Study Paves Way for Rainwater Harvesting in Mexico City

Tens of thousands of Mexico City’s most vulnerable residents are gaining access to clean water, thanks in part to a report led by a researcher at Columbia University’s Earth Institute.

In Mexico City, 32% of residents don’t have enough water to take care of their basic necessities. As the city’s overtapped aquifer empties, the land is sinking, damaging water pipes and exacerbating problems with water supply and quality. The city imports more than 40% of its water from other parts of Mexico, and distribution is tightly controlled in some areas.

Water scarcity is especially problematic in informal settlements on the outskirts of the city, where municipal pipes and electricity services don’t reach. Here, some of the city’s poorest pay the highest prices for water deliveries that might not arrive for weeks at a time.

Capturing and filtering rainwater offers a promising solution to some of the city’s water problems. Collected from rooftops and then cleaned and stored in large tanks, the rainwater can be used for bathing, cleaning, and other household needs.

Isla Urbana, a nonprofit organization, has been working to install rainwater harvesting solutions in Mexico City’s informal settlements for 11 years. In 2018, the organization wanted to evaluate the potential for expanding rainwater harvesting across the city, but they didn’t think the data to do so existed — in part because many informal settlements are not counted in the census, and because public data on roof area and water use didn’t have high enough spatial resolution. Isla Urbana co-founder Enrique Lomnitz mentioned this dearth of data to Elizabeth Tellman, a human environment geographer then working on her Ph.D. at Arizona State University. To her, the solution was obvious.

“We have tons of data. We have satellites everywhere. There are always creative ways to build a model, estimate a variable, do whatever you can from satellite data,” she said. Not long after, Isla Urbana secured funding from the international nonprofit organization Oxfam to do just that, and they asked Tellman to lead the study. She continued the research after taking up a position as a postdoctoral researcher at the Earth Institute. The results, made public in October 2020, are already being put into action by Mexico City’s mayor.

Where vulnerability meets opportunity

The goal of the study was to determine who in the city didn’t have access to water, and where rainwater harvesting could help.

First, to find out who was most vulnerable to water scarcity, Tellman and her colleagues created a “water precariousness index.” For each neighborhood of the city, the index took into account factors such as the percentage of homes with piped water, frequency of water delivery, education levels, occupants per room, and the number of informal settlements.

The researchers also estimated precipitation levels throughout the city, as well as water demand.

To determine which areas would be able to support Isla Urbana’s rainwater capture systems, they used laser rangefinder (LIDAR) data — processed by study coauthors with support from the National Autonomous University of Mexico’s (UNAM) Institute of Geography — to estimate the roof area of homes on each block. They also used satellite images to make sure trees (which have the same height as many roofs) were not erroneously adding to the count of available roof area in the city. Researchers including Bertha Hernández, who is finishing her PhD on water access in Mexico City at UNAM’s National Laboratory of Sustainability Science, led a field survey to collect data on water storage, costs, roof size, and other variables in informal settlements across the city.

Combining this data, the study identified areas where high need overlaps with areas with plenty of precipitation and ample roof space for collection systems.

Tellman and her UNAM colleagues analyzed the data block by block, across the entire city.

“We included the most water-scarce portions of the city in our study, and no other study had really done that before,” said Tellman. “Usually the informal settlement communities are totally left out of those assessments because there’s no data there. Other studies will take the official building footprints and census data to calculate roof area. And what we did instead is use satellites to calculate roof area.” The satellite images and other data helped the team to estimate how many homes were in the informal settlements, how many people likely lived there, and what water access was like.

Low income, high potential

The study identified eight boroughs with high degrees of water precariousness: Tlalpan, Iztapalapa, Xochimilco, Milpa Alta, Cuajimalpa, Álvaro Obregón, Magdalena Contreras and Tláhuac. These boroughs contain a high number of informal settlements that fall outside of the water-pipe networks. Instead, water is delivered by truck, and it’s not always there when residents need it. Others have issues with water quality. “These communities always are the last priority of the management of public services in the city,” said Hernández.

The disadvantaged residents of these boroughs also tend to spend the most money on water, with households in some informal communities in Iztapalapa and Xochimilco paying well over the equivalent of $500 USD per year — nearly three months of minimum wages in Mexico.

