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Water Supply, Forecasts, and Reservoir Storage

Monthly Reports: 

Water Supply Outlook Report

 Climate and Water Report

WSOR_May_2020

 

Water Supply Outlook Report Archive:

 

Climate And Water Report Archive:

 

Other Monthly Products: 

Surface Water Supply Index Summary

Reservoir Storage Tables

WSOR_May_2020

 

SWSI Summary Archive:

 

Reservoir Storage Table Archive:

 

Upper Colorado Regional Forecasts

Western Utah Regional Forecasts

WSOR_May_2020

 

Upper Colorado Regional Forecast Archive:

 

Western Utah Regional Forecast Archive:

 

Additional Resources

Reservoir Storage Resources

Streamflow Resources

Other Reports

 

What is the Water Supply Outlook Report? (WSOR)

Website under construction
Standby for WSOR description

 

What is the Climate and Water Report? (CWR)

The purpose of the Climate and Water Report is to provide a snapshot of current and immediate past climatic
conditions and other information useful to agricultural and water user interests in Utah. The report utilizes
data from several sources that represent specific parameters (streamflow data from the United States Geological
Survey, reservoir data from the Bureau of Reclamation, and other sources), geography including high
elevation United States Department of Agriculture (USDA), Natural Resources Conservation Service (NRCS)
Snowpack Telemetry (SNOTEL) data, and agriculturally important data from the USDA-NRCS Soil Climate
Analysis Network (SCAN). Data on precipitation, soil moisture, soil temperature, reservoir storage, and streamflow
are analyzed and presented. These data analyses can be used to increase irrigation efficiency and agricultural
production. As with all data and analyses, there are limitations due to data quality, quantity, and spatial
application.

 

What is the Surface Water Supply Index? (SWSI)

The Surface Water Supply Index (SWSI) is a predictive indicator of total surface water availability within a watershed for the spring and
summer water use seasons. The index is calculated by combining pre-runoff reservoir storage (carryover) with forecasts of spring and
summer streamflow which are based on current snowpack and other hydrologic variables. SWSI values are scaled from +4.1 (abundant
supply) to -4.1 (extremely dry) with a value of zero (0) indicating median water supply as compared to historical analysis. SWSI's are
calculated in this fashion to be consistent with other hydroclimatic indicators such as the Palmer Drought Index and the Precipitation
index.

Utah Snow Surveys has also chosen to display the SWSI value as well as a PERCENT CHANCE OF NON-EXCEEDANCE. While this is
a cumbersome name, it has a simple application. It can be best thought of as a scale of 1 to 99 with 1 being the drought of record (driest
possible conditions) and 99 being the flood of record (wettest possible conditions) and a value of 50 representing average conditions.
This rating scale is a percentile rating as well, for example a SWSI of 75% means that this year's water supply is greater than 75% of all
historical events and that only 25% of the time has it been exceeded. Conversely a SWSI of 10% means that 90% of historical events
have been greater than this one and that only 10% have had less total water supply. This scale is comparable between basins: a SWSI
of 50% means the same relative ranking on watershed A as it does on watershed B, which may not be strictly true of the +4 to -4 scale.

 

Reservoir Storage Tables Defined:

These tables define the current storage within the major Utah reservoirs. This includes the current storage, the capacity of each individual reservoir,
the percentage the reservoir was full this time the previous year, and the percent full the reservoir currently is. The green sections on the table
represent an over five percent increase in stored water from this time last year. The red sections represent an over five percent decrease from this
time last year.

 

What is a river forecast? (Upper Colorado & Western Utah Regional Forecasts)

Most of the annual streamflow in the western United States originates as snowfall that has accumulated in the
mountains during the winter and early spring. As the snowpack accumulates, hydrologists estimate the runoff
that will occur when it melts. Measurements of snow water equivalent at selected manual snowcourses and
automated SNOTEL sites, along with precipitation, antecedent streamflow, and indices of the El NiƱo /
Southern Oscillation are used in statistical and simulation models to prepare runoff forecasts. Unless otherwise
specified, all forecasts are for flows that would occur naturally without any upstream influences.

Forecasts of any kind, of course, are not perfect. Streamflow forecast uncertainty arises from three primary
sources: (1) uncertain knowledge of future weather conditions, (2) uncertainty in the forecasting procedure,
and (3) errors in the data. The forecast, therefore, must be interpreted not as a single value but rather as a
range of values with specific probabilities of occurrence. The middle of the range is expressed by the 50%
exceedance probability forecast, for which there is a 50% chance that the actual flow will be above, and a 50%
chance that the actual flow will be below, this value. To describe the expected range around this 50% value,
four other forecasts are provided, two smaller values (90% and 70% exceedance probability) and two larger
values (30%, and 10% exceedance probability). For example, there is a 90% chance that the actual flow will
be more than the 90% exceedance probability forecast. The others can be interpreted similarly.

The wider the spread among these values, the more uncertain the forecast. As the season progresses,
forecasts become more accurate, primarily because a greater portion of the future weather conditions become
known; this is reflected by a narrowing of the range around the 50% exceedance probability forecast. Users
should take this uncertainty into consideration when making operational decisions by selecting forecasts
corresponding to the level of risk they are willing to assume about the amount of water to be expected. If users
anticipate receiving a lesser supply of water, or if they wish to increase their chances of having an adequate
supply of water for their operations, they may want to base their decisions on the 90% or 70% exceedance
probability forecasts, or something in between. On the other hand, if users are concerned about receiving too
much water (for example, threat of flooding), they may want to base their decisions on the 30% or 10%
exceedance probability forecasts, or something in between. Regardless of the forecast value users choose for
operations, they should be prepared to deal with either more or less water. (Users should remember that even
if the 90% exceedance probability forecast is used, there is still a 10% chance of receiving less than this
amount.) By using the exceedance probability information, users can easily determine the chances of receiving
more or less water.