Lima, Peru’s capital, is running dangerously low on water, but archaeology may provide an answer in the form of a system of water storage developed by the area’s indigenous people 1,400 years ago.

Much of the world has seasons of wet and dry and must face the problem of making water supplies last until the next rains come. Few places have it as bad as Lima, however, the largest city in western South America. Located in an extreme desert, the city gets a tiny fraction of the rainfall necessary for life, instead depending on rivers flowing from the highlands, which dry up for months at a time. On the other hand, inland wet seasons can bring dangerous floods. As usual, climate change is expected to make both problems worse.

Building dams has become the worldwide response, but besides flooding precious river valleys, much of the water stored this way evaporates before it can be used. Ancient people in what is now Peru had a different approach. They diverted water from streams in the wet season into rock fractures. The water would then trickle through underground fissures, emerging downstream just when it was needed the most.

Researchers from Imperial College London have been studying the workings of these systems, today known as mamanteos after the Spanish for breastfeeding, and believe they could be scaled up to solve Lima’s modern-day problems.

A team led by Dr Boris Ochoa-Tocachi placed dyes in a recently restored mamanteo system during the 2014 and 2015 wet seasons. Located at Huamantanga 3,300 meters (11,000 feet) above sea level, this is one of the few mamanteos in use today. In Nature Sustainability they report dyed water surfaced an average of 45 days later, with some taking eight months.

"With the advent of modern science, you'd be forgiven for wondering how ancient methods could apply to modern-day problems. However, it turns out that we have lots to learn from our ancestors' creative problem-solving skills," Ochoa-Tocachi said in a statement. 

The authors extrapolate that 99 million cubic meters (2.8 million cubic feet) could be diverted each year through the rock systems of Lima’s catchment area. In the early part of the dry season, this would increase available water resources by 33 percent, providing nearby farmers with an extended period during which crops could grow.

Later in the dry season, when urban water supplies are running low, the water that takes the slowest path would provide a small but crucial top-up.

Such diversions would reduce the threat of floods when the wet season hits too hard. The authors do not consider redirecting rivers in this way a complete solution to Lima’s water problems but think it could be combined with small-scale dams to meet the growing city’s future needs.