2019 Dillon Reservoir Case Study
Key Findings
2 ASO snow surveys conducted in 2019
2 surveys were flown over the headwaters of the Dillon Reservoir collection system in 2019. These surveys were critical to the operation of Dillon Reservoir during a particularly wet runoff season.
ASO is critical to flood control operations
A June ASO flight indicated more remaining snowpack above Dillon Reservoir than it had room for, prompting a ramp up of releases.
Local SNOTEL sites provided incomplete information
Several local SNOTEL sites (Grizzly Peak, Hoosier Pass, Fremont Pass, and Copper Mountain), had mostly melted out, while ASO revealed that about 107kAF of water remained in the snowpack above Dillon Reservoir.
Denver Water, 2017
Project Background
Colorado had an unusual snow year in the spring of 2019. Several late-season storms brought peak snow water equivalent (SWE) well above average, resulting in higher-than-normal runoff in many of its river basins. 2019 was also the first year Denver Water piloted using ASO data to inform their operations. They have historically relied on SNOTEL measurements to monitor snowpack throughout the runoff season and inform the operation of Dillon Reservoir, Denver Water’s largest reservoir. Snowpack that accumulates in the Blue River Basin flows into Dillon Reservoir and is the source of 30% of the water supply delivered to Denver and its surrounding suburbs. In addition to water supply, the reservoir provides flood control for downstream areas including densely populated portions of Dillon and Silverthorne.
ASO Application
ASO, Inc. conducted an airborne snow survey for Denver Water on April 19th, 2019 over the headwaters of the Blue River, aiming to capture near-peak SWE for the entire Dillon Reservoir watershed. Data from this flight confirmed an unusually high snowpack and indicated a delayed melt. A second ASO flight was conducted on June 24th to measure snowpack once all of the basin’s SNOTEL sites had melted out, which revealed that about 107,204 acre-feet of water remained in the snowpack above Dillon Reservoir. Both flights provided key information about the timing and volume of snowmelt runoff not present in the SNOTEL measurements the agency typically uses to inform their decision making.
Results
Between the Dillon Reservoir storage contents and remaining snowpack, there was more water stored as snow in the basin than the capacity of Dillon Reservoir, necessitating a significant release.
An overtopping of the reservoir spillway could result in flooding in the downstream town of Silverthorne. Conversely, had reservoir managers acted conservatively, they may have released more water than necessary to make space for the coming runoff, and Dillon Reservoir may not have filled. Because of the ASO flight, Denver Water managers knew that they needed to begin ramping up outflows earlier than normal and continue them for additional weeks to avoid a peak release that was higher than acceptable.
If Denver Water had not conducted the June ASO flight and only relied on SNOTEL data, an unanticipated amount of snowmelt could have resulted in a large, unexpected reservoir release and significant negative impacts downstream. Alternatively, in the absence of ASO data, water managers may have chosen to be more conservative and draw down the reservoir farther than they would like to avoid this flooding issue. The ASO data allowed Denver Water to alter their operational plan, and thus optimize use of Dillon Reservoir, by continuing outflows longer than the forecast and hydrograph indicated to make room for the coming snowmelt and avoid downstream flooding. This also resulted in the runoff season ending continuing longer than the forecast and hydrograph indicated, thus ending with Dillon Reservoir as close to full as possible.
