MODFLOW-2005 and SWI2 models for assessing groundwater availability scenarios in volcanic aquifers on Kauai, Oahu, and Maui, Hawaii

Previously constructed steady-state numerical groundwater-flow models for the islands of Kauai, Oahu, and Maui, Hawaii (https://doi.org/10.3133/sir20205126) using MODFLOW-2005 with the Seawater Intrusion (SWI2) package, were used to examine the consequences of historical and plausible future withdrawals and changes in recharge. The volcanic aquifers of the Hawaiian Islands supply water to 1.46 million residents, diverse industries, and a large component of the U.S. military in the Pacific. Groundwater also supplies freshwater that supports ecosystems in streams and near the coast. Hawaii’s aquifers are remarkable given their small size, but the islands’ capacity to store fresh groundwater is limited because each island is surrounded by seawater, and saltwater underlies much of the fresh groundwater. The amount of fresh groundwater available for human use from Hawaii’s volcanic aquifers is constrained by the consequences of groundwater withdrawal. Restrictions placed on these consequences can translate to limitations on groundwater availability. Changes in recharge resulting from changes in land cover or climate can alter the effect of withdrawals. Therefore, five scenarios representing current conditions and various historical and projected future groundwater-withdrawal and recharge conditions were simulated using the previously published numerical models. The Current scenario represents conditions in 2010 which were used to calibrate the Kauai, Oahu, and Maui models. This scenario is the baseline to which all other scenarios are compared. Two historical scenarios (No Withdrawal and Predevelopment) represent selected aspects of conditions that existed in 1870, before the first modern well was drilled in 1879; these scenarios were simulated using all three models in this study. Two future scenarios (Future Rainfall and Increased Withdrawal) represent projections of future conditions and were simulated using the Oahu model only. Results of the simulations using the groundwater models of the islands of Kauai, Oahu, and Maui have implications for other islands in Hawaii. Results of the simulations enable quantification of the hydrologic effects of withdrawals and changes in climate, such as water-table depression, saltwater rise, and reduction of natural groundwater discharge to streams, springs, and the ocean. The effects can place limits on groundwater availability. This USGS data release contains all the input and output files for the simulations described in the associated model documentation report (https://doi.org/10.3133/pp1876).