The impending closure of Hawaii’s last sugar plantation, on Maui, raises an important environmental question: What happens to all that irrigation water?

The landowner, Alexander &Baldwin Inc., has pledged to keep the land in agriculture, and is considering several options, including growing food crops, grass for cattle and crops for biofuels.

But A&B won’t have the last word.

Years of legal struggles over water rights in Hawaii have elicited a fundamental guiding principle, enshrined in the Hawaii Constitution: Water must be managed as a precious public resource — with the emphasis on public.

In a landmark ruling in 2000, the Hawaii Supreme Court, addressing a dispute over the Waiahole Ditch irrigation system on Oahu, reaffirmed the public trust doctrine as it relates to the state’s water resources.

And the court placed responsibility squarely where the Hawaii Constitution (Art. XI, Sec. 7) says it belongs: with the state’s water resources agency.

The court said:

“As the primary guardians of public rights under the water resources trust, the Commission on Water Resource Management must not relegate itself to the role of a ‘mere umpire passively calling balls and strikes for adversaries appearing before it,’ but instead must take the initiative in considering, protecting, and advancing public rights in the resource at every stage of the planning and decisionmaking process.”

The end of sugar on Maui presents a golden opportunity for the water commission to do its duty. It has done so before, at times prodded by the Court.

In 1997 and 2001, the commission restored to Windward Oahu streams some of the water formerly used by Oahu Sugar Co. for sugar cane in Leeward Oahu.

In 2014, the commission restored water to four Maui streams, which had been diverted by Hawaiian Commercial &Sugar Co. (HC&S) and Wailuku Sugar for sugar cultivation.

In both cases, the results proved beneficial. Taro farmers could cultivate their crops again, native fish and flora replaced invasive species, and more water was available to recharge underground aquifers. It’s not wasteful to leave surface water where it naturally flows.

Even so, with its diversified agriculture plans, A&B could make a good argument to keep some water flowing to its many acres.

But how much water?

It’s likely to be less, possibly much less. Sugar cane is one of the thirstiest crops on the planet.

In the Waiahole case, the Supreme Court cited water commission estimates that diversified agriculture uses far less water than sugar — 1,800 to 5,400 gallons per acre per day, compared to 7,500 to 10,000 for sugar.

With the demise of sugar, more than 600 workers will lose their jobs at the end of the year when Hawaiian Commercial &Sugar shuts down; providing new work for those whose skills are matched to agriculture would benefit everyone.

And a larger, healthy agricultural sector can help reduce Hawaii’s dependence on imported food.

As in the past, the commission will face a challenging task. But as the Supreme Court noted, the commission’s job is not simply to weigh A&B’s interests against competing ones, including those who want the water restored to East Maui streams.

It’s to protect this precious public resource, for generations to come.