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Catchwaters may take the form of concrete canals, such as in Hong Kong, where there are many. Alternatively, they may take the form of a large concrete sheet, smothering a hill, and preventing rainfall from entering the rock strata, with a smaller channeling system for transport of the water to the storage tank - this latter system is in operation in Gibraltar.
Due to the expense of these systems, they are generally only to be found where there is an extreme shortage of freshwater, because of geographical or political issues.
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