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Low-Flow Characteristics for Streams in Southeast Kauai, Hawaii
Project Chief: Chui Ling Cheng
Existing surface-water diversion systems in southeast Kauai are vast and complex, and the degree to which they alter the flow of streams in the study area is uncertain because limited data are available for the diversion systems and intakes that are currently in operation. Surface water from streams mixed with groundwater pumped from wells is transported across drainage basins. In some of the heavily developed areas, the stream is used as a conduit for transporting water between several pass-through reservoirs, leaving no single reach of the stream with unregulated flow.
Kauai is among the most geologically complex of the eight main Hawaiian islands and Lihue basin is one of the most geologically complex areas on the island. Low permeabilities of the volcanic rocks of southern Lihue basin result in high inland groundwater levels and a high proportion of groundwater discharging to streams. This interaction between groundwater and surface water is critical to understanding available water resources on the island.
The objectives of this 4-year study on the study-area streams are to (1) quantify the amount of water available under natural, low-flow conditions upstream of existing surface-water diversions; and (2) characterize the low flows at selected sites downstream of diversions. Low-flow characteristics will be expressed as estimates of selected natural low-flow duration discharges between the 50 and 95 flow-duration percentiles.
Relevance and Benefits
This study will provide the public and decision makers with useful information on the hydrologic condition of the streams in southeast Kauai, Hawaii. Quantifying the availability of surface water in the area is critical to understanding how current and future changes in water demand will affect natural resources and the community. This effort is in agreement with the USGS Science Strategy related to understanding ecosystems and predicting ecosystem change. As water demand increases, it is crucial to properly manage the State’s limited water resources for current and future needs. The results of this study will aid in the establishment of instream flow standards, which is currently one of the State’s efforts in the management of its surface waters and the protection of biological resources in the streams. Information from this study will help communities and policy makers develop adaptive management strategies that consider the overall ecosystem health and sustainability.
The scope of the study includes a reconnaissance survey of the study area to provide insight into the surface-water diversion systems and the effects of these systems on stream low flows. This information will be used to determine the number of measurement sites at which streamflow data will be collected to determine selected natural low-flow characteristics. Low-flow characteristics, expressed as estimates of selected natural low-flow duration discharges between the 50 and 95 flow-duration percentiles, will be estimated using record-extension methods that relate flow measurements at the low-flow partial-record stations and concurrent daily flow at index stations. Streamflow measurements to identify gaining and losing stream reaches will also be quantified along the study-area streams.