Water Storage Tracking for Sierra Nevada

Mountain snowpacks provide an “extra” form of water storage in California and across the Western US, acting as natural reservoirs that hold winter precipitation (as snow) from the cold wet season for release as snowmelt in the warm dry seasons when water demands for human and environmental uses, including irrigation, are high. The combination of water stored as snow and water stored in human-built reservoirs therefore is a useful indicator of developing droughts, persistent droughts, and the termination of droughts in many water-supply systems of the western states. In a winter when reservoir storage is unusually low but snowpacks are unusually rich, it would be easy to imagine that a drought is occurring or soon to develop, if only the reservoir storage is reported. Conversely, in a winter when snowpack is lacking but reservoir storage is high (e.g., with the streamflows from a preceding wet year, like Lake Tahoe in water 2020), it is easy to anticipate that a drought is coming, if only snowpack is considered. Remarkably, in most reservoir trackers, snow and reservoirs are reported separately.

In the subsections below, we provide simple graphical summaries of the status of snow-plus-reservoir storage for the Sierra Nevada (see map at right) to facilitate tracking of the overall storage in the combination of these two “reservoir” types, to allow quick judgements regarding the status of “storage drought” (Dettinger and Anderson, 2015; or expanded version). Figures on this page are updated daily.

Additional observations and information about California snowpack monitoring is available from the California Cooperative Snow Surveys Program (CA DWR).

Sierra Nevada Storage

The following figure shows the most recent summary of reservoir water storage and reservoir-plus-snowpack water storage for the westward draining Sierra Nevada, based on daily California Department of Water Resources’ reports of storage in 28 reservoirs and of state-averaged snow-water content, The former is reported in acre-feet of stored water, and the latter is reported in inches of water content (how much liquid water would be released, per unit area, by melting all the snow on the ground at a given time). This depth of snow-water content is converted to acre feet of water by a simple scaling (between state-average inches of water content and long-term average April-1 total water from the Margulis et al. (2016) snow “reanalysis” data set. Taken together, adding one to the other gives a simple status report for total water in surface storage in the mountain range.

In the figure the blue shaded region is the long-term average reservoir storage of the 28 reservoirs (indicated by red symbols in the inset map of California and Nevada) from 2000-2015 and the grey is the average snow-pack over the same time period. The red curve is the current reservoir storage for the same reservoirs and the orange line is the current sum of reservoir plus snow-pack storage. A history of how these daily storage amounts have played out each water year since 2010 is available here.

Below are the same observations, except that they have been split out into three subregions to allow for a clearer depiction of where the snowpack storage is, and where the reservoir storage is, within the range.


For a longer term and larger scale perspective, the figure below shows the totals of reservoir storage in 12 “major” reservoirs around California (see also here) together with storage in another 152 reservoirs around the state, with April snowpacks. This was the scale originally discussed by (Dettinger and Anderson, 2015 and allows quick comparison to past droughts.

Upper Sacramento River, Yuba river, Feather River, and Lake Tahoe Basin Storage

Now, zooming in still further, below are corresponding storage summaries for three important water systems in the north central Sierra Nevada, the Upper Sacramento River drainage, the Yuba-Feather Rivers draining westward north of Interstate 80 and the Lake Tahoe Basin which drains eastward into the Truckee River, Reno, and ultimately Pyramid Lake and the Carson Sink in northwest Nevada.


See water-years 2020-2023 versions of this graphic here.

Mike Dettinger originated the concept and idea for the tracking of water supplies by tracking snowpack and reservoir levels together. The early development and original updates were supported by NOAA’s National Integrated Drought Information System (NIDIS) via the California Nevada Adaptation Program (CNAP) and the new display with daily updates was supported by the California Department of Water Resources via the AR Program.