In the ʻŌlaʻa rainforest of Hawaiʻi Volcanoes National Park, mist hangs in the air. Bright red blooms of ʻhiʻa trees mix in the canopy with ʻlapa trees that shimmer in the breeze. Underneath, towering fronds of the hāpu‘u fern shelter a forest floor slippery with mud. Moisture-loving plants like moss and ferns cover almost every tree surface, from downed logs to upright trunks and spreading branches. And on the forest floor lies wet leaf litter, with more wet fern, wet moss, and rare plants such as the Honohono Hawai‘i jewel orchid.

The ʻŌlaʻa Forest Reserve sits on the Big Island’s eastern side, on the windward slopes of Mauna Loa volcano. Like most areas on the island’s windward side, ʻŌlaʻa gets a lot of rain most of the year. “It’s a lush, verdant area,” said park botanist Sierra McDaniel. “When you visit, you’ll always need your rain jacket and rain boots. In summer, it rains often. In the winter, it rains all day, nearly every day.”

That wasn’t the case for most of 2010. McDaniel, who was already employed at the park, recalls the vegetation drying up and dying. “You would just go in there, and it would crunch,” she said. “You would never hear that otherwise. The leaf litter was dry, the ferns were brown and shriveling up, and leaves were falling off the trees. Moss covering the logs just crisped up. And the soil—it was just not wet. There was a dusty element to it, which was strange.”

From late 2009 to late 2010 only three inches of rain had fallen in the park’s rainforests. In more typical years, those areas would have received more than 50 inches. With dead ʻhiʻa trees turning into tinder, the park ceased chainsaw operations in some areas to prevent sparks from starting fires. Officials issued campfire restrictions and closed parts of the park.

By August, fire crews from the mainland flew in to help protect the park’s endangered plant and animal species and to prevent brush fires from threatening nearby residential areas. “We didn’t have a big fire that year, but if we had it would have been devastating,” McDaniel said.

On the Big Island’s drier, western side, coffee trees had been dying as well. Normally relying on rainfall to water their crops, Kona coffee farmers had started buying public water, drawn from groundwater, for irrigation. But they still lost trees.

Kevin Kodama, a hydrologist at the National Weather Service’s Weather Forecast Office in Honolulu, collaborates with the U.S. Drought Monitor in issuing the state’s drought alerts. He remembers 2010 well. For most of the year, more than 40 percent of the Hawaiian Islands experienced severe, extreme, or exceptional drought conditions. Between July and November, more than five percent of the state’s land area reached the exceptional drought category, defined as widespread crop and pasture losses, fire risk, and water shortages that result in water emergencies.

“Drought is defined by impacts,” Kodama said. “There is meteorological drought, which simply means you have some rainfall deficit.” When meteorological drought starts affecting forest vegetation, like it did in Hawai‘i Volcanoes National Park, it morphs into ecological drought. It picks up other names as it affects other things—agricultural drought, socio-economic drought, water supply drought. The U.S. Drought Monitor issues alerts for the state of Hawai‘i when a decrease in rainfall begins to impact different aspects of life in the state. And the 2010 drought affecting Hawai‘i’s coffee farmers, “that was a big impact, because Kona coffee is known worldwide,” Kodama said.

Kodama pointed out that other sectors had also been severely affected. “Ranchers are typically the first to get hit during droughts. Pasture conditions were already in poor shape around 2009,” he said. Herds had to be thinned or relocated to the mainland. There were reports of cattle being found dead in the pastures because there was no water or feed. Some cattle that survived the drought weren’t healthy enough to breed. “Once ranchers get droughts of that size it takes them years to recover, even if the rainfall returns,” Kodama said.

This is not the first time that Hawai‘i has experienced drought. “Drought is a naturally recurring phenomenon in Hawai‘i,” Kodama said. “It’s always gone on. And it will continue to go on.” However, he noted the 2010 drought was unprecedented. “In some places in the state, it’s considered the worst drought in modern times.”

Abby Frazier, a research fellow at the East-West Center who studies rainfall and drought in the tropics, says the drought started way before 2010. “Really, the rains stopped coming in 2007 to 2008,” she said. “Rainfall just stayed very low or at zero until 2014. And that’s what woke people up: not that it was dry and severe, but that it lasted for so long.”

