Winds funneled between Haleakala and Kohala lift flurries of dust on Luamakika, the small crater of Kaho`olawe, painting the new green water catchment building the color of the dry, reddish-brown dirt. The Kaho`olawe Island Reserve Commission spent around $2.8 million for the large roof and storage tanks that make up the catchment system on the island’s summit. With the natural watersheds having been lost long ago to grazing by goats followed by decades of the island’s use as a training target for military bombers, the new system is needed to bring water to the seedlings of the native aweoweo, pa`u o hi`iaka, kawelu, pili grass and `a`ali`i planted in the crater.
The commission concentrated its replanting on the summit, where it hopes the vegetation will eventually defend the bare ground from pounding rain and grip the soil in place. On a December visit to the island, I joined two other volunteers in planting hundreds of shrubs, trees, and grasses. Bales of pili grass defended the seedlings from the harsh wind, but no rain fell to nourish them.
Andre Perez, a restoration field coordinator, walked us to an ahu, or rock shrine, on the rim of the crater. Here, beyond empty blue channels, the mountains of the Big Island, Maui, Moloka`i, Lana`i, and O`ahu seem to encircle tiny Kaho`olawe. The shrine where we stood is one of three recently built on the crater, positioned to summon rain from a shrine placed on Haleakala, which blocks Kaho`olawe from the moisture-bearing tradewinds.
Within weeks of my visit to Kaho`olawe, I was looking back on Kaho`olawe from Haleakala. I had gone there to see first-hand what scientists from the U.S. Geological Survey and the University of Hawai`i are discovering about the watersheds on windward and leeward slopes of the mountain on Maui. Among other things, they are trying to see if forests at different elevations not only curtail erosion, but also recharge underground aquifers by absorbing moisture from the air.
Like Kaho`olawe, `Auwahi, an ahupua`a on the southern flank of Haleakala, is also in a rain shadow. Here, the dry forest has been mostly cleared by hooved animals and fires. Most rainfall in the area is brought by winter storms from the south. Unlike the starkly sunny Kaho`olawe, `Auwahi – whose very name means “smoky glow” – is often wrapped in clouds.
Martha Scholl, a hydrologist with the U.S. Geological Survey, describes three types of weather that can produce fog: “First, the strong, wet tradewinds that blow from the East and wrap around the mountain bring most of the fog. The second kind of fog-producing weather is from southwest winds in Kona storm patterns, where fog is blown in between the rain. Finally, a seabreeze-landbreeze circulation pattern sometimes happens, when the sun beats down and there are no tradewinds. The land heats up and the warm air rises, pulling water vapor from the ocean up the slope É where the vapors condense and become clouds.” This last pattern, she adds, “doesn’t make too much fog, though.”
Scattered remnants of village sites in `Auwahi reveal where ancient Hawaiians subsisted on the once-forested slopes. “People think that Hawaiians wouldn’t have lived up here if there wasn’t any water,” USGS hydrologist Stephen Gingerich points out. “Patrick Kirch, a University of California at Berkeley archeologist, has found evidence – such as petroglyphs – indicating places where there were springs that do not exist anymore.” Researchers, says Scholl, have reasoned that “Hawaiians could live here before ranching, and that is evidence that there used to be more water available here.”
Is there a chance that those ancient people enjoyed more rain?
“We don’t know whether the deforestation was enough to cause climate change,” says Scholl, but “it might well have done so.” Water-bearing clouds in Hawai`i get their moisture from two sources: the ocean, and the vapor that is transpired from forests. Since the forests have been erased, the amount of water that transpiration vapor now adds to the clouds is “obviously small,” she notes. That could mean less rain for `Auwahi and other deforested areas.
Scholl is participating with colleagues Gingerich and Lloyd Loope of the USGS and Thomas Giambelluca, a geographer at the University of Hawai`i at Manoa, on a two-year project that tracks the sources of moisture in Maui watersheds. They are trying to figure out the respective contributions of fog drip and rainfall in two areas, on either side of Haleakala, in hopes that eventually they can determine what moisture could be caught by trees if a forest grew on `Auwahi.
On cinder cones and ridges of the tumbling southern slope of Haleakala, amid cattle pastures and patches of stunted trees draped in lichen, the researchers have constructed a weather station specially designed to measure moisture in fog. Besides a standard rain gage, a one-meter by one-meter screen woven out of thin, plastic ribbon mounted on aluminum poles has been set up to face the prevailing winds. Moisture it collects from fog and rain that blows in at an angle is compared to levels of water in the rain gage.
A few feet from the gages, a visibility sensor has been mounted atop another aluminum pole. By emitting a beam of light, the sensor is able to “see” water droplets soar in the air, even when they may not be caught in the fog net.
