A 1,300-year-old hydrological computer
On the southern slopes of Spain's Sierra Nevada mountains, a vast network of water channels called acequias keeps the terraced farms of the Alpujarras green. This gravity-fed system is a piece of engineering, with some channels dating back approximately 1,300 years to the transition between the Visigothic and al-Andalus periods. The network, totaling around 3,000 kilometers in the Sierra Nevada alone, captures snowmelt from peaks as high as 1,800 meters and directs it across the mountainous landscape.
The system operates on a simple but effective principle. The main channels, or acequias de careo, are intentionally unlined earthen ditches. This design is a form of managed aquifer recharge. As water flows along the channel, a significant portion seeps into the ground, saturating the subsoil and replenishing underground aquifers. This "sown" water re-emerges months later in springs and streams at lower altitudes, ensuring a water supply during the dry summer months when it is most needed. The careos also generate high-altitude pastures, giving them their name from the verb carear, meaning to guide livestock to graze. From these main channels, smaller acequias de riego deliver water directly to agricultural terraces.