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Summary: Distributed Hydrology Soil Vegetation Model (DHSVM) was successfully applied in Mākaha watershed to investigate the hydrologic impact of groundwater pumping on the streamflow. DHSVM was calibrated (1971–1980) and validated (1981–1990) against observed streamflow during pre-development conditions. Monthly observed and simulated streamflows after the development (after 1990) were compared to quantify the decline in streamflow. Simulation results indicate that declines in streamflow could be attributed to: (i) an increase in the fraction of gross rainfall converted into evapotranspiration (ET); and (ii) groundwater pumping. Groundwater pumping from additional groundwater development in the form of new pumping wells starting in 1991 has significantly reduced the streamflow, especially during the summer. The fraction of gross rainfall converted into ET has significantly increased by 7% during 1991–2009 compared to the pre-development period. Based on the model results, in the absence of the groundwater development in the upper part of Mākaha watershed, the observed cumulative streamflow could have been 11% higher than its current level. On an annual basis, total additional groundwater pumping accounts for 26% of the recharge with a significant seasonal variability (12% in wet and 180% in dry season). Relative to recharge, higher groundwater pumping during the dry season could have been the reason behind the significant streamflow decline during the summer. This study clearly emphasizes the importance of temporal patterns in groundwater withdrawal which are often ignored while determining safe yield. [ABSTRACT FROM AUTHOR]

Land-use/land-cover changes in Mäkaha valley have included the development of agriculture, residential dwellings, golf courses, potable water supply facilities, and the introduction of alien species. The impact of these changes on surface water and ground water resources in the valley is of concern. In this study, streamflow, rainfall, and ground-water pumping data for the upper part of the Mäkaha valley watershed were evaluated to identify corresponding trends and relationships. The results of this study indicate that streamfiow declined during the ground-water pumping period. Mean and median annual streamfiow have declined by 42% (135 mm) and 56% (175 mm), respectively, and the mean number of dry stream days per year has increased from 8 to 125. Rainfall across the study area appears to have also declined though it is not clear whether the reduction in rainfall is responsible for all or part of the observed streamfiow decline. Mean annual rainfall at one location in the study area declined by 14% (179 mm) and increased by 2% (48 mm) at a second location. Further study is needed to assess the effect of ground-water pumping and to characterize the hydrologic cycle with respect to rainfall, infiltration, ground-water recharge and flow in the study area, and stream base flow and storm flow. [ABSTRACT FROM AUTHOR]