Using the data collected by these devices, researchers can more precisely monitor the frequency and intensity of rain and fog and are able to tell how much of the moisture that comes to `Auwahi arrives in the form of fog.
What they have found so far is that during the dry summer, about the only water `Auwahi got came as fog blown in by strong trades. Because there are so few trees on the slopes to slow the clouds and hang on to their moisture, this means that most of the water that blows up the slope keeps right on going. In other months since the study began a year ago, rainfall has been the primary source of moisture at `Auwahi, with fog drip contributing proportionately less than it does in the summer.
On the opposite, windward side of Haleakala, in the dense alpine shrublands of Waikamoi above the inversion layer that limits rainfall, the scientists have set up another weather station with instruments identical to those in `Auwahi. In contrast to `Auwahi, every month, the fog net in Waikamoi absorbs more moisture than the rain gage receives – up to three and a half times that of rain.
How much of that fog-borne moisture collects on leaves and finds its way eventually into the groundwater? As it turns out, rainwater can be distinguished from water obtained from fog because heavier oxygen isotopes are more abundant in the latter. Most of the water in `ohi`a sap and the trickling Waikamoi Stream comes from the fog that drifts in. The moisture in the soil, on the other hand, can be either predominantly from fog or from rain, depending on the frequency and intensity of storms hitting the mountain and of intervening dry periods.
In `Auwahi, Scholl suspects that “there is not a whole lot of fog getting into the soil. The isolated trees at `Auwahi can’t catch enough cloud water to drip down to their roots and do them much good.” Water from a heavy cloudburst at `Auwahi stayed in the soil for many months, instead of being quickly displaced by fog drip seeping into the ground.
This data intrigues Gingerich. “We are trying to see how the cloud forests work on the windward side,” he says, “and how they could work on the leeward side if there was a forest with trees that collected fog like our instruments do. The study has raised as many questions as answers, which usually happens with these things. Would fog transpired from trees be recycled in higher elevations? Would fog be thicker if there was a forest? Do trees dry up areas by transpiration or are fog-collecting trees an additional help to recharge aquifers, and is this an added incentive to reforest areas?”
Giambelluca points out that in other studies, forests cool air by using the sun’s energy to transpire water, while barren ground radiates the heat of the sun. Fog condenses when air cools, so the forests from lower elevations in Waikamoi could make the fog layer wider and more frequent. Also, the researchers speculate that water transpired by lower forests thickens the fog and is collected above by the alpine vegetation.
As I gaze from `Auwahi, I see Kaho`olawe enveloped by a shell of clouds. Beams of late sunlight blaze into the surrounding ocean. Could plants on Kaho`olawe not only hold the soil, but also bring moisture – life – to the bomb-stricken island of near-perpetual drought?
Giambelluca has looked into this question. He mentions that workers have occasionally seen fog on Kaho`olawe, even though the island does not reach the elevation where most Hawai`i fog develops. Long ago, he adds, the fog drip “could have played a role in the ecosystem” and, because the island sees so little rain, fog drip “could have been a high percentage of the water coming in.”
In Kaho`olawe, though, what plants do above ground is less important than what they do below the surface. At present, most of the rain that falls runs directly off into the ocean. The roots of the old forests would have broken up the soil, allowing rain to percolate into the ground. Before goats stripped the island, there were springs, and an early ranch was supplied by coastal wells. Now that those springs are extinct and the wells brackish, a desalination plant, and weekly airlifts of thousands of liter bottles of drinking water are needed to quench the thirst of the workers today who clear bombs and plant vegetation.
“Kaho`olawe is one of the best examples in the world to show how deforestation has changed all the watershed functions,” says Giambelluca. Over the past century, between six and ten feet of soil has blown away or washed into the sea, disturbing marine life. With the absence of transpiring plants, sun energy heats the soil and air. “The bare soil is very inhospitable to the young seedlings,” Giambelluca says, not only because of the boiling temperatures, but also because of the soil’s hardness and the winds that thrash and spin, unobstructed, past the crater.
The absence of plants has created a cycle of desolation. Still, Giambelluca is pleased when I tell him we planted `a`ali`i, a tree that he says “seems to survive almost everywhere.” If these plantings take hold, they may cool and soften the crater’s soil, block the wind, and nurse future seedlings.
The furious wind and the glaring sun have flayed Kaho`olawe for eons. For just as long, rain has shunned the isle. Still, trees grew and springs bubbled until humans intervened. Restoring Kaho`olawe will involve more than removing the bombs from the tortured soil and building catchments, although that is a prerequisite. In the end, the task of nurturing the broken land back to health is a job best left to the fog-grabbing, earth-grasping trees.
— Emma Yuen
Volume 13, Number 9 March 2003
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