With global temperatures inexorably rising, how much groundwater pumping will the state’s aquifers be able to sustain in the coming decades?
A study by the U.S. Geological Survey on groundwater recharge, released in May, may help the state Commission on Water Resource Management determine that, but revising the agency’s sustainable yield estimates to account for climate change will be no simple task.
“Most of the data sets suggest a drier future for the Hawaiian islands,” USGS hydrologist Heidi Kāne told the Water Commission last month during a presentation on her agency’s final estimates for mid-century and end-of-century groundwater recharge for the Hawaiian islands.
Some models, however, suggest a wetter future, she added.
The divergence, also reflected in preliminary data the USGS released in 2022, is due to the two types of models used to predict how global climate changes will affect specific regions: statistical downscaling models use local weather data to help determine how a region might respond to global climate changes, while dynamical downscaling models rely on projected regional climate conditions derived from global climate models.
The USGS came up with several groundwater recharge scenarios for Maui, Hawaiʻi, Oʻahu, Kauaʻi, Lanaʻi, and Molokaʻi aquifers using the two different approaches. The agency then narrowed its focus on two statistical downscaling scenarios (a projection for the years 2041-2071, and one for 2071-2099), as well as the “wettest” projections produced by the downscaling model. It also did a drought simulation for Lanai, as well.
These scenarios were chosen “to capture the widest range of wetting and drying,” Kāne said. Those scenarios were then compared to a reference climate of 1978-2007 to quantify how much recharge a given area would gain or lose.
The statistical downscaling scenarios predicted overall declines in recharge on all islands, while the dynamical ones suggested that most of the aquifers throughout the islands would see greater recharge. However, even under the most favorable projection from the “wet” scenario, the island of Hawaiʻi was predicted to see an overall decrease in recharge.
For the aquifers in western and central Maui — where the county’s development plans direct most of the future growth on the island — decreases in recharge as high as 72 percent were projected in the mid-century and the “dry” climate scenarios and, to a lesser extent, for the “wet” climate scenario, Kāne reported.
For the “wet” scenario, only the Ukumehame aquifer in the Lahaina aquifer sector was predicted to see a drop in recharge. The other five were predicted to see slight increases, of up to 9 percent. (The increases in recharge predicted for the Launiupoko and Honokowai aquifers — 0.9 percent and 5.1 percent, respectively — may be of little comfort to resource managers, given that commission staff determined in 2022 that total existing and authorized planned use for those aquifers exceed the sustainable yields by 115 percent and 170 percent, respectively.)
Under the mid-century and “dry” scenarios, all but three East Maui aquifers — Honopou, Waikamoi, and Kawaipapa — would see a drop in recharge. And the increases expected in those three aquifers were also slight, ranging from 0.6 percent to 3.4 percent.
The wet climate scenario projected greater recharge in East Maui’s aquifers, ranging from 13 to 21 percent.
For Kauaʻi, all aquifers were predicted to see less recharge under the statistical downscaling scenarios, with the Kekaha aquifer suffering the most. Under the mid-century scenario, it would see a drop in recharge of 62 percent. That number would grow to 82 percent by the end of the century.
The wet climate scenario showed a drop in recharge of up 13 percent for three leeward aquifer systems (Napali, Waimea, and Wainiha) and wetting of up to 20 percent for the remaining aquifer systems on the island, Kāne reported.
Similarly on Oʻahu, the “wet” scenario found that only three aquifers on the island would see drops in recharge (Mokuleia, Makaha, and Lualualei), while the mid-century and dry scenarios predicted recharge declines in all aquifers, especially along the west and northwest coasts.
West Oʻahu’s Nanakuli aquifer was predicted to see the greatest drop in recharge: 66 percent by mid-century, and 84 percent by the end. Neighboring Lualualei aquifer was not far behind, with predicted drops of 65 percent and 81 percent, respectively.
Molokai was also expected to see extensive drying — up to 64 percent by the end of the century — on the western side of the island and no increases in recharge for any aquifers under the statistical downscaling scenarios. However, the “wet” scenario predicted increases in recharge of up to 62 percent in one aquifer, with only two aquifers — Kahanui and Haʻupu — getting drier.
Lanaʻi had the starkest contrast between the statistical and dynamical downscaling model results. The former projected decreases in aquifer system recharge as high as 71 percent estimated, which aligned closely with the USGS’s drought climate scenario for the island. However, groundwater recharge would increase as high as 73 percent indicated for seven aquifer systems in wet climate scenario, Kāne said.
Finally, Hawaiʻi island, like Maui, was projected to see an increase in rainfall along the north eastern coast under the mid-century and dry scenarios, but most of the island would see less rainfall. The “wet” scenario also showed drying across 16 aquifer systems, with wetting in just eight of them.
“These three scenarios tend to align better than they do on the other islands,” Kāne said.
Next Steps
When it came time to discuss the findings, commissioner Aurora Kagawa-Viviani seemed critical of the projections produced by the dynamical downscaling model, known as the Hawaiʻi Regional Climate Model (HRCM).
“This is a longstanding debate. They admit HRCM … does well simulating historical patterns over Hawaiʻi island, but it does poorly over Kauaʻi and, most particularly, Maui and Oʻahu. So there’s these two competing projections and that is often a source of discussion and confusion. But there’s a saying. I don’t want to say, ‘Garbage in garbage out,’ but the rainfall data that’s used is really influential when looking at projected recharge,” she said.
Kane conceded that there was greater uncertainty with the HRCM, noting that the model tries to predict what the climate conditions will be in the 2080-2099 time frame.
“They’re looking very far out into the future and there is some greater uncertainty in that,” Kane said.
Jonathan Scheuer, a consultant for the Department of Hawaiian of Hawaiian Home Lands, suggested a way forward.
“If we have two models and sometimes they agree and sometimes they disagree, let’s look only at where they agree,” he said.
Of the 110 aquifer systems in the state, there are five that all three models agreed would become wetter and 35 where they agreed it would become drier, he pointed out.
Commission chair and director Dawn Chang pointed out that the data will also be useful in guiding which of the Department of Land and Natural Resources’ forest lands that are included in agricultural leases planned for transfer to the state Department of Agriculture need to be retained.
“We want to keep a lot of those in forest reserve for water recharge. It does help us make those kinds of decisions now for future projection of our watershed,” she said.
When the commission will take any action based on the USGS data remains to be seen.
“[T]he Commission on Water Resource Management is in the beginning stages of discussions with the USGS and other stakeholder groups to determine how best to incorporate the new data and update sustainable yields statewide. The Commission is still assessing the report findings and has not made a determination on weighting any of the future recharge scenarios. Areas such as West Maui and West Hawai‘i, where groundwater resources may not be fully understood, will likely be considered a higher priority,” the commission staff stated in an email to Environment Hawaiʻi.
— Teresa Dawson
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