🛰 How We Estimate Reservoir Levels
A plain-language explanation of our satellite monitoring system
The problem with official data
The state of Colorado keeps official records of how much water is in each reservoir. But those records are updated slowly — usually months behind. In the middle of irrigation season, that's not much help. We needed a way to see what's happening now.
Satellites that can see water
There are satellites orbiting Earth that take pictures of the ground every few days. When you look at those pictures with the right tools, you can clearly see where the water is — and more importantly, how much surface area the water covers.
A full reservoir has a large water surface. A half-full reservoir has a smaller one. We measure that surface area from the satellite image and use it to estimate how much water is stored.
Turning an area into a volume
Knowing the water's surface area is only half the puzzle. To get a volume (in acre-feet), we need to know how that reservoir fills up — every reservoir has a different shape. A wide, shallow reservoir fills very differently than a deep, narrow one.
We use years of official state records — times when we know exactly how full each reservoir was — to teach the system what a given surface area means for each specific reservoir. The more historical data we have, the better the estimate.
What "Good estimate" vs. "Approximate" means
Not every reservoir is equally easy to read from a satellite. Some reservoirs have a clear relationship between water area and storage — when the area goes up, the volume goes up in a predictable way. Those get a "Good estimate" label.
Other reservoirs — particularly wide, flat ones — don't change their water surface much even as their water level rises and falls. For those, we still provide a best guess, but we label it "Approximate" so you know to take it with a grain of salt.
How often do we update?
The satellites pass over Colorado roughly every 5 days when skies are clear. Clouds, smoke, and snow on the water can block or confuse the reading, so not every pass produces a usable image. In practice, we typically get updated estimates twice a week during irrigation season.
What this is — and isn't
These estimates give NPIC staff and shareholders a timely picture of system storage between official state readings. They are not official water measurements and should not be used for legal or regulatory purposes.
For official reservoir records, contact NPIC directly at (970) 568-3612 or the Colorado Division Engineer, District 3 in Greeley at (970) 352-8712.
Estimation system last run: 2026-05-12