Low-flow characteristics of streams in the Lahaina District, West Maui, Hawai'i

The purpose of this study was to characterize streamflow availability under natural low-flow conditions for streams in the Lahaina District, west Maui, Hawaiʻi. The study-area streams included Honolua Stream and tributary Pāpua Gulch, Honokahua Stream and tributary Mokupeʻa Gulch, Kahana Stream, Honokōwai Stream and tributaries Amalu and Kapāloa Streams, Wahikuli Gulch and tributary Hāhākea Gulch, Kahoma Stream and tributary Kanahā Stream, Kauaʻula Stream, Launiupoko Stream, Olowalu Stream, and Ukumehame Gulch. The results of this study can be used to assist in the determination of technically defensible instream-flow standards for the study-area streams.

Low-flow characteristics for natural (unregulated) streamflow conditions were represented by flow-duration discharges that are equaled or exceeded between 50 and 95 percent of the time. Partial-record sites were established on 10 main streams and 5 tributary streams, mainly upstream from existing surface-water diversions. Flow characteristics were determined using historical and current streamflow data from continuous-record streamflow-gaging stations and miscellaneous sites, and additional data collected as part of this study. Based on strategically scheduled observations, six of the study-area streams were ephemeral streams that were observed to remain dry at least 50 percent of the time: Pāpua Gulch, Honokahua Stream and its tributary Mokupeʻa Gulch, Kahana Stream, and Wahikuli Gulch and its tributary Hāhākea Gulch. For the remaining streams with measurable flow, Honolua, Honokōwai, Kahoma, Kanahā, Kauaʻula, Launiupoko, and Olowalu Streams, and Ukumehame Gulch, flow-duration discharges were computed for the 30-year base period (water years 1984–2013), using two record-augmentation techniques. The 95-percent flow-duration discharges ranged from 0 to 4.8 cubic feet per second (ft³/s). The 50-percent flow-duration discharges ranged from 0.47 to 9.5 ft³/s.

This study also estimated the streamflow gains and losses downstream of surface-water diversions using seepage-run measurements. A majority of the streams lost flow downstream from diversions. Measured seepage-loss rates ranged between 0.045 and 1.6 ft3/s per mile of stream reach. Seepage gains mostly occurred upstream from diversions and the measured seepage-gain rates generally ranged between 0.75 and 5.1 ft3/s per mile of stream reach. Under natural-flow conditions, Honolua Stream is estimated to flow to the ocean less than 80 percent of the time and Honokōwai Stream is estimated to flow to the ocean less than 50 percent of the time. Kahoma Stream, Kauaʻula Stream, Olowalu Stream, and Ukumehame Gulch are estimated to flow to the ocean at least 95 percent of the time.