KIHEI — As planners chart out Maui island’s water use over the next 20 years, uncertainty hovers over the region with the largest population but the least groundwater resources.

Spanning 229 square miles of Kahului, Paia, South Maui and Upcountry, the Central Maui aquifer sector covers more than 100,000 residents but produces 26 million gallons per day, the lowest of the six aquifer regions. And, two years after the closure of Hawaiian Commercial & Sugar Co., it’s still unclear how a decrease in irrigation will impact the central aquifers, said Alex de Roode, planner with the Water Resources and Planning Division.

“The demand that’s in the central aquifer sector area can’t be met by the natural resources in it,” de Roode said. “So it is reliant on resources from Wailuku and Koolau. With that irrigation now not coming in, we have to see what happens over time. It’s so new still.”

De Roode spoke Tuesday night at a hearing on the Maui Island Water Use and Development Plan, which is being updated. The plan looks at future water needs over the next 20 years and discusses how Maui can meet those demands.

Of the six aquifer sectors on Maui, the central area is the largest. In 2015, it had a population of 103,970 and was expected to reach 138,164 by 2035. The area includes four aquifers — Kamaole, with a sustainable yield of 11 mgd; Makawao and Paia at 7 mgd each; and Kahului at 1 mgd.

By contrast, the Koolau aquifer sector has a sustainable yield of 175 mgd, followed by the Hana area at 122 mgd, the Wailuku area at 36 mgd and the Kahikinui and Lahaina areas at 34 mgd each.

Public water supply for the central area primarily comes from outside sources, while agricultural irrigation comes from a combination of East Maui streams, surface water from the Wailuku area and groundwater from the central area.

The uncertainty in the region has to do with water availability, now that HC&S no longer irrigates vast fields of sugar cane. The irrigation created an “artificial increase in groundwater recharge” for the aquifers in the central area, according to the draft plan.

“With that irrigation now being drastically reduced or eliminated in many parts, there’s a lot of uncertainty in terms of availability and yield from those aquifers,” de Roode said. “Wells that had previously had water to meet irrigation demand, it’s uncertain how much they’ll yield in the future.”

Of course, he noted, the decrease in irrigation is good for the East Maui streams that are being restored in the wake of the plantation’s closure. The Department of Water Supply will have to continue monitoring the central aquifer systems to know just how much impact the reduced irrigation will have.

Another factor is how far Alexander & Baldwin takes its diversified agricultural plan. The company’s plans include 4,000 acres of livestock pastures, 500 acres of feedstock crops and 250 acres to study the viability of pongamia as an energy crop — which could increase to 2,000 acres if successful.

In 2016, the last year of the sugar plantation, the central area required close to 238 mgd, which included domestic, industrial, agriculture and other uses. In 2017, that dropped to about 79 mgd, and is expected to climb to about 165 mgd by 2035.

De Roode said future plans to meet water needs include seeking long-term funding for maintaining and expanding watershed protection in East Maui, developing a basal well in the Makawao aquifer, expanding distribution from the Kahului wastewater treatment facility and exploring wells in East Maui.

During the public hearing Tuesday, Justin Kekoa Kekiwi, a lineal descendant of Honua’ula moku, expressed concerns about the central area’s reliance on outside water sources.

“We got to realize that we live in one desert — South Maui — and what’s going on right is developers are coming up with future proposed developments, and we basically stealing water from the moku of Wailuku, Iao and Waihee aquifers,” he said.

Kekiwi complained about resorts in Wailea using potable water to irrigate their lawns. He also was concerned about upcoming luxury developments that he said would use water and not benefit everyday residents.

Eva Blumenstein, planning program manager with the Department of Water Supply, said that Kekiwi’s concerns echoed many others.

“Population centers and planned growth areas on Maui are generally in dry coastal zones with limited natural water resources,” she said via email. “Surface and groundwater needs to be imported to some extent from water resource ‘rich’ regions to provide for reliable potable supply.”

She said the draft plan tries to look toward other resources, such as “expansion of recycled water distribution, conservation and stormwater capture.”

Blumenstein added that some resorts do rely on potable water from the department, while others use a combination of brackish groundwater for irrigation and potable water for pools and other water features.

“Irrigation and other outdoor water uses at resorts can be substituted with alternative sources to offset potable water for actual potable needs,” Blumenstein said. “One of the most cost-effective strategies to target outdoor uses at resorts in dry areas include xeriscaping — using climate-adapted plants to rely on ambient (natural) rainfall.”

Rob Weltman, vice president of the Kihei Community Association and the president of the Sierra Club Maui Group, said Tuesday night that he got rid of the lawn at his own home and replaced it with native plants, which “drastically reduced” his water needs. He supported native plants and “non-green landscaping” as water-saving solutions, and he said he hopes there will be more options for using recycled gray water from showers, laundry and sinks in the future.

“We do have to realize that South Maui is a consumer of water. It’s not a producer of water like many of the other districts,” Weltman said. “I think that should be reflected in the way we do business here.”

Weltman added that “Maui is going through a radical, radical change in the way it produces and consumes water.”

“We’re diverting water back to restore traditional and cultural values and habitats,” he said. “And for the island to continue to thrive, we need to find other ways to produce that water. The first rule is reduce consumption before you look at ways to increase production.”

De Roode said the department offers a number of conservation and rebate programs. Customers who have toilets that use 3.5 gallons per flush or higher can trade them in for a free toilet that uses only 0.08 gallons per flush. The department also offers free rain barrels that customers can hook up to gutters and use for irrigation, as well as free “high-efficiency fixtures” that can replace older fixtures in showers and sinks.

* Colleen Uechi can be reached at cuechi@mauinews.com.