The Lihu`e Basin is the source of drinking water for more than half of Kaua`i’s population of 58,000. But according to a recent study by the U.S. Geological Survey, the basin has experienced sharp declines in water levels and well productivity, raising concerns “about the future reliability of ground-water sources.”
The USGS identifies three factors contributing to the decline. First, there is increasing demand for water from the basin. Second, rainfall in the area has been lower than average in recent years. Most significant, though, is the third factor: the phase-out of sugarcane cultivation in the area. Irrigation of the cane fields, the USGS says, “artificially increased recharge by about 25 percent over natural conditions.”
According to the lead author, Scot Izuka, “even if there was agriculture to replace sugar, they wouldn’t use the inefficient, furrow-irrigation method used in the early days of sugar.” Sugar planters were themselves switching from furrow irrigation to drip, which was three times as efficient – and therefore contributed much less to groundwater recharge. Also, says Izuka, “we didn’t consider the effect of increased demand, since this was primarily a recharge study. We’re working now on a model to see what happens when we put everything together – pumping, changes in recharge, etc. That won’t be done for probably another year.”
Water levels have declined to the point that well and tunnel production has been affected. “For example,” the report states, “at the Garlinghouse Tunnel, one of the most productive sources of public water in the area of the Kilohana-Puhi wells, two pumps could be operated continuously prior to 1970, yielding about 1,500 gal/min for extended periods. By the 1990s, however, only one pump could be operated (at about 800 gal/min) without causing water levels to decline to the level of the pump intake.”
When the USGS put together a model of the groundwater combining no irrigation with extreme drought conditions, the result was a calculated “worst-case scenario,” with a decrease in recharge of up to 84 percent from the 264 million gallons a day of recharge when cane fields received furrow irrigation in a wet year. “Of course, droughts are passing things,” Izuka says. “One would imagine eventually they will go away and we’ll have higher-than-normal rainfall to compensate.”
The study, he says, shows the extent to which irrigation can have a substantial effect on recharge. “We haven’t done studies like this in every area where there used to be sugar, but it would be interesting to do that.”
The report’s conclusions do not bode well for the area served by the Lihu`e Basin. “Results of the water-balance analysis indicate that recent variations in precipitation and irrigation in the Lihu`e Basin have caused large reductions in ground-water recharge, and that plausible scenarios of future land-use changes and drought could result in even greater reductions in ground-water recharge.”
(The report, “Effects of Irrigation and Rainfall Reduction on Ground-Water Recharge in the Lihue Basin, Kaua`i, Hawai`i,” was written by Scot Izuka, Delwyn Oki, and Chien-Hwa Chen. It is available online at [url=http://pubs.usgs.gov/sir/2005/5146]http://pubs.usgs.gov/sir/2005/5146[/url].)
— Patricia Tummons
Volume 16, Number 11 May 2006
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