Understanding how much water is available in East Maui is a crucial issue this year – one of the driest in a long time – but the Waiokamilo question has been on the table since 1985.

At that time, the Native Hawaiian Legal Corp. initiated a contested case at the board over water licenses for streams at Huelo, Honomanu, Keanae and Nahiku – the heart of Maui's taro lands today.

That action to set in-stream flow standards remains open, but in a separate petition six years ago, the corporation got an order for the board to see to it that at least 6 mgd (computed as a year-round rolling average) is saved from diversion by EMI so it can flow to the taro lo'i.

Nobody thinks there is 6 mgd in Waiokamilo Stream, but since it has not been measured, the board has reached an agreement with the U.S. Geological Survey to set up a temporary gauge just below Dam #3 (the lowest diversion on the Waiokamilo-Akeke Springs catchment). The gauge is to be operated for a year, with an option for a one-year extension.

Alan Murakami of Native Hawaiian Legal Corp. (NHLC) said Tuesday he does not believe Waiokamilo Stream averages 6 mgd. But the legal advocate for Native Hawaiian taro growers doesn't believe EMI needs what is there.

Whatever is being diverted "is way beyond what they need for their purposes," he said.

Measuring stream flow can be a simple or rather complicated business, according to Rick Fontaine, the surface water specialist in the Hawaii office of the Geological Survey.

Water managers would always like more measurements, but the key to understanding is a long series of them, Fontaine said.

"You don't identify trends with short periods of records."

That's one of the virtues of a USGS system – it keeps long records. A few years of measurements can be misleading.

At West Wailua Iki Stream, where there has been a gauge for 95 years, the long-term median flow has been 13 cubic feet per second.

The Geological Survey commonly measures cfs. The county Department of Water Supply often measures a million gallons per day. A million gallons per day is a little over 1.5 cfs, so West Wailua Iki Stream flows about 9 mgd about half the time, and less than 9 mgd the other half. (Waiokamilo, where there is no gauge, is thought to average less than 6 mgd year-round, but in July its flow would normally be much less.)

But the variation from day to day, month to month and year to year can be enormous.

The lowest flow ever recorded on West Wailua Iki was 300,000 gpd, says Fontaine. That was in July 1922.

But the highest flow ever measured came the next year – 6,400 mgd (or 6.4 billion gallons per day).

The highest was about 18,000 times as high as the lowest. And West Wailua Iki is far from the most variable stream in the islands.

Peak flows are important, said Fontaine, because of a question that comes up at meetings of the state Drought Task Force: How much water, over and above "normal" or "full" stream flows, rushes down and is available for "harvest."

The issue of water harvesting, or diversions, is the element in the taro growers demand for 6 mgd in Waiokamilo.

Since the flows are uneven, that affects how large stream flow needs to be to allow a diversion to capture the surplus. The Department of Water Supply is studying whether the answer to Upcountry's summer water shortages would be a new reservoir, two or three times bigger than the 100-million-gallon Kahakapao Reservoir.

Fontaine has not done a study, but taking a recent random year as a test case (which may or may not be typical), the gauge shows that about 25 percent to 30 percent of all the water that flowed in West Wailua Iki Stream passed down on just five days – during periods of heavy rainfall when flooding occurs.

That proportion of high flows, or "flashiness," affects how big a reservoir you would need to efficiently harvest the surplus. The size of the transmission system (ditches, usually) is probably even more important, Fontaine said.

To capture as much flood water as possible requires a diversion system that is otherwise oversized when stream flows are normal.

When measuring a stream at low flows, hydrologists look for a place where the stream is narrow.

Where flows are higher, a V-shaped weir can be flung across the stream, which channels the water. The flow is measured by how high the restriction makes the water rise.

The purpose of the Waiokamilo gauge will be to allow the department's monitor to oversee the adequacy of the diversions, ensure compliance with the board's order, verify the board's understanding of the facts of the stream's characteristics and resolve minor complaints.

Steve Holaday, general manager of Hawaiian Commercial & Sugar Co., said he doubts Waiokamilo will be found to have 6 mgd on average.

Murakami does not think so either, but he wonders whether there are other diversions, higher up, that NHLC does not know about.

The East Maui watershed is enormous, with only a few permanent stream gauges and even fewer rain gauges.

It can be hard to know what the water situation is by measuring all the pieces and adding them up, Fontaine said.

However, sometimes there is a "good integrator" that reveals the overall state of the system.

The main integrator in East Maui is the Wailoa Ditch, which receives most of the diverted water. Its flows vary throughout the year from the maximum 199 mgd to as little as 25 mgd over the past year. The record low was 10 mgd.

Whether the Wailoa Ditch is the best integrator is unknown, Fontaine says.

That is one of the questions that the Maui Drought Task Force is looking at.

Another is how to define drought.

For a farmer with crops in the field, said Fontaine, a drought might be a few days without rain. For a water department pumping groundwater, a few days or even a few weeks or months without rain would not be noticeable.

But better definitions of drought are needed in order to make official declarations, which in turn can turn on the spigots for disaster assistance money from the federal government; or for long-term mitigation projects (like reservoirs) from the national or state governments.

"It's not simple," said Fontaine, but it can be done. "Certainly we need more information."

Harry Eagar can be reached at heagar@mauinews.com.