Almost 99% of Hawaiʻi’s drinking water comes from water stored underground in aquifers across the islands, and groundwater supplies are impacted by changes in both land cover and climate. The NSF funded ‘Ike Wai (knowledge of water) Project at the University of Hawaiʻi spans multiple scientific disciplines and connects researchers to better understand how future land use and climate change might change groundwater recharge, and how, this in turn would influence water management decisions.. Following the stakeholder-driven approach used in the Pacific RISA Maui Groundwater Project, we worked with the State Department of Forestry and Wildlife, the Koʻolau Watershed Partnership, the City and County of Honolulu, and the State of Hawaiʻi Department of Planning to come up with a set of possible futures of conservation, urban, and agricultural land in the most heavily utilized aquifer on Oahu: the Pearl Harbor aquifer. Pacific RISA co-lead Laura Brewington developed the future land cover scenario maps as inputs to the groundwater model, which was run by the US Geological Survey. The scenarios reflected both transit-oriented development (dense development around the projected corridor for the Honolulu light rail project) and sprawl-type development (a business-as-usual approach), in combination with varying degrees of agricultural intensification or reduction and native forest protections.

Sustainable yield estimates and resulting differences in replacement costs were estimated for the six land cover scenarios crossed with two water demand scenarios under a potentially drier future climate. The results showed that both climate and land cover change were important drivers of changes in groundwater recharge. The degree of watershed protection, through preventing the spread of high-water-use, invasive plant species, had a much stronger impact than urban development. Specifically, protecting all of the aquifer’s native forests increased sustainable yields by 7–11% (30–45 million liters per day) and substantially decreased treatment costs compared with no forest protection. Furthermore, the greatest benefits to recharge occurred in the upper elevations of the watershed, which impacted the Waipahu-Waiawa and Waimalu subaquifers most substantially.