SBCT Final EIS - Govsupport.us

Chapter 3 ⎯ Affected Environment3.1.2 WATER RESOURCESThe proposed project components are located on two Hawaiian Islands — Oahu and Hawaii. The projectarea is located in a tropical climate. Rainfall in the Hawaiian Islands is very unevenly distributedand is highly dependent on elevation as well as location. The maximum rainfall occurs at elevationsbetween 2,000 to 3,000 feet amsl. Above this elevation, rainfall decreases rapidly so that the high elevationsare relatively dry (USAG-HI 2004).Affected Environment for water resources includes description of surface water and groundwater resources.These include surface water occurrence, flooding, surface water quality, groundwater occurrence,and groundwater quality. Wetlands on the affected areas are presented in Section 3.1.9.The ROI for surface water resources includes the watersheds containing the training and deploymentareas on Oahu and Hawaii. The ROI for surface water is not necessarily the same as the ROI forgroundwater. Because groundwater often crosses topographic watershed boundaries, the ROI for surfacewater is expanded to include the aquifers underlying these watersheds and any aquifers downgradient(in the direction of groundwater flow) from the training and deployment areas. The ROIs forboth surface water and groundwater include the downstream and downgradient near-shore areas alongthe coast where surface water and groundwater, respectively, discharge to the sea.3.1.2.1 Schofield Barracks Military ReservationThe uneven distribution of rainfall has implications for surface water runoff and groundwater recharge.The upper portion of each watershed can receive significantly more rainfall in a given stormthan the lower portion. Many of the watersheds on the islands are small, and there is often little storagecapacity, resulting in rapid runoff and common flooding. Streams on Oahu are generally perennialat higher elevations, where there is greater precipitation, and at lower elevations, where the topographyintercepts the groundwater table (Nichols et al. 1996). At intermediate elevations, streamstend to be intermittent due to a combination of high infiltration, diversion of the flows, and highevaporation rates at low elevations (USAG-HI 2004).Surface Water OccurrenceThe average annual precipitation at SBMR is 43.75 inches. Monthly averages range from 1.63 to3.78 inches during the dry season (April through October) and from 4.14 to 6.21 inches during thewet season. SBMR lies within an area in which the 100-year 24-hour rainfall is estimated to be about16 inches.SBMR lies near the drainage divide between the Kaukonahua watershed and the Waikele watershed.The principal surface water feature of the Kaukonahua watershed is the Wahiawa Reservoir (LakeWilson), which lies just outside the eastern boundary of SBMR, east of Highway 99. The reservoirstores drainage from tributaries of the Kaukonahua Stream that originate in the Koolau Range. Thereservoir receives small amounts of surface drainage from the eastern side of SBMR and is used foragricultural irrigation.The main drainages at SBMR are the Waikoloa Gulch and the Waikele Stream. The Waikoloa Gulchdrains the area just north of the cantonment and joins the Kaukonahua Stream below Wahiawa Reservoir.Two other streams that drain the north part of SBMR (Mohiakea Gulch and Haleanau Gulch) aretributaries to the Kaukonahua Stream. Kaukonahua Stream drains northward through the area underlainby the Waialua aquifer system, joining the Poamoho Stream to form the Kiikii Stream, whichdischarges to Kaiaka Bay just east of Waialua. Streams in lower reaches of SBMR tend to be inter-February 2008 3–18 2/25th SBCT Final EIS