Technical Memorandum Report No. 3
Irrigation Water Supplies on the Islands of Oahu and Maui
Doak C. Cox
March 21, 1966
Background
Hawaiian geohydrology is best understood if continental concepts are forgotten and the Islands are considered as essentially porous and permeable lava sponges set in the ocean and saturated with sea water at their bases. Plentiful rain falls with relative uniformity in time on the windward sides or the tops of these spongy islands, but only in winter storms on their leeward sides. Infiltration rates are high and stream channels short and steep, so that most streams are intermittent or at least flashy. Within the sponges are, however, some impermeable septae retaining water at high levels and in places diverting it back to the surface as stream-stabilizing spring flow. The ground water escaping high-level retention in the sponges sinks to sea level and accumulates there floating on and displacing the seawater and discharging laterally in coastal springs. Surface storage capacities are slight due to the steepness and permeability of valleys. Development of surface water for irrigation is rarely feasible except where the streams are spring-fed. Spring flows may be augmented by tunnel development of high-level ground water. Where coastal prisms of sediments restrict discharge to the ocean, the basal ground water bodies may exceed 1000 feet in thickness and have heads in excess of 25 feet above sea level. Deep drilled wells, flowing or pumped, may in such places be used for development. Where such restriction does not exist, the basal ground-water bodies tend to be so thin that specially developed systems of shafts and skimming tunnels, known as Maui wells, are required to recover the fresh water floating on the salt.