Sierra Nevada villages stored snowmelt underground by letting acequias seep into the hill

BBC Future reported in October 2022 that the Moors built a 3,000km irrigation network across the Sierra Nevada between the 8th and 10th centuries. Each spring snowmelt is diverted from river headwaters into porous earthen channels called acequias, which empty at simas, where water seeps into the ground, a process called "sowing water". The water replenishes aquifers and feeds springs that emerge further down the mountain, where it is "harvested" for summer crops.

Hydrogeologist Sergio Martos-Rosillo of the Spanish Geological Survey says research shows rivers have fewer peak flows and a more constant flow through the year, and that sowing water doubles aquifer recharge. He describes the approach as ecohydrology: imitating nature to regulate water instead of concrete structures. The acequia at Mecina-Bombarón, nearly 10km with 15 simas, has served the village for over a thousand years.

The system is threatened: from the 1960s people left rural areas and an estimated 15-20% of the range's acequias were abandoned. Since 2014 the University of Granada's Memolab project has restored 14 and cleaned 30 others.

Sow water high in the valley so it seeps underground and re-emerges lower down in summer, using the mountain as a reservoir.

Sowing water high in the valley slows its journey off the mountain, extending availability into the dry season.

Local acequieros know the underground routes water takes, so they can choose where it will resurface.

Communal governance and shared maintenance keep the channels working and the distribution fair.

Because it works with soil and aquifers, it avoids concrete structures that alter flow up and downstream.

Storing a resource inside the natural system can beat building concrete storage, but it relies on knowledge people must keep passing on.

The scientist interviewed says the approach could be replicated in mountain areas with similar geology, and notes interest in several Andean countries and a Peruvian system of about 1,400 years ago. A hot, late-snow 2022 spring cut the sowing season short, and the BBC says few people still hold the skills to maintain acequias. Some figures, such as the doubling of recharge, are researchers' own findings reported by the BBC.