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[[Image:RockCatchment.jpg|thumb|right|300px|Rock catchment project in Kitui, Kenya.]]

The technique applied for utilizing rocks for water supply is called rainwater harvesting and the structures built for harvesting rainwater from rocks are called rock catchments. These are naturally occurring catchments of bare rock that have high runoff coefficients (around 0.9). Water can be stored as an open reservoir behind a retaining structure, with storage capacities ranging from 20 – 4,000 m3, or can be stored directly in a covered storage tank that collects water directly from the catchment. The capacity of rocks to supply water is significant, because a rock surface of 1 hectare (10,000 square metres = 2.47 acres) can provide 1,000 cubic metres (1 million litres) from every 100 millimetre of rain. Even low and erratic showers can supply large volumes of water provided the rock area is sufficiently large enough to compensate for the low rainfall.