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North-central Oahu consists of 152 square miles and includes mountainous terrain, the Schofield Plateau, and a narrow coastal plain. Half the area is above an altitude of 1,000 feet and receives 71 percent of the 580 mgd (million gallons per day) average annual precipitation. Recharge to groundwater reservoirs from precipitation is estimated to be about 225 mgd.Streamflow to the ocean is negligible, except during floods, owing to diversions by way of numerous mountain ditches into reservoirs. As it is used for irrigation of sugarcane, a large part of the diverted water percolates into underlying ground-water reservoirs.Ground water is available from wells drilled into lava flows of the Koolau and Waianae Volcanic Series and from shallow sedimentary material in the coastal plain. About 200 mgd discharges to the ocean from ground-water reservoirs underlying the coastal plain.Water use in a dry year is about 93 mgd 75 mgd for irrigation and 18 mgd for domestic, industrial, and miscellaneous uses. Two-thirds of the total is from ground-water sources, and one-third is from streams and reservoirs. Long-term ground-water pumpage for all uses averages 48 mgd, of which nearly 42 mgd is for irrigation. Average annual streamflow diversion is 57 mgd, and irrigation application is approximately 36 mgd.Salt-water intrusion of the shallow sedimentary material or of the deeper water-bearing lava is not yet a major problem. Water from a few heavily pumped coastal wells has become moderately saline, but a reduction in pumping at these sites, a dispersal of wells, or an inland placement of new wells would minimize intrusion.

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