Technical Report No.110
GROUNDWATER RECHARGE AND COASTAL DISCHARGE FOR THE NORTHWEST COAST OF THE ISLAND OF HAWAII: A COMPUTERIZED WATER BUDGET APPROACH
Brian Y. Kanehiro,Frank L. Peterson
The basic objectives of this study were to determine the average annual recharge to the groundwater body and the fresh groundwater discharge at the coastline, as well as the aquifer characteristics for an and area located on the northwest coast of Hawai’i island between Kiholo Bay in the south and Puako in the north, and encompassing the land from the coast to approximately the 762-m (2500-ft) elevation level. Of equal importance was the development of methodologies, namely, a computerized water budget and groundwater tidal analyses to calculate these quantities.
The probable recharge to the groundwater body in this region was computed to be 143.83 x106 M3/yr (38 bil gal/yr). Due to the lack of hydrologic data for the area, a range of values was determined, with the maximum average annual recharge being 264.95 x106 M3 (70 bil gal) and the minimum being 719.15 x105 M3 (19 bil gal). Since there are 26 km (16 miles) of coastline in this area, this implies a probable average daily flux of 15,006 m3/km (6.38 mil gal/mile) of coastline.
Three different calculations were obtained for the hydraulic conductivity of the groundwater aquifer in the coastal region: two by tidal analysis, and a third based on the coastal discharge as determined by the water budget. The Werner and Noren method of tidal analysis gave an average value for Kz (hydraulic conductivity times average saturated depth of aquifer) in the vicinity of ‘Anaeho’omalu Bay of 585,270 M2/day (6.3 x 106 ft2/day); the method by Cox gave a value for K in the same area of 1,027 m/day (3,369 ft/day). The coastal flux method gave a value for K of 2,771 m/day (9,092 ft/day).