EXPOSURE OF PERCHED GROUNDWATER BODIES AFFECTS STREAM BASEFLOW ON YOUNG VOLCANIC ISLANDS

The shield-building phase of Haleakalā and Mauna Kea volcanic islands consists of thick accumulations of thin vesicular pāhoehoe and ʻaʻā lava flows which have high hydraulic conductivity (e.g., estimated range 152 to 1524 m/d) and permeability. Overlaying the shield-building phase on Haleakalā is the post-shield Kula Volcanics (15-930 ka) with thicknesses of 15-60 m near the coast, and on Mauna Kea is the Hāmākua Volcanics (65-265 ka) with thicknesses of 24-168 m near the coast. At low elevations (<1000 m a.s.l.), these series are undivided and consist of basaltic flows, some cinder cones, and minor pyroclastic deposits. These phases often have patterns of short periods of erosion, soil formation, and ash deposition followed by new lava flows created numerous opportunities for the formation of perched water bodies. Thus, perched groundwater is regularly found above the interface between shield-building and post-shield phases, intercepting water infiltrating from upslope regions. In many locations, stream channel incision has exposed these perched bodies, producing springs that contribute groundwater to streamflow. We quantified low flow conditions in 12 streams on Mauna Kea, whose watersheds are composed almost entirely of Hāmākua Volcanics, and 14 streams on Haleakalā, whose watersheds are almost entirely composed of Kula Volcanics, to quantify the relationship between streamflow and a stream’s exposure to perched water bodies.

We found that low flows were significantly related to length of stream channel normalized by drainage area on both Mauna Kea and Haleakalā. Interestingly, mean annual rainfall only had a significant affect on Mauna Kea, while on Haleakalā, results were independent of rainfall. The extent that perched groundwater bodies contribute to baseflow in stream ecosystems is important for quantifying the availability of water under drought conditions and critical for making water resource management decisions.