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Vol. 16, no. 1
February/March 2013


Who'll Map the Rain? 

Story by Jon Letman


The light was fading,
the wind was raging and the temperature had fallen below fifty degrees. Rain was crashing down in what’s known in Hawaiian as Kahakiki, a violent downpour. A long and fruitful day of hiking on the rugged eastern edge of Maui’s Haleakala National Park had gone from hot and sunny to storm-driven whiteout, leaving two hikers struggling up a steep slope covered with dense ‘ohi‘a forest.


The hikers—Dr. Tom Giambelluca, a geography professor at the University of Hawai‘i at Manoa, and his then-graduate student Trae Menard—had spent the day slogging through native forests to repair climate stations used to measure, among other things, rainfall. Trudging beneath the weight of their heavy packs, the two were at seven thousand feet with only vague directions to a simple structure encouragingly called “grassland cabin.” The rain was pounding so hard, water was flowing six or eight inches deep over the ground. Caught in the maelstrom, the pair thought, “This might be the end.” But eventually they found the cabin—a crude plywood structure—and there they remained trapped as hundred-mile-an-hour winds and ceaseless kahakiki blasted the mountain for three days straight in the worst storm to hit the park in sixteen years.


“I’m a field guy,” Giambelluca says today, recalling that March 1996 storm. “I’ve had a lot of experiences with Hawaiian rainfall—not all of them good.” Nonetheless, Giambelluca still calls Hawai‘i “the most interesting place in the world to study rainfall.” He has dedicated his career to helping people better understand Hawai‘i’s weather and climate, and what he and Menard (who is today director of forest conservation for The Nature Conservancy of Hawai‘i) experienced firsthand in 1996 offers proof: His own data have now proved that East Maui’s high montane native forests, with 404 inches of rain a year, are the rainiest place in Hawai‘i.


Giambelluca was the lead researcher on both the original Rainfall Atlas of Hawai‘i, commissioned in 1986 by the state Division of Water and Land Development, and the 2011 Rainfall Atlas of Hawai‘i—an updated web version of the paper original. The online 2011 atlas is a highly accessible resource available to everyone interested in Hawai‘i’s rainfall, designed to be useful to both the most sophisticated researcher and the greenest newbie.


“The atlas is not confined to scientists and academics. It is a way for everybody to understand their climate better,” says graduate student Abby Frazier, who was on the eighteen-member team that worked on the atlas; she scrutinized the accuracy of data coming from rain gauges across the Islands. “Anyone can interact with this information in a way that wasn’t possible before.”


Try the atlas out yourself and you’ll see. If you’re planning to go bird-watching in Hakalau in March, the atlas will give you an idea whether you should bring rain gear. If you want to grow tropical fruit trees on Kaua‘i and you’re wondering which gets more rain, Kalaheo or Kîlauea, the atlas will give you an answer in seconds. If you’re just a weather lover who wants to better understand rain patterns on Hawai‘i’s seven high islands, you have access to the most complete, up-to-date rainfall information there is. Say, for example, you’re interested in Kihei. Open the atlas and click on the little green square in the Kihei region of Maui. Immediately you’ll see that Kihei receives a mean annual rainfall of 10.89 inches. If you want to compare Kihei with the mountains of West Maui, click on the Kukui station: You’ll see that a mere fifteen miles away, the mean annual rainfall is almost 366 inches.


University of Hawai‘i associate geography professor Dr. Qi Chen was co-principal investigator, charged with fusing enormous volumes of data based on rain gauges, radar and other sources. And while the atlas contains a fascinating history of rainfall record-keeping in Hawai‘i as well as a detailed explanation of the project’s methodologies, its real heart is its interactive map. Users can choose from nine “base” maps that show such things as satellite imagery, topographic details, bathymetric references and surface landmarks. The maps can be overlaid with mean monthly and annual rainfall statistics and a color-coded layer that tracks rainfall over the thirty-year base period that the atlas covers (from 1978 to 2007).


Dr. Jonathan Price, an assistant professor in the UH Hilo geography department, calls the atlas “the next generation of rainfall data.” Price played an important role in creating the atlas by using his knowledge of native vegetation and ecosystems to fill in the gaps for very remote areas where there are no rainfall stations or historical records. “We were able to derive a relationship that says, ‘This type of vegetation requires about this amount of rainfall,’” Price explains. In the absence of data from climate stations, scientists could make an educated estimate of how much rain falls in any given area by looking at what’s growing there.


“Our climate situation in Hawai‘i is about as complicated as anywhere,” Price says. “It’s so complex that even a climate station a couple of miles away is just not good enough in some places. In two miles you can have extraordinarily different climate.”