Why big storms and deep snows don’t always equal full reservoirs
Editor's note: This story has been updated to reflect March 2021 snowpack and conditions.
There’s a lot of attention paid to Colorado’s high country through the winter months.
Ski resorts trumpet how deep the snow has piled up on their runs after every storm. Local news stations in Denver devote precious minutes of air time comparing this year’s snow depths to the previous year and the average for the date.
Water planners throughout Colorado and across the West also keep close eyes on the mountain forecasts and gauges this time of year.
But a robust snowpack doesn’t automatically mean full reservoirs.
“I look at snowpack like your gross paycheck,” said Nathan Elder, Denver Water’s manager of water supply.
“But then you have things taken out of it, like taxes, Social Security and health care. What’s left is what actually goes into your bank account. For water, that’s the runoff, that’s the water that actually goes into the reservoirs.”
For instance, in 2018 the snowpack across the South Platte River basin peaked at 92% of normal, but the runoff in the basin was only 63% of normal.
And beyond the weather, other issues can affect how much water is available for Denver Water and other utilities. The long-term drought that’s plagued the Colorado River is leading the seven states that rely on the river, including Colorado, to craft drought contingency plans to ensure enough water remains in lakes Powell and Mead, two mammoth reservoirs that are key to managing water supplies in the river basin.
Denver Water gets half of its supply from the Colorado River Basin, so the utility and the 1.5 million people it serves in metro Denver are directly affected by the changing conditions — both natural and political — in the basin.
In mid-March of this year (2021), Colorado’s snowpack was at 90% of normal. In the mountains above Denver Water’s collection system, the drifts were 95% of normal.
While Elder’s calculations indicate the water supply in the areas that feed Denver Water’s system has been “slightly below normal but trending up” in recent weeks — with more precipitation in the forecast, the next few weeks will be critical in determining how 2021 water supply shapes up.
“We see things improving from last fall, when much of the state was in extreme or exceptional drought, but we have a lot of weather still in front of us. There’s still a lot of time to have things change,” Elder said.
Despite the early season snowfall, the U.S. Drought Monitor Map classified approximately 40% of the state as being moderate to severe drought.
Several factors can eat away at a healthy winter snowpack, reducing the amount of water that ultimately ends up in reservoirs. Among them:
- Wind: Winds can pick up the snow and move it around. And a warm, Chinook wind can melt the snow away.
- Evaporation: Water is always moving into the air, or evaporating. How much snow disappears to evaporation increases when temperatures rise.
- Soil moisture: Mother Nature gets first dibs on the water in the snow, with the soil soaking up moisture like a sponge. Only when the soil is saturated does the remaining water run into streams and reservoirs.
The timing of the runoff also is important, because a longer, later runoff that stretches through late spring and into the summer is like a steady paycheck that covers the bills — it delays the need for a water utility to dip into its savings account, via its network of storage reservoirs, to meet the needs of its customers.
The spring runoff typically starts at the end of April and peaks weeks later in the middle of June, Elder said.
But scientists have found that a dark layer of windblown dust or soot on the surface of snow increases the amount of heat it absorbs from sunlight, speeding the pace at which it melts.
A dense forest also affects the timing of the runoff. Tree branches can shade the snow, slowing down the melting process. Conversely, less shade means more sunshine can reach the snow, speeding the melting process.
“You just don’t know what the spring will bring,” Elder said. “We do a range of calculations to see what actions we might need to take — depending on how deep the snow is and what the runoff might be.”