The children at Koko Head Elementary will soon be studying ancient Hawaiian farming, but first they'll have to harvest water.

In a lesson that mixes science with sustainability — and a bit of philanthropy — they plan to irrigate a taro lo'i with rainwater that ordinarily would have washed away. A catchment system donated recently by Green Builders Depot will collect and store water that flows from a drainage downspout on a walkway roof.

The Rainwater HOG, a catchment system popular in Australia, can hold 51 gallons of water. It's shaped like a tall, green plastic shoebox with a spigot at the bottom. There's a filter at the top to keep out debris, and the plastic is dark enough to prevent algae from growing inside.

The unit installed at Koko Head is a few doors away from the classroom where Jordan Ushijima, the school's environmental sciences teacher, has stressed the need for water conservation all year. His lessons are folded into what the Koko Head fourth-graders learn about Hawaiiana and the taro they plant every year.

"This way we can use rainwater instead of the tap water," he said. "That carries over to what the kids are learning in terms of sustainability and our resources not being endless. We have to conserve water and conserve our resources."

Ushijima got some of his students to write letters to Green Builders Depot last month after they learned the local supply company would donate one Rainwater HOG for every five sold. But Ushijima was already familiar with the concept — he has a homemade version that uses a 55-gallon plastic drum at his Mānoa home.

It's common-sense environmentalism that puts a free resource to use.

"The taro doesn't care if it's rainwater or hose water," Ushijima said. "And it would just go down the drain and into the ocean."

Although it's clean, the water inside the Rainwater HOG isn't meant for drinking. Those who own them use the water for irrigation or washing cars. In Australia, where severe water shortages prompted its creation, people even use the catchment system to flush their toilets.

The catchment system at Koko Head won't be needed until next school year, when the fourth-graders plant their taro, but since the elementary school is located in one of the drier O'ahu neighborhoods, that could be a blessing. The area only receives about 4 to 5 inches of rain a year, said Michael Reeves, vice president and founder of Green Builders Depot, when he explained the system to the students.

Even though the math was probably over their heads, the students understood the potential when Reeves told them that one inch of rain on a 1,000-square-foot-roof will produce 650 gallons of water.

"Is that amazing?" he said. "So you see how much water Hawai'i loses and how much water we could save.

"If everybody took a part in doing this, we could protect our environment, our natural resources, and we could grow a lot more plants naturally."

Reeves told the students to touch the plastic, which can be completely recycled, and reminded them not to climb on it.

"The more people who use these water hogs will help Hawai'i sustain more water, so that when you grow up and are as old as me, you will have enough," he said. "Everybody's part in conserving the most precious resource in the world is using these catchment systems."