on a national level, Buytaert said, researchers should include the specific geography and environment of individual cities.

“A lot of policy actions are being taken, but we have to be aware about what the future holds in order to make sure that those actions are sustainable and future proof,” he said.

The region could work to keep future water scarcity at bay by using several tactics, Buytaert explained. First, he said, would be to educate citizens about the issues of water scarcity and how to consume less water. Quito, for instance, uses approximately 250 liters (66 gallons) of water per person per day while the United Kingdom uses about 100 liters (26.5 gallons) per person per day – less than half of Quito’s consumption. Second, officials could safeguard current crucial water supplies, such as wetlands, through conservation efforts.

These approaches are just a few of many that could combat the problem of future water scarcity. “The solution will be complex and will really be a combination of many small things that can be done,” Buytaert said.

To probe the regional situation more deeply, Buytaert said, the next step will be to look at additional influences on water resources, such as vegetation changes and land degradation. Integrating those stressors into models would give an increasingly accurate picture and timeline for when, where, and how water scarcity will impact the tropical Andes.

— Jessica Orwig, AGU science writing intern