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Sevier Water Supply Outlook, April 1, 2009

Prepared by B.Bernard
NOAA, National Weather Service
Colorado Basin River Forecast Center
Salt Lake City, Utah


Sevier Summary

Snow Water Equivalent & Precipitation - conditions across the Sevier River drainage were affected last month by warm temperatures and below average precipitation which melted much of the low and mid elevation snows across this basin. Snow water equivalents decreased from 95% of average to 82% of average with no real spike in streamflows. Monthly precipition was below average, totalling 78% of average. Season precipitation declined as well, totalling at 100% of average. Currently only high elevation snow remains in the Sevier Basin.

Soil Moisture & Streamflow - Currently we have no models soil moisture grids for the Sevier River Basin. Streamflows were 89% of average for the Sevier at Hatch.

Short Term Precipitation Forecasts - Weather guidance from National Weather Service Models are indicating a period of cool and wet weather during the first 6-10 days of April. Short term CPC guidance shows no signal during the remainder of April.

General Discussions - Longer range predictions from NOAA's Climate Prediction Center are now factoring in a weak La Nina episode, predicted to last through the spring of 2009. Please reference the web link above for further details and effects within the Sevier River Basin.

Volumetric forecasts were lower this month due to declining snow water equivalents. Overall the volumetric forecasts now range 71% of average on the Sevier River Nr Gunnison, Below San Pitch Reservoir to 94% of average on the East Fork of the Sevier River Nr Kingston. Both of these forecast points represent regulated/observed forecasts rather than corrected natural flow forecasts.

*Median of forecasts within each basin.

Sevier Basin Conditions

* Percent usable capacity, not percent average contents.
Click for multi-month Graph.

Sevier Specific Site Forecasts (kaf)

Click site name for graph.
Hatch April-July36488762
Kingston, Nr April-July23288540
Ef Sevier
Kingston, Nr April-July25339450
Marysvale, Nr, Piute Dam, Blo April-July52758290
Gunnison, Nr, San Pitch, Blo April-July347571114
Chicken Ck
Levan, Nr April-July1.73674.5
Oak Ck
Oak City, Nr, Little Ck, Abv April-July1.21.4841.7
Beaver, Nr April-July15.43111539
Minersville Res, Minersville, Nr April-July141810822
Coal Ck
Cedar City, Nr April-July12.1189320

Differences between the full period forecasts and the residual forecasts may not exactly equal the actual observed volumes due to rounding conventions (see Definitions section).

Sevier Reservoir Contents (kaf)

Last Year
Last Year
untitled Otter Ck
Otter Creek Res, Antimony, Nr 52.5 32.4 62 33.9 65
untitled Sevier
Piute Res, Marysvale, Nr 71.8 44.3 62 53.6 75
untitled Gunnison Reservoir
20.3 14.4 71 10.2 50
untitled Sevier Bridge Reservoir, Juab, Nr
236.0 127.3 54 158.7 67
untitled Beaver
Minersville Res, Minersville, Nr 23.3 11.4 49 11.2 48
TOTAL 403.9 229.8 57 267.6 66

Precipitation Maps


10% exceedance forecast: Given the current hydrometeorological conditions, i.e current snowpack, soil moisture and streamflow, the volume that has a 10% chance of being exceeded. Previously referred to as "Reasonable Maximum Forecast".

50% exceedance forecast: Given the current hydrometeorological conditions, i.e current snowpack, soil moisture and streamflow, the volume that has a 50% chance of being exceeded. Previously referred to as "Most Probable Forecast".

90% exceedance forecast: Given the current hydrometeorological conditions, i.e current snowpack, soil moisture and streamflow, the volume that has a 90% chance of being exceeded. Previously referred to as "Reasonable Minimum Forecast".

Acre-Foot (af): The volume equal to one acre covered one foot deep (43,560 cubic feet). See kaf below.

Average: The arithmetic mean. The sum of the values divided by the number of values. Values from 1971-2000 are used for this publication.

Categories: Much Above Average=Greater than 130%, Above Average=111-130%, Near Average=90-110%, Below Average=70-89%, Much Below Average=Less than 70%.

CBRFC: Colorado Basin River Forecast Center.

Forecast Period: The period from April 1 through July 31, unless otherwise noted.

kaf: Thousand Acre-Feet. See Acre-Foot above.

Inflow: The volume of water that flows into a reservoir or lake.

Median: The middle value of an ordered set of values. Half of the values are higher and half of the values are lower. When the set contains an even number of values the median is the average of the two middle numbers.

NOAA: National Oceanic and Atomospheric Administration.

NWS: National Weather Service.

Rounding Conventions:
RangeRound to
1000+3 significant digits

Streamflow: The volume of water that flows past a specific stream site.

Water Year: The 12-month period, October 1 through September 30. The water year is designated by the calendar year in which it ends. Thus, the year ending September 30, 2008, is called the "2008 water year."

Additional Information

Water supply forecasts take into consideration present hydrometeorological conditions and use average basin temperatures and precipitation for the forecast period. As the forecast season progresses, a greater portion of the future hydrologic and climatic uncertainty becomes known and monthly forecasts become more accurate. For more information on the tools we use, consult Water Supply Forecasting Tools.

Volume forecasts represent adjusted flows; that is, observed flows with upstream water use taken into account. Adjusted flows will closely approximate natural or unimpaired flows. However, not all upstream diversions or impoundments are measured or quantifiable. For specific adjustments used with each forecast point, consult the Guide to Water Supply Forecasting.

The Water Supply Outlook is issued monthly January through May by the Colorado Basin River Forecast Center. It represents a coordinated effort between the National Weather Service, Natural Resources Conservation Service, Bureau of Reclamation, U.S. Geological Survey and local water district managers.

Note: Data used in this report are provisional and are subject to revision.

For more information, or to be included on the mailing list, please contact:

Colorado Basin River Forecast Center
2242 W North Temple
Salt Lake City, UT 84116
(801) 524-5130

Hydrologist: B.Bernard