(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)
About a year ago, a Volcano Watch article described recent changes at Lake Waiau, the tiny lake just below Mauna Kea’s summit that is Hawai`i’s only alpine lake. Despite Lake Waiau’s small size (normally 0.7 hectares, or 1.7 acres), it plays an important part in local ecology and in Hawaiian culture. The article noted that the lake has been shrinking at an alarming rate. Over the past year, this decrease in lake size has continued, and the lake is now almost entirely gone.
Office of Mauna Kea Management Rangers, working cooperatively with the Department of Land and Natural Resources (DLNR), Division of Forestry and Wildlife (DOFAW) which manages the Mauna Kea Ice Age Natural Area Reserve, have been monitoring the lake closely and have tracked this remarkable reduction in the lake size with repeat photography.
HVO has recently been watching these changes, as well, as part of our broad mission to monitor Hawaii’s active (and recently active) volcanoes. HVO scientists have compiled numerous high-resolution satellite images to document the surface area of the lake since about 2000.
The results are compelling. Prior to 2010, the lake surface area fluctuated between about 5,000 and 7,000 m2 (1.2-1.7 acres), with the variability presumably due to recharge from winter storms balanced by loss due to evaporation. Sometime in early 2010, however, the lake surface area began to shrink and, by late September 2013, had declined to just 115 m2 (0.03 acres) – that is, about 2% of its normal surface area.
Geography professor Donna Delparte, formerly of University of Hawai`i at Hilo and now at Idaho State University, has also been monitoring the recent changes. Her group has made detailed measurements of lake geometry using advanced techniques, such as laser scanning and photogrammetry. Prior to 2010, the maximum depth of the lake was about 3 m (yards), but today the lake is less than 30 cm (1 foot) deep. This means that the current volume of the lake is less than 1% of its normal (pre-2010) value.
Using air photos to extend the time series of lake surface area back to the 1950s, we see no other drops of such scale. Historical photographs taken through the last hundred years, and written reports going back to the early 1800s, give no indication of the lake ever being as small as it is today. This suggests that the current reduction in size is unprecedented in modern times, but we cannot say this with absolute certainty, because there were large time gaps between the recorded observations in the 1800s and early 1900s. Nevertheless, the reduction in lake size that we see today appears to be highly unusual.
What could be driving such dramatic change? An obvious culprit would be the ongoing drought in Hawai`i that began in 2008. The Mauna Kea visitor center weather station shows very little precipitation for several consecutive months in early 2010, which may have been a trigger for the level drop that was sustained by low precipitation over the subsequent few years. The National Drought Mitigation Center shows that the drought across Hawai`i intensified in early 2010, consistent with this local weather data.
Could other factors be contributing to the potentially unprecedented nature of these changes? Lake Waiau is a “perched” water body, in which water is held in a depression by an impermeable substrate. This substrate consists of layers of silty clay, interbedded with ash layers, and it has been proposed that permafrost also underlies the lake.
It has also been proposed that permafrost surrounds the lake and provides a catchment that directs water into the lake. Could changes in the presumed permafrost have altered the water balance in the lake over the past few years? So far, there is no hard evidence to support this possibility, but we cannot yet count it out. We simply don’t know at this point, and more research needs to be done. If you have historical photos of the lake that you are willing to share, please contact HVO (askHVO@usgs.gov).
Given its cultural significance and its uniqueness, the disappearance of Lake Waiau would be a great loss for Hawai`i. The future is far from certain for Lake Waiau, and DLNR, rangers, and scientists will continue to watch this situation closely.