Rainfall, the primary source of water resources of basaltic islands such as Hawaii, is distributed contrastingly between low and elevated areas. Typically, in mountain areas rainfall averages 200 to 300 in. a year, whereas in coastal areas rainfall averages only 20 to 30 in. a year. In spite of the prolific rainfall in mountain areas, direct runoff in some watersheds amounts to as little as 20% of rainfall. A permeable basaltic rock terrane accounts for the great absorption of rainfall and the consequent creation of large bodies of groundwater in basaltic rocks. High-level groundwater bodies, present at elevations of as much as 1,000 ft in volcanic-dike complexes of wet mountain areas, are called dike water. Unseen below the surface, groundwater presumably moves from thes high-level bodies into even greater and more extensive bodies of groundwater standing basally a few feet to several tens of feet above sea level as buoyant bodies of fresh water on salt water. Salt water saturates the island at depth. The problems of developing dike-complex water supplies relate primarily to hydrologic yield and utilization of natural storage, but the problems of developing basal-lens supplies involve the added constraint of the quality of water produced under pumping conditions. Infiltration tunnels, Maui-type shafts, and drilled wells are utilized in developing dike-complex and basal-lens water supplies. The method of development chosen depends upon the hydrology of the area, water supply design requirements, and economics.

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