“Water precariousness is highest in areas with the lowest income families,” said Delfín Montañana Palacios, director of socio-environmental education at Isla Urbana. “And what is really a thing of injustice is their water is the most expensive water in the city.”

Within the eight boroughs identified, the study points to 287 communities where rainwater harvesting could be most beneficial. According to the researchers’ calculations, installing 105,000 rainwater capture systems in these communities has the potential to reduce water vulnerability for 415,000 people.

Translating data into action

Whereas most studies end with a report and recommendations, Isla Urbana helped to translate the findings into real-world change by designing a rainwater harvesting program. The nonprofit began discussing the results with Claudia Sheinbaum Pardo in 2018, while she was running for city mayor. Now the mayor of Mexico City, Sheinbaum Pardo has used the study and Isla Urbana’s proposal to launch a government program that aims to install 100,000 rainwater capture systems by 2024.

In 2019 and 2020, the program offered to install rainwater harvesting systems, free of charge, for residents in five of the boroughs identified in the study. The program also trains the recipients in how to use and maintain the systems.

Isla Urbana installed the program’s first 10,000 systems in 2019. Montañana Palacios said that he and his colleagues have seen first-hand how capturing rainwater can transform a family’s life.  Many of these families face water scarcity for 12 months out of the year. But in just four hours, a team can install a rainwater harvesting system that supplies the majority of a household’s water for eight months out of the year, when rain is plentiful, said Montañana Palacios. For those months, the family only needs to buy drinking water.

In addition to saving stress and money, the systems mean that “you don’t need to spend time in solving the situation that you don’t have enough water to live day-by-day,” said Montañana Palacios. That’s especially important for women in the family. When the family orders a water delivery, the delivery truck might arrive anytime within the next 10-30 days; and if no one is home when the truck arrives, the family has to re-place the order and wait all over again. The task of staying at home to wait for the truck usually falls to the women in the household.

Mexico City hired a different organization to install another 10,000 systems in water-scarce areas during 2020, bringing the total number of systems up to 20,145 as of December. However, it will likely be challenging to reach the program’s goal of installing 100,000 systems by 2024, said Montañana Palacios. Isla Urbana’s original plan targeted 20,000 new installations in 2020, but progress has slowed, likely due to funding issues, he said.

In 2021, the government program will expand into other municipalities with less vulnerable populations, marking a shift from the study’s recommendations and Isla Urbana’s program design. The government program, in fact, largely avoids installing systems in the informal settlements, due to legal and political concerns. This means that many vulnerable areas have been left out. In 2019-2020, only 3% of the installed systems benefitted informal settlements.

Interactive Map: Rainwater Harvesting in Tlalpan, Mexico City

This map displays informal settlements, water stress index, and rainwater harvesting systems installed in 2020 (blue) and prior to 2017 (green). Read more / view large. Pan and zoom enabled. Hover over map to view water stress index. Buttons at lower left show/hide layers.

A tall order for a more water-secure future

Even 100,000 new rainwater harvesting systems installed in the most water-scarce neighborhoods wouldn’t solve all of Mexico City’s water problems. For example, Hernández says that she would like to see water distributed more equitably by the government. Currently, wealthier districts can receive as much as two times more water than less wealthy regions. She also thinks the government should recognize the informal settlements, so that they could be included in the piped networks and in other social programs.

Together with Isla Urbana, Tellman hopes to work with Oxfam on scaling up the project to other cities in Mexico and across the country as a whole. And when Tellman starts as a professor at the University of Arizona in August, she plans to work with colleagues to address water access problems in the Navajo Nation. More than 2 million Americans do not have access to running water, and Native American households are 19 times more likely than whites to lack indoor plumbing, according to a 2019 report from the U.S. Water Alliance and Dig Deep.

“What’s exciting about rainwater harvesting in Mexico City and the scale at which it’s happening is that it could be a model for other [places] around the world,” said Tellman.

Water problems will only grow as the planet continues to heat up and population grows. It is estimated that by 2025, 2 billion people will experience water scarcity, and two-thirds of the world’s population will experience some degree of water stress.

Tellman’s research has helped to show the huge potential of changing the way water is managed, said Montañana Palacios. Seeing that potential is the first step toward implementing social, cultural, and infrastructure transformations that will be needed to ensure that all people have access to clean water, he said.