Sometimes, modern problems require ancient solutions.  
 
A 1,400-year-old Peruvian water-diverting method could supply up to 40,000 Olympic-size swimming pools’ worth of water to present-day Lima each year, according to new research published in Nature Sustainability.
 
It’s one example of how indigenous methods could supplement existing modern infrastructure in water-scarce countries worldwide. 
 
More than a billion people across the world face water scarcity. Artificial reservoirs store rainwater and runoff for use during drier times, but reservoirs are costly, require years to plan and can still fail to meet water needs. Just last week, the reservoirs in Chennai, India, ran nearly dry, forcing its 4 million residents to rely on government water tankers.  
 

Animation showing monthly rainfall in the tropical Andes. Humid air transports water vapor from the Amazon and is blocked by the Andean mountain barrier, producing extreme differences between the eastern and western slopes. (B. Ochoa-Tocachi, 2019)

Peru’s capital, Lima, depends on water from rivers high in the Andes. It takes only a few days for water to flow down to Lima, so when the dry season begins in the mountains, the water supply rapidly vanishes. The city suffers water deficit of 43 million cubic meters during the dry season, which it alleviates with modern infrastructure such as artificial reservoirs. 
 

Panoramic view of the Andean highlands in the Chillon river basin where Huamantanga is located. The city of Lima would be located downstream in the horizon background. (S. Grainger, Imperial College London, 2015)

Artificial reservoirs aren’t the only solution, however. Over a thousand years ago, indigenous people developed another way of dealing with water shortages. Boris Ochoa-Tocachi, a postdoctoral researcher at Imperial College London and lead author of the study, saw firsthand one of the last remaining pre-Inca water-harvesting systems in the small highland community of Huamantanga, Peru. 

Water diverted, delayed
 
The 1,400-year-old system is designed to increase the water supply during the dry season by diverting and delaying water as it travels down from the mountains. This nature-based “green” infrastructure consists of stone canals that guide water from its source to a network of earthen canals, ponds, springs and rocky hillsides, which encourage water to seep into the ground. It then slowly trickles downhill through the soil and resurfaces in streams near the community.  
 
Ideally, the system should be able to increase the water’s travel time from days to months in order to provide water throughout the dry season, “but there was no evidence at all to quantify what is the water volume that they can harvest from these practices, or really if the practices were actually increasing the yields of these springs that they used during the dry season,” said Ochoa-Tocachi. 
 

A diversion canal as part of the pre-Inca infiltration system during the wet season. Canals like this divert water during the wet season to zones of high permeability. (M. Briceño, CONDESAN, 2012)

To assess the system’s capabilities, the researchers measured how much it slowed the flow of water by injecting a dye tracer high upstream and noting when it resurfaced downstream. The water started to emerge two weeks later and continued flowing for eight months — a huge improvement over the hours or days it would normally take. 
 
“I think probably the most exciting result is that we actually confirmed that this system works,” Ochoa-Tocachi added. “It’s not only trusting that, yeah, we know that there are traditional practices, we know that indigenous knowledge is very useful. I think that we proved that it is still relevant today. It is still a tool that we can use and we can replicate to solve modern problems.” 

Considerable increase in supply
 
The researchers next considered how implementing a scaled-up version of the system could benefit Lima. Combining what they learned from the existing setup in Huamantanga with the physical characteristics of Lima’s surroundings, they estimated that the system could increase Lima’s dry-season water supply by 7.5% on average, and up to 33% at the beginning of the dry season. This amounts to nearly 100 million cubic meters of water per year — the equivalent of 40,000 Olympic-size swimming pools. 
 
Todd Gartner, director of the World Resources Institute Natural Infrastructure for Water project, noted that this study “takes what we often just talk about — that ‘green [infrastructure] is as good as grey’ — and it puts this into practice and does a lot of evaluation and monitoring and puts real numbers behind it.” 
 
Another benefit of the system is the cost. Ochoa-Tocachi estimated that building a series of canals similar to what exists in Huamantanga would cost 10 times less than building a reservoir of the same volume. He also noted that many highland societies elsewhere in the world have developed ways of diverting and delaying water in the past and could implement them today to supplement their more expensive modern counterparts. 
 
“I think there is a lot of potential in revaluing these water-harvesting practices that have a very long history,” Ochoa-Tocachi said. “There are a lot of these practices that still now could be rescued and could be replicated, even though probably the actual mechanics or the actual process is different than the one that we studied. But the concept of using indigenous knowledge for solving modern engineering problems, I think that is probably very valuable today.” 



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