The National Security Agency has many secrets, but here’s a new one: the agency is refusing to say how much water it’s pumping into the brand new data center it operates in Bluffdale, Utah. According to the NSA, its water usage is a matter of national security.

The agency made the argument in a letter sent to officials in Utah, who are considering whether or not to release the data to the Salt Lake Tribune. Back in May, Tribune reporter Nate Carlisle asked for local records relating to the data center, but when he got his files a few months later, the water usage data was redacted.

The situation shows just how important the new data center will be to the agency’s operations, including its widely discussed efforts to eavesdrop on internet communication

The situation shows just how important the new data center will be to the agency’s operations, including its widely discussed efforts to eavesdrop on internet communication. If it revealed how much water it’s using in Bluffdale, the agency believes, outsiders could get a good idea of the scope of NSA surveillance.

“By computing the water usage rate, one could ultimately determine the computing power and capabilities of the Utah Data Center,” wrote the NSA’s associate director for policy and records, David Sherman, in an undated letter filed with Bluffdale in response to the Tribune’s public records request. “Armed with this information, one could then deduce how much intelligence NSA is collecting and maintaining.”

The reality is that Sherman’s argument requires a pretty big leap of logic. Data center engineers can get rough ideas of compute power based on how much power a building consumes, but figuring this out on water is another matter. Some data centers, like Facebook’s facility in Prineville, Oregon, use custom-made swamp coolers to mist the air and cool down servers. Others push hot air into evaporative cooling towers, which are kept cold by running water.

“There are many different ways to cool a data center,” says Jonathan Koomey, a research fellow at the Steyer-Taylor Center for Energy Policy and Finance at Stanford University. “Without knowing more about the actual facility then I don’t think anyone’s going to give you solid [computing capability] numbers.”

But, oddly enough, water usage has become a very contentious issue for the NSA. An anti-government group called the Tenth Amendment Center is calling for Utah to simply cut off the NSA’s water supply, saying that water is the $1.5 billion data center’s “Achilles Heel.” And last month, a state Republican lawmaker named Marc Roberts said he would introduce a bill that would do such a thing.

But to the local paper, tracking water usage is just part of having an informed debate about the impact of the NSA’s data center, says Carlisle. “We are the second driest state in the nation,” he says. “We’re just in the habit of accounting for water in this state because we have to. There’s just not enough water.”

Early planning documents estimated that the NSA’s data center, which opened last year, would guzzle about 1.7 million gallons of water per day. That number was revised downward to 1.2 million gallons — made available at a discounted rate — according to other local documents uncovered by the Tribune. NSA did not immediately respond to a request for comment on this story.

Carslile is appealing Bluffdale’s denial of his records request. That hearing will happen today at 9am, with the Utah State Records Committee. But don’t expect the NSA to give up its water numbers without a fight.

Updated March 19, 2014. 10:10 a.m. PDT: After his hearing this morning, Carlisle tweeted that the Utah State Records Committee has rejected the NSA’s argument and ordered the records released.

Below: a Tenth Amendment Center video advocating for a water cut-off.