Similarly, consumption of agricultural irrigation water, said to represent almost 80 percent of all usage in Utah, is also built on estimates, through a separate process based on land acreage.
DWR Director Eric Millis said his agency makes modifications to the raw data only when they arrive with obvious gaps or errors. Corrections are made in tandem with the water suppliers that provide the data, he said. The final product, according to Millis, provides about as good a picture of the state's water use as possible, given limitations on data collection.
"Once we've gone through the process of checking and working with the water supplier, we've got the data to be the best that it can be," Millis said. "Whether it's 100 percent accurate or not is another question."
The data in question were the center of a monthslong records battle initiated in November by the nonprofit Utah Rivers Council. Zach Frankel, the group's executive director, said the council requested the data after discovering discrepancies between water-use numbers obtained directly from water managers and what was being reported by the state.
Frankel said erroneous water-usage numbers have helped convince Utahns that the state is running out of water and must invest in multibillion-dollar water projects such as the Lake Powell Pipeline and the Bear River Project.
"They don't have the science to back up their claims," Frankel said. "They want to just keep fighting to spend money, and we're fighting because we don't think that money needs to be spent."
Erroneous data have long presented a problem for the DWR, Millis said. And while the imprecisions were considered acceptable in the past, he said, the need for more accurate water numbers is growing.
"We've known for a long time that there have been problems with the data," he said, "and as we go into the future, we have realized that we need more and better data. ... Years and years ago, you could say we were getting by with the information we had we weren't as close to the limits of our water supplies. But now things are much more critical."
The Utah Division of Water Rights is tasked with collecting water use data from the state's water providers, be they municipal drinking water systems, water wholesalers such as conservancy districts, or neighborhood irrigation systems for home lawns and gardens.
Officials at the Division of Water Resources then review data for accuracy, and often find basic errors ranging from misplaced decimal points to incorrect conversions between unit measures such as gallons and acre-feet, Millis said.
According to the DWR, analysts within the agency made corrections to 2015 data from 269 of 475 Utah water suppliers.
In addition to correcting errors, the DWR also often resorts to in-house estimates to fill in missing data. Those gaps may occur due to a lack of water meters especially in the case of the secondary water systems some Utahns use on their lawns. Or the gaps might result when water providers fail to provide requested data.
For the 2015 data set, DWR staffers had to estimate the amount of water used by 323 secondary water systems that lacked meters. Between that and the 36 water systems that did not submit data, the agency was forced to estimate some portion of water used by 75 percent of the state's 475 water providers.
Millis said his agency is working to combat bad data,
DWR has created an online form Millis described as the "Turbo Tax solution" to make it easier for water providers to report their water usage. The program is able to flag potential errors in the data before they are submitted to the state.
The state's 2015 data will be the first submitted to an independent third-party review, scheduled for completion later this year.
The state is also working on a new rule that would penalize managers of metered water systems who refuse to provide water-use data, or refuse to cooperate with the DWR when errors are identified.
Once that rule is in place, these water systems could find their license to deliver drinking water in jeopardy.
"We've always had problems with suppliers that have either not reported," Millis said, "or that reported information that wasn't correct and maybe they weren't willing to change it."
James Greer, an assistant state engineer in the Division of Water Rights, said that most of the systems that fail to report do so out of a lack of resources.
"Previously it was a voluntary program," Greer said. "We would do our best to get entities to report the data, but in some instances they would never get to it. It was a low priority for the systems themselves, and we didn't have the resources to go collect it."
Some water providers have refused outright to provide data to the state, added Greer, though he said he could not specifically identify any of the water systems that have failed to comply with state data requests.
Twitter: @EmaPen Holes in Utah's water data
# of 2015 water-use records corrected by the state Division of Water Resources: 269 out of 475.
# of 2015 water-use records that contain estimates by the DWR: 359 out of 475.