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Whatever happened to the September swoon?

Warmer late-summer temperatures challenge Denver Water supply managers.

Weather forecasters consider September the start of “meteorological fall.” And even without the fancy term, most of us think of the month as the unofficial kickoff to autumn.

School starts, football season is underway, and temperatures begin a welcome cool down.

Well, about that last part: September is not as cool as it used to be. 

A car thermometer registers another hot September day. Photo credit: Denver Water.

And that means customers are watering their yards later into the year, creating more demand for water and making things trickier for Denver Water’s water supply managers. 

You may recall how September started in 2022, with a string of days in the mid-to-upper 90s, including back-to-back 99-degree days in the Denver area. (Denver Water’s downtown weather station hit 100.) 

In fact, on Sept. 20, 2022, 9News meteorologist Chris Bianchi reported on what's now X, formerly known as Twitter, that Denver had broken a record for number of 90-degree days (10) in a single September. 

A wave of heat-related records were broken across the West in 2022, including here in the Denver metro area. Image credit: Chris Bianchi via Twitter.

9News meteorologist Cory Reppenhagen on Sept. 5, 2022, also reported on the unusual heat records in Denver that are part of the metro area’s rising September heat trend. 

September 2023 is tredding on the same warming ground, with Bianchi noting on X, that this year the month was off to the "2nd-warmest start to September on record in Denver. Avg temp of 78.6° is 9.1° above average."

"September is basically August 2.0 these days," he lamented.

All told, toasty Septembers appear to be one more element of a changing and warming climate.

A screenshot from a Sept. 5, 2022, report by 9News meteorologist Cory Reppenhagen about September’s changing weather patterns in the Denver metro area. Image credit: 9News meteorologist Cory Reppenhagen.

“We have a pretty good idea of what water demand in the summer months will look like, but September is becoming a wild card,” said Nathan Elder, the utility’s water supply manager. 

“We have seen higher demand and increased reservoir releases in recent Septembers,” he added. “But we also know we can see snow during the month, which makes planning for the month and setting up winter operations difficult.” 

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Additionally, Elder said, warmer fall temperatures can dry soil in the collection system, which means more snow is needed the following winter to fill reservoirs. 

“This is a trend we are really keeping our eye on because it can have significant impacts on water supply late in the season and going into the next spring,” Elder said.

One other challenge tied to warmer Septembers: a longer fire season, such as in 2020, when the state’s two largest-ever fires exploded late in the year, including the East Troublesome Fire that roared through Grand Lake in mid-October.

Dillon Reservoir is Denver Water’s largest reservoir. It sends water to the Front Range via the 23-mile-long Roberts Tunnel under the Continental Divide. Photo credit: Denver Water.

Some numbers that tell the story: 

  • Denver Water is seeing more demand for water in September. Last year, demand was about 30 million gallons per day above the 30-year average. In 2022, the volume was 15 million gallons over the same average. The difference between the two years can be attributed to the fact that precipitation in 2022 was closer to normal and 2021 was drier.
  • September 2022 was Denver's third-warmest on record, at an average of 69 degrees, surpassed only by 2015 and 2019, according to National Weather Service data highlighted by Bianchi, the 9News meteorologist.
  • Flows in the Roberts Tunnel, which delivers water from Dillon Reservoir to the Front Range, are rising in September, with 260 cubic feet per second seen in recent years versus the long-term average of 160 cfs. That’s a sign Denver Water needs to send more water to the metro area to meet the higher September demand. 

That doesn’t mean, however, that Denver Water is pulling more water from its West Slope reservoirs. In fact, overall water movement through the Roberts Tunnel over the course of a year is flat, as lower winter demand and tunnel shutoffs have helped balance out that September bump.

Warmer-than-normal temperatures during the day and at night raise the overall average temperature of the month. Image credit: Chris Bianchi via Twitter.

Overall, September reservoir releases across the system have been higher than average in recent years, a sign that Denver Water must rely more on storage to meet higher late-summer demand.

Cheesman Reservoir on the South Platte River system serves as one example, with average September releases during the last five years of 286 cfs versus the longer-term average of 211 cfs. 

Denver Water’s records show September is warming at a higher rate than any other month during the watering season. And Reppenhagen’s reporting found that “September (weather) is changing the most out of all the months, warming by 1.5 degrees compared to the previous 30-year period of record.”

As Reppenhagen has pointed out, warmer Septembers are extending the summer growing season, a development that he notes was predicted by some of the earliest computer modeling examining the effect of rising carbon dioxide in the atmosphere.

Denver Water customers are using more water later in the year as Septembers warm. Photo credit: Denver Water.

That is borne out by water use. 

Outdoor demand in September has grown about 23% when comparing the recent-five year stretch of 2017-2021 to the period between 2000 and 2016. In the same five-year span, September use has only been 3% below June levels.

From a practical standpoint, warmer Septembers make work tougher for Denver Water’s planners.

When that extra pull on reservoirs in September is combined with lower soil moisture, it means the utility has to lean even harder on the winter months to provide enough snowpack to fill reservoirs the following spring. 

The shift also means planners are relying less on historical water demand models and focusing far more on data from more recent years to get a better idea of how much water Denver Water customers will need in September. 

Overall, Denver Water customers have been good at conserving. Demand is generally flat or even declining during most months of the year. September is an outlier: as noted above, the five-year average for the month is actually on the rise.

Adjusting your sprinkler control system to account for weather and the time of year can save water. Cooler nights send a signal to your landscape that it’s time to wrap things up for the winter and it doesn’t need as much water. Photo credit: Denver Water.

But a reminder to customers: Even with a warmer September, the need for outdoor watering declines because nighttime is cooler, and grass can get by with less water. 

As Denver Water takes many steps to adapt to a warming climate, it will incorporate these late-summer temperature increases into its many-faceted approach to ensure it can supply the Denver region with water. 

In the shorter term, Elder has a more visceral suggestion: “September should be kicked out of fall,” he said. “Move it permanently to summer.”