The islands recover quickly after short droughts, but the 2007 to 2014 multi-year drought caught some sectors off guard. Recent studies show Hawai‘i has been experiencing decreasing rainfall over the last century. With the 2010 wake-up call, how is the state preparing for the next big, bad drought and possible dry years to come?

Hawai‘i has two faces
The secret to understanding where fresh water can be found in Hawai‘i is in knowing the land and its relationship with the enormous ocean that surrounds it. Frazier has been studying the islands’ rainfall patterns and is interested in how these have changed over time and how they might change in the future. She said, “The windward and leeward profiles of the islands define a lot in terms of rainfall and in terms of drought vulnerability.”

Each Hawaiian island is essentially a mountain made up of one or more shield volcanoes partially submerged in the North Pacific Ocean. When trade winds blow from the northeast, they push cool, moist air toward the islands’ eastern and northeastern slopes. This wind rises as it meets the windward sides of the mountains, and as this happens, moisture in the air condenses, producing clouds and rain. “You have very little moisture left when the wind comes over the leeward side,” Frazier said. “It’s a dramatic difference because our mountains are so tall.”

This interaction between the trade winds blowing from the ocean and tall mountains near the coast is called the rain shadow effect. One way to get a feel for the effect is to look at the Hawaiian Islands on Google maps with the satellite layer turned on. The image shows the islands’ northeast sides generally greener than the south and west sides. The islands’ mountain shape, the trade winds, and the rain shadow effect influence how different and diverse climate can be on the islands. The Big Island, for example, has a humid, tropical climate on its windward side, an arid climate on its leeward side, and sometimes has snow on the summits of Mauna Loa and Mauna Kea.

There are other cyclical climate patterns that influence rainfall over the islands. An El Niño event marks the warm phase of a recurring climate pattern across the tropical Pacific that shifts back and forth irregularly every two to seven years, but triggers predictable disruptions in temperature, rain, and winds.

“We can predict when an El Niño event is coming,” Frazier said. “About six months out, we know we are probably going into an El Niño, which means we’re going to have a dry winter. That typically lasts only a few months, and then the rains come back.”

However, scientists have seen a change in Hawai‘i’s rainfall patterns that span longer than El Niño’s periodic visits. Recent studies have suggested a downward trend in Hawai‘i’s rainfall over the last century. Researchers have also found a downward trend in stream base flow and an upward trend in the number of consecutive dry days and wildfire incidents over the same period.

This is concerning because rainfall is the source of all fresh water in the islands. Rain flows down Hawai‘i’s mountains, filters through watersheds and feeds streams and rivers, which in turn fills reservoirs. Rain also percolates into the soil to recharge groundwater reserves from which Hawai‘i’s public water supply is drawn. Rain waters pastures and farmlands and, on a smaller scale, fills catchment tanks in off-grid households.

Frazier’s own study found that the last decade has been the driest on record for Hawai‘i in one hundred years. “Our findings also show the most severe droughts are typically associated with El Niño events, and in recent decades the leeward coast of Hawai‘i Island has been the most drought-prone area in the state,” Frazier said.

Another study suggests an even drier future for leeward sides of the islands. “Regional projections show that unusually severe dry seasons will become increasingly common on the leeward side of all Hawaiian Islands,” Frazier said.

Short droughts vs multi-year droughts
Kodama said brief droughts lasting a few months should mostly affect groups that directly depend on rainfall. “It first affects the agricultural sector,” he said. “And a lot of times it impacts the ranchers because they depend on unirrigated pastures.” Local residents who use rainwater catchments, or covered water tanks, for home use are also affected.

A prolonged, multi-year drought similar to or worse than 2007 to 2014, however, could force more residents and businesses that rely on rainfall to turn to groundwater. It could also cause a surge in water use by groups—such as residents and irrigated farms—that already use groundwater.

“Once you have a prolonged drought, people will start using more water. It’s a compounding effect,” said Neal Fujii, drought and water conservation coordinator for Hawai‘i’s Commission on Water Resource Management.

On O‘ahu, where most of Hawai‘i’s population lives, long-term droughts can mean uncomfortable changes in residents’ lives and expensive alternatives for the state in protecting the viability of the island’s groundwater. “The Honolulu Board of Water Supply is the largest distributor of drinking water on O‘ahu,” Fujii said. “During a drought they might ask water users to voluntarily restrict their usage. During a really bad drought this could become a mandatory restriction.”

