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More so than snow, get to know the flow

Streamflow is the great integrator in determining Denver’s water supply conditions each year.

Just when you thought you’d read everything about how Denver Water gets its water supply, we have a new one for you. (After all, we do pride ourselves on being thorough on this site.)

We’ve told you about mountain snowpack and how it is the main source for our water supply.

Denver Water Winter Park employees, Tim Holinka (right) and Conor Peters, measuring snow in January 2018. Photo credit: Denver Water.

We’ve talked about how weather patterns and climate change can drastically impact that snowpack.

We’ve even shown you how to follow along to see how our snowpack is doing and what that means for reservoir levels and the water available to meet customer needs in any given year.

Now there’s one more factor we need to tell you about. We present to you: the great integrator!

We’ll pause while you catch your breath.

Yes, there is another very important element that Denver Water planners consider when they are forecasting what water supplies look like each year. It’s called streamflow.

“We call streamflow the great integrator because it brings so many factors together in forecasting our water supplies,” said Nathan Elder, interim manager of water supply at Denver Water. “Our source water makes it from the mountains to our reservoirs through a number of rivers and streams. And that streamflow can be influenced by soil moisture, temperature, precipitation and evapotranspiration to varying extents. All those factors can determine how much water actually makes it into our reservoirs.”

It might be helpful to think about these factors like your personal finances.

If reservoirs are the savings accounts in which you store water, streamflow is the paycheck you earn. In years where warm temperatures and dry soil absorb water creating low streamflows (like when unexpected costs take a chunk out of your paycheck), we have to dig deeper into those savings accounts to pay the bills (or in Denver Water’s case, meet customer needs). While in years of strong streamflow, we can rely mostly on that streamflow (or paycheck) to serve our customers – carrying over that savings account for future dry years.

“Essentially, Denver Water uses streamflow forecasts to predict how much runoff we receive,” Elder said. “Then we determine the amount we store in our reservoirs and the amount that goes straight to meeting customer demand.”

The Natural Resources Conservation Service, Colorado Basin River Forecast Center and Missouri Basin River Forecast Center have models that integrate snowpack, weather conditions, future precipitation, soil moisture, evapotranspiration and other influencers to predict streamflow.

Denver Water collects water from more than 30 streams in Grand County, including Ranch Creek, pictured here. Photo credit: Denver Water.

But it isn’t an exact science, because factors like snowpack, soil moisture and evapotranspiration are difficult to measure on a large scale. Therefore, the predictions show a range of possibilities, rather than a single number.

You can see the latest streamflow predictions in our weekly Water Watch reports (Page 6). The report focuses on the two basins where Denver Water gets its water supply: the South Platte River and Colorado River basins.

“Based on the most recent forecasts, our streamflows should be pretty good this spring,” Elder said. “The Colorado River basin has seen conditions more conducive to good runoff than the South Platte, but both are much better than the southern portion of the state, where it has been very dry.”

When you combine those forecasts with our reservoir storage that has benefitted from several consecutive wet years and efficient use by customers, Denver Water’s overall water supply is in good shape going into the summer, meaning we don’t anticipate any additional restrictions outside of our annual summer watering rules.

But now you have another element to keep your eye on during future runoff seasons. The great integrator is always a factor.