Groundwater Pumping Effects On Streamflow in a Flashy Mountainous Hawaiian Watershed.

Hawaii streams are known for generating intense and short-duration runoffs that often cause flash floods. Many of the perennial streams go dry during the year; excessive groundwater pumping is one of  the contributing factor to streamflow decline. The  distributed hydrology soil vegetation model (DHSVM) was applied in the mountainous 13 km² Makaha watershed, Hawaii to: 1) assess model applicability in predicting hydrological processes of a flashy watershed under tropical condition,  and 2) evaluate model suitability as a tool to assess the link between groundwater pumping and streamflow. The model was calibrated (1971-1980) and validated (1981-1990) to the pre-pumping watershed conditions against measured daily streamflow. Simulations were performed at 1 h time step and  10 m grid resolution.  Model initial states were generated by running it for five years (1966-1970) as warm-up period. Model performance was evaluated by comparing the daily measured and simulated streamflow using Nash-Sutcliffe efficiency, correlation coefficient, and root mean  squared error for each year of simulation. Monthly measured and simulated streamflow during extensive pumping (after 1990) periods were compared to quantify the decline in streamflow.  DHSVM model performed reasonably well in simulating daily streamflow; however, it had less success in simulating extreme rain events. Comparison of simulated and observed streamflow during the extensive pumping period (1991-2010) indicates decline in streamflow which can be attributed to groundwater pumping.