Because water moves more slowly below ground than on the surface, the captured rainfall emerges some time later from underground springs, effectively shifting water availability into the dry season.

Now, for the first time, researchers have measured and modeled how restoring and using additional amunas—which exist in different parts of the Andes highlands, although many have been abandoned—could help Lima, the second largest desert city in the world. With a population pushing 10 million, Lima has dams and reservoirs to help it through the dry season, but the city still falls short: The average dry season deficit is 43 million cubic meters of water. Lima’s total annual demand is 848 million cubic meters.

By diverting 34.7 percent of wet-season stream