Managing water during the mountain snow runoff has plenty of challenges and it’s going to become more complex due to climate change.
“As water planners, we prefer to see predictable weather patterns,” said Nathan Elder, water supply manager at Denver Water. “Unfortunately, every year is different and with climate change we’re seeing more variability and that makes it tougher to manage our water supply.”
That challenge may be most acute during runoff season, that critical — and brief — window of time when snow melts, flows into streams and fills reservoirs. Climate change may lead to changes in runoff timing that, in turn, require more nimble reservoir operations.
“We’re seeing more swings between wet and dry years, more variation in year-to-year stream runoff and earlier runoff,” said Laurna Kaatz, climate program manager at Denver Water. “We’re also expecting to see more extreme weather events like extreme heat and enhanced drought, but we could also see more intense rainstorms and flooding especially if heavy rain falls on top of a lot of snow.”
Timing is everything
The timing of the snow runoff in Summit County, which is home to Dillon Reservoir, provides an example of how climate change impacts not only water collection but also recreation and flooding.
Rapid snowmelts caused by rain falling on snow could lead to a greater risk of flooding below Dillon Dam.
During a gradual runoff, Denver Water can take steps to minimize the risk of flooding below the dam, however, if there are more instances of warm weather combined with rain falling on snow, large amounts of water can fill Dillon quickly and send water through the dam’s overflow spillway. This scenario can lead to high water levels on the Blue River through Silverthorne.
“We do our best to minimize high flows out of our reservoirs, but if there is a fast runoff, we can only do so much and there’s a greater chance for flooding downstream if there’s a major rain-on-snow event,” Elder said.
Changes in runoff and precipitation also impact when Dillon Reservoir fills — or doesn’t fill — which plays a role in boating season and water levels for the Dillon and Frisco marinas.
The timing of the runoff also impacts Denver Water’s ability to make the most of its water rights.
“Later runoff allows us to use our water rights to match higher customer demand during the summer watering season,” Elder said. “Early runoff means we have to let some water go downstream before we can put it to use on the Front Range. This also impacts how much water we can store for times of drought.”
Extreme weather events
Colorado has seen several big swings in weather over the last 20 years, suggesting the kind of uncertainty that may be more pronounced as climate change intensifies and the resulting complexity in managing the snow runoff.
Most recently, the winter of 2017-2018 was exceptionally dry across the state but was followed by above average snow in 2018-2019.
The years 2012 through mid-2013 were another period of drought, followed by record flooding in September 2013. Two wet years followed in 2014 and 2015.
The dramatic weather turnaround in 2002 and 2003 is another example of how extreme weather impacts Denver Water’s water supply and planning.
Those years marked a major period of drought. In 2003, Denver Water was preparing to have water restrictions and Dillon Reservoir was more than half empty and critically low. But in March 2003, the Front Range and central mountains got hit with a major snowstorm that filled Denver Water’s reservoirs.
“A drought could last one year or several and then be followed by big snow years,” Elder said.
“We could get most of our water for the year from one or two big storms, so we have to be prepared for these situations.”
Swings in weather patterns and extreme events could have Denver Water planning for drought conditions with watering restrictions for customers and end up with a surplus of water after a big storm.
Planning for climate uncertainty
Denver Water has relied primarily on historical weather patterns and data to plan for how much water it will collect from mountain streams. Now the utility is incorporating climate change into its long-range preparation through scenario planning.
“One component of scenario planning involves creating a variety of potential climate scenarios instead of simply assuming patterns will stay the same over the next 50 to 100 years,” said Jeff Bandy, a water resource manager at Denver Water. “This approach helps us plan for potential changes in climate and evaluate our system’s reliability.”
Denver Water takes data from global climate models and uses the information to create various outcomes on streamflow and precipitation in its water collection system.
The planning team develops scenarios that include variables such as warmer temperatures, more precipitation and shifts in timing of precipitation, all of which result in changes to volume and timing of runoff in Denver Water’s watersheds.
“We evaluate the scenarios and determine if future infrastructure projects or operational changes are needed,” Bandy said.
Enhancing data collection
Denver Water collects water from 4,000 square miles in Colorado’s central mountains and foothills. With such a large area, getting accurate and timely information about weather and streamflow conditions is critical to water supply management.
“We use a lot of different data sources to manage and forecast water supply and a lot of these data sources are based off historical climate data,” Elder said. “With a changing climate, the current data sources are no longer as reliable as they used to be. This makes it more difficult to manage our reservoirs.”
In preparation for more weather extremes and variability, Denver Water is investing in new technology to get a more accurate picture of the snowpack above Dillon.
“In April we started using NASA’s Airborne Snow Observatory which uses a plane to measure snowpack over the mountains in our watershed,” Elder said. “The more we know about the snow, water content and runoff, the better decisions we can make when it comes to managing our water supply for our customers and the communities where our reservoirs are located.”
What can customers do?
The best way communities can be prepared for the impacts of climate change is to use water wisely.
“Our water supply is vulnerable to climate and our customers play a major role in how we manage our system,” Elder said. “That’s why we always ask our customers to be efficient with their water all year long and even in wet years.”
Water is a limited resource in Colorado so climate change will impact communities on both sides of the Continental Divide.
“Climate change means water change and that’s important to us all,” Kaatz said. “So, it’s our goal at Denver Water to make sure we’re thinking about it and actively preparing for the changes we’re going to experience.”