Water extraction from the ground using pumps and wells may have significant impacts on the flow of water in streams on the surface, according to new research. Results suggest that groundwater pumping has caused total stream flow in one Hawaiian stream to decline by 5.4mm per year since 1960.
The management of water resources has traditionally viewed surface water (water in streams, rivers and lakes), as separate from groundwater (water from underground streams and aquifers). The two are connected but their interaction is difficult to measure. Small island watersheds with varied land use and climate, such as in Hawaii, are uniquely suited to investigate this interaction. In Hawaii the rainfall recharges groundwater aquifers that supply 99 per cent of Hawaii's drinking water and half of its freshwater.
The study focused on the Makaha valley in the island of O'ahu which has an average rainfall of between 600 mm near the coast and more than 2000 mm inland. Between 1968 and 1995 a total of eight wells were drilled for municipal or irrigation purposes. The study examined data going back to the 1950s on rainfall, the flow of water in streams and the amount of water pumped from the wells to investigate the interaction between groundwater and surface water.
Results confirmed a decrease in the total stream flow (stream water that comes from surface run off and water in the ground) from 1960 to 2008 by 5.4 mm per year, where flow decrease was reported in units of equivalent depth (volumetric flow divided by watershed area). There has also been a decrease in the base stream flow (stream water that comes from just ground water) by 1.7 mm per year. During this time there were two significant downward shifts at around 1972 and 1992. There have been decreases in rainfall but the significant downward trend in rainfall has been from 1979 to 2008 which doesn´t align with the shifts in stream flow.
By comparing the trends in the amount of water pumped and the stream flow, it appears that pumping was responsible for the two downward shifts. In 1978 an extra well was introduced and a further six wells were brought on line between 1991 and 1995. It appears that groundwater pumping has reduced the amount that is discharged into streams by reducing the amount of water in the aquifer. The extent of year-round flow in the stream has receded from elevations above 305 m in the early 1990s to elevations above 424 m in 2006. This implies that stream flow in the Makaha valley may disappear entirely if wells continue to be developed and if rainfall continues to decrease.
The results have important implications for water resource management. Over the last 40 years the amount of freshwater extracted for irrigation or for municipal purposes has increased, but at the expense of declining water levels overall. Greater understanding of the interactions between surface and groundwater is needed to effectively manage water resources. Since it is difficult to model these interactions, the researchers suggest using methods similar to those used in this study that apply real results to investigate the relationships. This could be a first step before proceeding with more complicated modelling studies.