Scientific Investigations Report 2005-5146

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2005-5146

Effects of Irrigation and Rainfall Reduction on Ground-Water Recharge in the Lihue Basin, Kauai, Hawaii

Prepared in cooperation with the
County of Kauai Department of Water

By Scot K. Izuka, Delwyn S. Oki, and Chien-Hwa Chen

Abstract

Recent declines in water levels and productivity in some wells and tunnels in the Lihue Basin, Kauai, Hawaii, have raised concerns about the future reliability of ground-water sources. The trend of declining water levels coincides not only with increases in ground-water development, but also with decreases in applied irrigation and periods of lower-than-average rainfall. Water-balance computations indicate that the sugarcane industry had, at its peak, artificially increased recharge by about 25 percent over natural conditions. Periods of decreased precipitation and irrigation, concurrent with declines in observed ground-water-levels, caused substantial reductions in ground-water recharge relative to periods of normal rainfall and full irrigation. Simulations of recent decreases in irrigation, a recent drought, and hypothetical future scenarios of droughts and irrigation cessation indicated basin-wide recharge decreases of 7 to 83 percent relative to the condition of normal rainfall and full irrigation.

For the period during the observed decline in ground-water levels, the water-balance simulations indicate that the effect of the recent drought was greater than the effect of reduced irrigation. Effects of droughts, however, are temporary conditions that will eventually be mitigated by wet periods, whereas loss of irrigation in the Lihue Basin may be permanent and have a greater long-term effect. Effects of irrigation also may appear to be small relative to basin-wide recharge, but the effects of irrigation changes are concentrated in former irrigated sugarcane fields, and many wells with recent declining water levels are near these former sugarcane fields.

Contents

Abstract
Introduction
Description of Lihue Basin
Changes in Ground-Water Recharge
Summary and Conclusions
References Cited
Appendix A

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