Last Updated: September 15, 2021Consistent with the Upper Basin Drought Response Operations Agreement (DROA) provisions to protect a target elevation at Lake Powell of 3,525 feet, this September 2021 24-Month Study includes releases from the upstream initial units of the Colorado River Storage Project Act to deliver an additional 181 thousand acre-feet (kaf) to Lake Powell by the end of December 2021. The additional releases began in July and will continue to be implemented on the following schedule:
The releases detailed above are in addition to the already established releases determined by operational plans for each of the identified facilities. The additional delivery of 181 kaf is equivalent to Lake Powell’s elevation of approximately three feet. Water year 2021 releases from Lake Powell to Lake Mead will not be adjusted as those are determined consistent with the Interim Guidelines.The unregulated inflow volume to Lake Powell during August was 294 thousand acre-feet (kaf) (59% of average). The release volume from Glen Canyon Dam in July was 801 kaf. The end of August elevation and storage of Lake Powell were 3548.96 feet (151 feet from full pool) and 7.51 million acre-feet (maf) (31% of live capacity), respectively.
To view the most current reservoir elevation projections, click on: Lake Powell Elevation Projections.
To view the most current monthly release projections, click on: Lake Powell Release Projections.
To view the 2022 progession of snowpack above Lake Powell, click on Lake Powell Snow Chart.
To view the current inflow forecast relative to past inflows, click on Lake Powell Inflow Forecast.
The operating tier for water year 2021 (October 2020 through September 2021) was established in August 2020 as the Upper Elevation Balancing Tier, consistent with Section 6.B of the Interim Guidelines. Consistent with Section 6.B of the Interim Guidelines, Lake Powell’s operations in water year 2021 will be governed by the Upper Elevation Balancing Tier. With an 8.23 maf release from Lake Powell in water year 2021, the April 2021 24-Month Study projected the end of water year elevation at Lake Powell to be below 3,575 feet. Therefore, in accordance with Section 6.B.1 of the Interim Guidelines, Lake Powell will continue to release 8.23 maf through the remainder of the water year 2021.
The anticipated release volume for September 2021 is 623,000 af with fluctuations anticipated between about 7,060 cfs in the nighttime to about 12,656 cfs in the daytime, and consistent with the Glen Canyon Dam, Record of Decision (dated December 2016). The October anticipated release is 480,000 af with fluctuations between about 6,220 cfs to around 8,965 cfs with a three-hour evening weekend peak on Saturday of 9,437 cfs.
In addition to daily scheduled fluctuations for power generation, the instantaneous releases from Glen Canyon Dam may also fluctuate to provide 40 megawatts (mw) of system regulation. These instantaneous release adjustments stabilize the electrical generation and transmission system and translate to a range of about 1,100 cfs above or below the hourly scheduled release rate. Under system normal conditions, fluctuations for regulation are typically short lived and generally balance out over the hour with minimal or no noticeable impacts on downstream river flow conditions.
Releases from Glen Canyon Dam can also fluctuate beyond scheduled releases when called upon to respond to unscheduled power outages or power system emergencies. Depending on the severity of the system emergency, the response from Glen Canyon Dam can be significant, within the full range of the operating capacity of the power plant for as long as is necessary to maintain balance in the transmission system. Glen Canyon Dam currently maintains 30 mw (approximately 800 cfs) of generation capacity in reserve in order to respond to a system emergency even when generation rates are already high. System emergencies occur fairly infrequently and typically require small responses from Glen Canyon Dam. However, these responses can have a noticeable impact on the river downstream of Glen Canyon Dam.
Inflow Forecasts and Model Projections
The forecast for water year 2021 unregulated inflow to Lake Powell, issued on September 1, 2021, by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume this year will be 3.56 maf (33% of average).