“If it were a decade-long drought, water distributors would start rationing water. Water purveyors would look for alternate water supplies, like desalinated seawater,” Fujii said. “But that would be at a great cost because electricity is so expensive. Water suppliers would also need to be creative with recycling sewer water or other types of wastewater.”

Responsible for managing the state’s water supply, the Commission on Water Resource Management would start prioritizing water permits in water management areas. “Do golf courses take priority over agriculture? We’ll have to make decisions like that,” Fujii said. “Drinking water for humans would be the last thing you would cut back. All the other sectors would probably be restricted.”

The long-term drought scenario is dire and depressing. State water officials and water researchers want to head it off by conserving Hawai‘i’s groundwater, generally regarded as a resilient source of fresh water. As a first step in maintaining a source of water for island use, particularly during a drought, water managers need to know how much groundwater is available and what the demand for water is in the local area.

“But we aren’t even sure of specific groundwater levels on different islands just yet,” said Gwen Jacobs, principal investigator of a $20-million University of Hawai‘i project—funded by the National Science Foundation EPSCoR Program—to study the islands’ aquifers.

Keeping aquifers resilient
Groundwater is water found underground in the cracks and spaces in soil, sand, rock—and in Hawai‘i’s case, piles of old lava flows and volcanic ash. Rainwater drains into soil and percolates downward through permeable rock where it is stored and moves slowly through geologic formations of soil, sand, and volcanic rocks. These porous rocks and sand that hold the water are called aquifers.

Because aquifers are refilled or recharged by rainfall, Hawai‘i’s recharge mirrors the islands’ rainfall patterns—recharge is high on the windward slopes and lower on the leeward slopes. The aquifers have a limited capacity to store fresh water because each island is small and surrounded by saltwater. So, unlike aquifers in the mainland that can siphon fresh water from surrounding aquifers, each Hawaiian island’s aquifer system is self-contained. Around the islands, saltwater underlies much of the groundwater because fresh water is less dense than salt water. When too much groundwater is withdrawn from an aquifer—through pumps, for example—salt water can move into the aquifer, contaminating drinking water sources.

Called ‘Ike Wai, the Hawaiian words for knowledge and water, the project led by Jacobs in partnership with the University of Hawaiʻi Water Resources Research Center and others is collecting new hydrological and geophysical data to understand the structure and flow in two important Hawaiian aquifers, the Hualālai aquifer underneath the Kona Coast, and the Pearl Harbor aquifer beneath the southern coast of O‘ahu.

The data will be used to build several hydrological models that can help researchers understand the details of the aquifers, how the water flows into them, through them, and out of them. Ultimately, the model can be used to assist water resource managers in making decisions on how best to conserve groundwater for all of the state’s aquifers, each with its own complex system.

Jacobs and her colleagues hope that the data they are collecting, the model they are building, and the decision tools they are developing will prepare Hawai‘i for big challenges ahead. “Two things that will impact our aquifers in the longterm are changes in the rainfall pattern which affects the amount of aquifer recharge, and sea-level rise, because as the sea level gets higher, that can increase the amount of salty water in the aquifer,” she said.

State water resource managers are also preparing for these challenges, hoping to get changes in place even before the next long drought occurs. “We are asking county water departments across the state to incorporate climate change into their supply and demand plans,” Fujii said. “The commission is also looking at how climate change is going to affect water availability.”

The state is putting together water shortage plans for water management areas to prepare for bad droughts in the future. Regarding water availability, Fujii said, “It’s ongoing research. As new findings or discoveries about the aquifers are made, we would look at them, consider them, and make changes accordingly.”

In 2013, Hawai‘i’s water demand was 449 million gallons per day (mgd). By 2030, the state would need an additional 100 mgd to accommodate projected population growth and development. It is aiming to meet this future demand through a combination of conservation, water reuse, and improving aquifer recharge by increasing forest protection and green infrastructure.

Jacobs thinks the next bad droughts could be game changers. “What would agriculture look like, for example, if you don’t have enough rain?” she said. What would future droughts bring to the islands? Will they pass unnoticed? Or will they endanger the islands’ caches of water, redefining the lush mountains of rainforests of every tourist’s dream?

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