Southeastern Oahu comprises the eastern end of the Koolau Range and is divided into two roughly equal parts by the crest of the range. The northside of the crest is commonly called the windward side and the southside, the leeward. Precipitous cliffs aproned by a gently sloping landscape are the main topographic features on the windward side. The leeward side is a gentle lava-flow slope incised by steep narrow valleys.
The main Koolau fissure zone, including the caldera, lies on the windward side. The leeward side includes minor rift zones that are perpendicular to and intersect the main fissure zone. Dikes in the main fissure zone strike from nearly east-west in the eastern end to about N. 55? W. in the western part. Dikes in the minor rift zones strike from north-south to slightly northeasterly.
Water use is about 18 Mgal/d (million gallons per day) of which only 4 Mgal/d is obtained locally from ground-water sources. About a third of the 14 Mgal/d deficit is imported from sources northwest of the study area on the windward side and the remainder from sources in the Honolulu and Pearl Harbor areas on the leeward side. The 4 Mgal/d being developed represents only about 3 percent of the area's rainfall compared to a development-rainfall ratio of 20 percent for the rest of the island.
Streams are short and flashy. Perennial streamflow to the sea occurs only in Maunawili Valley and in the Waimanalo area. Mean annual discharge is estimated at 20 Mgal/d in the windward side and at 15 Mgal/d on the leeward side. Low flow, expressed as the flow that is equaled or exceeded 90 percent of the time, is 5 Mgal/d windward of the crest and zero leeward of it. Most fresh ground water occurs in lava flows of the Koolau Volcanics. It is impounded by dikes in the rift zones and floats on saline ground water as lenses outside the rift zones. Small but important bodies of freshwater are perched in volcanic rocks of the Honolulu Group in Maunawili Valley. Fresh ground water occurs in near-shore calcareous sands that overlie a clay horizon in the Waimanalo area. Deeply buried talus and alluvium also carry fresh ground water in the Waimanalo area.
Wells tapping saline ground water in fresh lava flows of the Honolulu Group provide water for a sea-life park in the Makapuu area. The same aquifer is tapped by wells for disposal of the saline waste water.
The current development scheme in the windward side that utilizes only the free-flow equilibrium discharge of dike-impounded water is inefficient and does not cope with the annual weather cycle. The flow available for development under this scheme is greatest in the rainy winter months when demand is the lowest and least in the summer months when demand is the highest. A more optimal scheme would be to change this natural flow pattern by depleting storage by pumping to increase flow in the high-demand summer months and allowing the depleted storage to recover naturally in the low-demand winter months. Depleting storage would lower water levels which would provide more room for infiltration and provide less opportunity for evapotranspiration.
The basal-water reservoir in the leeward side is isolated hydrologically from abutting reservoirs outside the area and can and should be fully exploited. The existing development of the basal-water reservoir is small compared to the natural ground-water flow and that part not being developed is wasting to the sea. Because the area is hydrologically isolated, development will not be detrimental to or reduce the ground-water supply outside the area.