In addition to the September 2021 24-Month Study based on the Most Probable inflow scenario, and in accordance with the Drought Response Operational Agreement (DROA) Reclamation has conducted model runs in September to determine a possible range of reservoir elevations under probable minimum and probable maximum inflow scenarios. Probable minimum and probable maximum model runs are conducted in January, April, August, and October. The probable minimum inflow scenario reflects a dry hydrologic condition which statistically would be exceeded 90% of the time. The most probable inflow scenario reflects a median hydrologic condition which statistically would be exceeded 50% of the time. The probable maximum inflow scenario reflects a wet hydrologic condition which statistically would be exceeded 10% of the time. There is approximately an 80% probability that a future elevation will fall inside the range of the minimum and maximum inflow scenarios. Additionally, there are possible inflow scenarios that would results in reservoir elevations falling outside the ranges indicated in these reports.
The DROA coordination will continue until either (i) the minimum probable projected elevation remains above 3,525 feet for 24 months or (ii) the process moves to the next step when the most probable projected elevation indicates Powell elevations below 3,525 feet and a Drought Response Operations Plan is developed.
The September forecast for water year 2022 ranges from a minimum probable of 4.74 maf (44% of average) to a maximum probable of 16.0 maf (148% of average) with the most probable forecast for water year 2022 of 8.20 maf (76% of average). There is a 10% chance that inflows could be higher than the current maximum probable forecast and a 10% chance that inflows could be lower than the minimum probable forecast.
Based on the current forecast of 3.56 maf unregulated inflow, the September 24-Month Study projects Lake Powell elevation will end water year 2022 near 3,542.07 feet with approximately 7.03 maf in storage (29% of capacity). Note that projections of elevation and storage for water year 2022 have significant uncertainty at this point in the season. Projections of end of water year 2022 elevation using the minimum and maximum probable inflow forecast from and results from the September 2021 model runs are 3,506.01 feet and 3,615.68 feet, respectively. Under these scenarios, there is a 10% chance that inflows will be higher, resulting in higher elevation, and 10% chance that inflows will be lower, resulting in lower elevation. The annual release volume from Lake Powell during water year 2022 will be 7.48 maf as determined under Section 6.C.1 of the Interim Guidelines.
Upper Colorado River Basin Hydrology
Upper Colorado River Basin regularly experiences significant year to year hydrologic variability. During the 21-year period 2000 to 2020, however, the unregulated inflow to Lake Powell, which is a good measure of hydrologic conditions in the Colorado River Basin, was above average in only 4 out of the past 19 years. The period 2000-2020 is the lowest 21-year period since the closure of Glen Canyon Dam in 1963, with an average unregulated inflow of 8.62 maf, or 80 percent of the 30-year average (1981-2010). (For comparison, the 1981-2010 total water year average is 10.83 maf.) The unregulated inflow during the 2000-2020 period has ranged from a low of 2.64 maf (24 percent of average) in water year 2002 to a high of 15.97 maf (147 percent of average) in water year 2011. In water year 2018 unregulated inflow volume to Lake Powell was 4.6 maf (43 percent of average), the third driest year on record above 2002 and 1977. Under the current most probable forecast, the total water year 2021 unregulated inflow to Lake Powell is projected to be 3.56 maf (33 percent of average).
At the beginning of water year 2021, total system storage in the Colorado River Basin was 28.88 maf (48 percent of 59.6 maf total system capacity). This is a decrease of 2.77 maf over the total storage at the beginning of water year 2020 when total system storage was 31.64 maf (53 percent of capacity). Since the beginning of water year 2000, total Colorado Basin storage has experienced year to year increases and decreases in response to wet and dry hydrology, ranging from a high of 94 percent of capacity at the beginning of 2000 to the now current level of 48 percent of capacity at the beginning of water year 2021. Based on current inflow forecasts, the current projected end of water year total Colorado Basin reservoir storage for water year 2021 is approximately 22.94 maf (38 percent of total system capacity). The actual end of water year 2021 system storage may vary from this projection, primarily due to uncertainty regarding this season’s runoff and reservoir inflow.