Rainfall, Groundwater Pumping, and Streamflow

October 8, 8:00pm - 8:00pm
Manoa Campus, Agricultural Science Room 219

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Titled: Effects of Rainfall Variability and Groundwater Pumping on Streamflow in Upper Makaha Valley. Doctoral Defense Presentation by: Alan Mair, a Doctoral Candidate of Watershed Hydrology. The main goal of this study was to evaluate the effects of rainfall variability and groundwater pumping on streamflow in upper Mākaha valley. Historical rainfall, streamflow, and groundwater pumping data were compiled and analyzed for long-term trends and shifts. A field investigation was also conducted from 2005 to 2008 to supplement the historical hydrologic data and involved the monitoring of a variety of hydrometeorological parameters at multiple locations. Rainfall and ET parameters were measured using conventional tipping bucket rain gages and weather stations. Throughfall was measured using trough gages, designed for remote monitoring and steep terrain, in stands dominated by each of three species (Schinus terebinthifolius, Coffea arabica, and Psidium cattleianum). Continuous water level monitoring was conducted in a high-level marginal dike zone aquifer using pressure transducers. These data were used to assess rainfall spatio-temporal variability, and the relationships between rainfall, streamflow and pumping using time series analyses. Stream total flow and base flow significantly declined from 1960-2008, while groundwater pumping significantly increased over the same period. The water level in the high-level aquifer has declined as much as 14 m since the 1960s indicating depletion of groundwater storage. No significant rainfall trends were observed from the 1960s to the present; however, significant decline was observed over periods since 1979-80. Watershed yield has undergone two significant and successive downward shifts, first around 1972 and then in 1992. A regional rainfall maximum was observed west of Mt. Kaala, contrary to long-term isohyets maps. Mean throughfall was 60.5%, 58.2%, and 42.9% under canopies dominated by S. terebinthifolius, C. arabica, and P. cattleianum, respectively. The results suggest that groundwater pumping from the mid-valley began having a negative effect on streamflow as early as 1972, while pumping from the upper valley, which began in 1991, began impacting streamflow almost immediately. Further analyses are needed to better distinguish and quantify the impacts of pumping and rainfall trends.

Event Sponsor
Natural Resources and Environmental Management Department- CTAHR, Manoa Campus

More Information
Professor Ali Fares, 956-6361, AFares@Hawaii.Edu

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