干旱区生态环境知识资源中心

Basaltic lava is generally porous and cannot hold water to form lakes. Here we investigate two impermeable cinder cones in the alpine desert of Maunakea volcano, Hawaii. We present the results of the first ever geophysical survey of the area around Lake Waiau, the highest lake on the Hawaiian Islands, and establish the existence of a second body of standing water in a nearby cinder cone, Pu’upohaku (similar to 4000 m above sea level), which has a sporadic pond of water. Based on unpublished field notes from Alfred Woodcock (*1905-dagger 2005) spanning the years 1966-1977, more recent observations, and our own geophysical survey using electric resistivity tomography, we find that perched groundwater resides in the crater perennially to a depth of 2.5m below the surface. Hence, Pu’upohaku crater hosts a previously unrecognized permanent body of water, the highest on the Hawaiian Islands. Nearby LakeWaiau is also perched within a cinder cone known as Pu’uwaiau. Among other hypotheses, permafrost or a massive block of lava were discussed as a possible cause for perching the water table. Based on our results, ground temperatures are too high and specific electric resistivity values too low to be consistent with either ice-rich permafrost or massive rock. Fine-grained material such as ash and its clay-rich weathering products are likely the impermeablematerial that explains the perched water table at both study sites. At Pu’uwaiau we discovered a layer of high conductivity that may constitute a significant water reservoir outside of the lake and further be responsible for perching the water toward the lake.