Influence of groundwater pumping and rainfall spatio-temporal variation on streamflow.

Summary Groundwater pumping and surface water management structures, i.e. dams and water flow diversions, have been raising serious concerns about the declines in groundwater levels and streamflow in different watersheds throughout the world. Small island watersheds with complex land use, and strong spatio-temporal climate and edaphic variability (e.g. Hawaiian watersheds) offer an ideal environment to help improve our understanding of groundwater–surface water interaction and the hydrological processes it involves. In this study, we investigate the spatio-temporal relationships between streamflow, rainfall, and groundwater using long-term (>40 years) data records from a small Hawaiian watershed. A suite of Mann–Kendall tests were used to evaluate trends and shifts in time series data. The impact of groundwater pumping in the valley on streamflow was also investigated; multiple linear regression analysis was used to quantify the effect of pumping on streamflow. Stream base flow and total flow have declined significantly since 1960, while rainfall showed no statistically significant trends since the 1960s. Groundwater pumping has significantly increased since 1960, and our findings indicate it is a significant contributing factor to streamflow decline. Watershed yield experienced two successive downward shifts: first around 1971–1972 and then again around 1991–1992. The first downward shift appears to be related to the pumping of groundwater from the mid-valley area which began in 1968. The second downward shift is the result of pumping in the upper valley, which began in 1991. Regression models and double mass curve analyses indicate that pumping may have captured significant amounts of groundwater that otherwise would have comprised stream base flow. Streamflow has been reduced by 19–22% since 1971 and as much as 36% since 1991.