Time series analysis of daily rainfall and streamflow in a volcanic dike-intruded aquifer system, O'ahu, Hawai'i, USA

A conceptual model of groundwater occurrence was developed for a dike-intruded aquifer system in MAkaha valley, O'ahu, Hawai'i, USA, and used to explain the impacts of water resource development on groundwater levels and streamflow. Time-series analyses were applied to two subsets of daily rainfall, total flow, and baseflow-from the third (1946-1968) and fourth (1968-1991) periods of development-to evaluate changes in streamflow response. Autocorrelation, cross-correlation, squared coherency and phase functions were used to estimate the decorrelation lag time, and the correlation length, linearity with frequency, and frequency response delay between rainfall and streamflow. The decorrelation lag time for total flow and baseflow declined by 16-20 days (d). The correlation length between rainfall and streamflow declined 4-7 d for total flow and 10-13 d for baseflow. The squared coherency function indicates an overall decline in linearity between rainfall and total flow across most frequencies. The changes in hydrologic response following increased water-resource development is consistent with a model of groundwater depletion resulting in less groundwater discharge and more induced groundwater recharge. Changes in streamflow response are inconsistent with observed changes in rainfall and imply that streamflow decline is more likely to be the result of groundwater pumping.