Examples of Water Reliability from 2021 West-Wide Assessment
Local context is important when interpreting the results for each basin.
The complex interaction between inflow, storage, water demands, and operating policies were evaluated in the 2021 Assessment. The impact of a drought on the managed system depends in part on the magnitude and duration of the drought, but also on the size of the system’s reservoirs relative to their inflow, and the extent of carryover storage.
For example, Lake Mead and Lake Powell have the combined capacity to store approximately three times their annual inflow, which allows these reservoirs to store water in wet years to meet demands in drought years.
At Shasta and Millerton Lakes, observed historical droughts had a larger influence than paleo droughts on the reservoirs' end-of-September storage. This provides confidence in the ability of the systems to withstand a greater variety of droughts.
In another example, Upper Klamath Lake has limited ability to provide carryover storage, and operating policies allow for a relatively small amount of active storage in comparison to the lake’s total capacity, so drought conditions may not have a clearly visible impact on storage.
In the Upper Missouri River Basin, 11 percent of the active storage of Canyon Ferry Reservoir is unallocated, which serves as a buffer against the impact of drought.
Anderson Ranch Reservoir in the Columbia River Basin (Boise River), which can store a little less than its average annual inflow, experiences more impact to storage during paleo droughts than during observed historical drought events.
As these examples illustrate, metrics such as end of- September storage may be consistently evaluated across reservoirs, but local context is important for interpreting the results in each basin.
To evaluate a reservoir’s ability to meet its management objectives, another analysis in the 2021 Assessment evaluated reservoir inflow, outflow, and reservoir storage data. The research focused on five reservoirs (noted in the image to the left) to identify severe, sustained, and seasonal droughts covering both paleohydrologic and observed periods.
The analysis showed that the minimum active storage pool at which restrictions are imposed is generally a large fraction (about 60 percent) of the active storage capacity and represents a strategy that imposes restrictions while maintaining a relatively large fraction of storage in order to hedge against droughts (see 2021 Assessment, Chapter 5 ).
