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Lima, Peru, which lies on the dry Pacific side of the Andes Mountains, is the second-largest desert city in the world. (Cairo, Egypt, is the largest.) To endure the region’s 7- to 9-month dry season, Lima’s 10 million inhabitants are almost entirely reliant on water collected from the glaciated Andes or transported from the lush Amazon rain forest to the east. But the glaciers are melting, and existing dams and reservoirs, which hold a total of 330 million cubic meters of water, can quench Lima’s thirst through only a single year of drought.

A team of hydrologists, engineers, and social scientists is hoping to strengthen the water security of Lima and other Peruvian cities through analysis of a 1,400-year-old nature-based system developed by pre-Inca mountain communities. The technique uses a canal system that diverts water from streams to small ponds or spreads it over rocky hillslopes that act as natural sponges. This slows the flow of water down the mountains, preserving it into the dry season.

The team’s analysis determined that if the system were scaled up to its maximum capacity, it could divert, infiltrate, and recover up to 100 million cubic meters of water and increase the region’s dry-season water volume by up to 33%. Lead author Boris Ochoa-Tocachi of Imperial College London presented the team’s findings at AGU’s Fall Meeting 2019 in San Francisco, Calif.

Quantifying the Benefit of Green Infrastructure

Like most modern cities, Lima relies on gray infrastructure like reservoirs and dams for water diversion and storage. Gray infrastructure alone, however, has its drawbacks. It is often expensive and challenging to implement. It also has a static threshold, unable to adapt to shifting environmental conditions.

Natural (green) infrastructure can be much more dynamic and cost-effective than gray infrastructure. Green infrastructure is a broad category that can include planting native grasses to prevent erosion and maintaining wetland health to hold and filter water. Crucially, communities can use it in addition to the dams and reservoirs already in place, amplifying their effectiveness and providing a buffer when their threshold is exceeded.

“Sometimes we think that scientific knowledge is more valuable than indigenous and ancient knowledge. With this research, we tried to really show how both can complement each other.” To understand the most effective ways to implement green infrastructure, researchers are examining techniques already used by indigenous cultures around the world. But few studies quantify the hydrological effects of natural interventions like those used by indigenous Andean mountain communities.

“Sometimes we think that scientific knowledge is more valuable than indigenous and ancient knowledge,” Ochoa-Tocachi said. “With this research, we tried to really show how both can complement each other.”

Going with the Flow

Ochoa-Tocachi and his team conducted workshops, field visits, and interviews with over 100 members of a village called Huamantanga in the Andean Highlands near Lima. The village is one of the last to maintain the water-saving canals known as amunas, and the team was able to locate 11 operational infiltration canals through participatory mapping.

Then they injected a red dye into one of the canals to track the water’s progress over time. Samples from local springs showed that water from the canal was retained underground for between 2 weeks and 8 months, which means that at least some of it was stored for the entirety of the dry season.

Once they had quantified the capacity of existing amunas, the researchers modeled what it might look like to upscale the system and apply it to the Rímac River basin, one of Lima’s primary water sources. They determined that 35% of the water flowing through the Rímac River during the wet season could be similarly diverted, increasing the river’s dry-season flow by 33% at the beginning of the dry season. These effects could increase the capacity of current gray infrastructure to withstand drought conditions.

“Indigenous knowledge of water management is particularly beneficial because it’s closely tied to the place where it was developed, and it’s been honed to be very responsive to local conditions.” “The beauty of indigenous knowledge is in its specificity,” said Kate Brauman, lead scientist for the Global Water Initiative at the University of Minnesota’s Institute on the Environment, who was not involved in the study. “Indigenous knowledge of water management is particularly beneficial because it’s closely tied to the place where it was developed, and it’s been honed to be very responsive to local conditions.”

Ochoa-Tocachi is hoping that his team’s findings will help to inform policy decisions in the region as melting glaciers remove a previously relied upon natural buffer. “If the glaciers are retreating, the only way to counteract the loss of this buffer is through the use of natural infrastructure,” he said.

Peru has embraced green infrastructure in recent years, but the projects that receive funding aren’t always backed by evidence. For instance, a recent review paper coauthored by Ochoa-Tocachi found that a policy of planting nonnative trees in high-altitude native grasslands is actually decreasing, rather than increasing, water availability. Next year, he and his team will begin a review of the benefits of native grasses for water security and erosion prevention to incentivize their preservation.

“The work that Dr. Ochoa-Tocachi and his team are doing is critical because we need more robust evidence of exactly how effective green infrastructure is, and under which conditions,” Brauman said.

—Rachel Fritts (@rachel_fritts), Student in the Graduate Program in Science Writing, Massachusetts Institute of Technology, Cambridge