Mountain snowpacks provide an “extra” form of water storage in California and the interior West, 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 reservour 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), it is easy to imagine 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 and Upper Colorado River Basin (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) in these two key western systems.
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.
In water year 2020,reservoir storage began well above normal due to carryover from the wet 2019 water year. Snowpack began somewhat late and soon fell beyond normal levels. Because precipitation lagged (as did snowpack), runoff into the reservoirs was less than normal and reservoir storage fell below normal overall by March. Finally in March and April significant storms arrived and added to the snowpack and raised reservoir levels slightly above normal again.
Below are the same observations, except that they have been split out into three subregions (indicated by the dots in the inset map) to allow for a clearer depiction of where the snowpack storage is, and where the reservoir storage is, within the range as a whole.
Zooming in still further, below are corresponding storage summaries for two important water systems in the north central Sierra Nevada, 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 amd the Carson Sink in northwest Nevada.
Yuba-Feather Rivers Storage
Lake Tahoe Basin Storage
UPPER COLORADO RIVER BASIN STORAGE
A similar, albeit somewhat more complex, set of graphics showing the amount of water stored in 20 long-reporting reservoirs above Lake Powell in the Upper Colorado River Basin (UCRB), in snowpack above Lake Powell, and in Lake Powell, is presented below. Lake
Powell is one of two very large reservoirs–along with Lake Mead–that are used to manage the Colorado River for water users in all seven Southwestern states. Lake Powell storage is considerably greater than the total storage in the 20 reservoirs considered here, and is comparable to the long-term average yearly maximum volume of water stored in the snowpacks of the UCRB. Lake Powell is managed to provide reliable water supplies to all the rivers of the always dry but significantly populated Lower Colorado River Basin.
The upper panel of the UCRB graphic compares storage in UCRB reservoirs (above Powell) to long-term norms (inset graphic) and compared storage in UCRB snowpack and reservoirs to their long-term norms. The lower panel compares UCRB reservoirs storage and UCRB snowpack-plus-reservoirs storage to UCRB reservoirs-plus-Lake Powell storage. For comparison to developments this year, the corresponding graphics for water years 2016 thru 2020 are available here.