Ground-Runoff Harvesting to Increase Water Availability in Isolated Households on Hilly Mediterranean Islands: A Case Study in a Micro-Catchment of Ibiza (Spain)

Mediterranean islands suffer from a lack of freshwater due to persistent and recursive droughts, limited groundwater availability and mass tourism. In Ibiza (Spain), private estates disconnected from the water distribution network consume about 21% of the total freshwater demand on the island. We conducted a study to evaluate the potential of ground-runoff harvesting (GRH) as a sustainable and inexpensive solution to increase freshwater availability in isolated households in Ibiza. The study involved an innovative modular tank of 40 m3 buried in the garden of a private property. The tank intercepted runoff forming in a 12,300 m2 hilly micro-catchment. We found that an extreme rainfall event with an intensity of 65 mm/h was able to create sufficient runoff to fill up the tank in one hour. A curve-number-based rainfall-runoff model was used to simulate the experimental results and to obtain a first-cut estimation of the potential of GRH at the scale of the island. The analysis indicates that, if installed in all forest areas in Ibiza with a similar slope to the study area, a volume of 1.31×106 m3 of freshwater could be harvested per year on the island just from extreme precipitation events. Such a volume of water is equivalent to about 5% of the island’s total freshwater budget. The study concludes that GRH is a highly valuable, yet still unexploited opportunity to save large freshwater volumes in dry-climate areas like Ibiza. GRH should be promoted across Mediterranean islands, and it can be easily incorporated within local water regulations.