Reservoirs-plus-Snowpack Water Storage in the Sierra Nevada

April 19, 2024

Current measures of water stored in Sierra Nevada reservoirs and snowpacks are now continually updated and available from CW3E. Mountain snowpacks provide an “extra” way that water gets stored in California and across the Western US, acting as natural reservoirs that hold winter precipitation (as snow) from the cold wet season until spring and summer when the water is released as snowmelt when water demands for human and environmental uses, including irrigation, are high. Thus the combination of water stored as snow and water stored in human-built reservoirs is a useful indicator of development, persistence, and termination of droughts in many western water-supply systems. As the climate warms in coming decades, the “typical” mix of water in snowpack versus reservoir is projected to change, with far less snow holding far less water in future winters. Whether the water no long stored in snow ends up residing instead in reservoirs for more of the year in the future will depend on a variety of tradeoffs associated with winter-weather and hydrological changes, infrastructure constraints, and flood- and resource-management responses that tracking of the combined storage amounts may help to highlight.

This approach to tracking water-supply status was initially developed for drought early-warning and tracking purposes, but has also demonstrated value in wet years. Notably, last year, these figures attracted widespread attention when they showed that the 2023 snowpack in drainages above the ancient Tulare Lake bed contained as much as three times the total amount of space in the reservoirs in that drainage even if all the water currently in the reservoirs was released to make space. This highlighted that nearly all of the water that entered the reservoirs that year was going to have to be routed out of the Tulare Basin or else flooding would certainly occur. However, that much water could not be routed out of the Basin with existing infrastructures and so flooding did occur, and Tulare Lake refilled, covering large expanses of agricultural land and impacting communities. In the three preceding drought years (2020-2022), by contrast, snowpacks never grew to sizes that would refill Sierra Nevada reservoirs, and so water in storage declined each year.

Since 2015, scientists at Scripps have been developing and circulating simple graphics that showed how the combination of these two forms of storage evolved, in order to better communicate water resource status as the water years progressed. CW3E researcher Mike Dettinger demonstrated the value of tracking of water supply status by tracking these combined storages during the 2012-2015 California drought (Dettinger and Anderson, 2015), and has been producing and circulating them on a roughly monthly (or more frequent, depending on circumstances) basis since. The development and original updates were supported by NOAA’s National Integrated Drought Information System (NIDIS) via the California Nevada Adaptation Program (CNAP).

This spring the process of updating and generating these diagrams was automated so that now they will be updated on a daily basis, with support from the California Department of Water Resources via the AR Program. Updated storage diagrams will be available at:

along with background information, explanations, and archives of the diagrams from previous years (to put the current year’s progress into historical contexts).

Dettinger, M.D., and Anderson, M.L., 2015, Storage in California’s reservoirs and snowpack in this time of drought: San Francisco Estuary and Watershed Science, 13(2), 5 p.,