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San Juan Water Supply Outlook, May 1, 2009

Prepared by Tracy Cox
NOAA, National Weather Service
Colorado Basin River Forecast Center
Salt Lake City, Utah


San Juan Summary

-Model Soil Moisture/Observed Streamflow...Soil moisture for most of the upper basins in the San Juan were below average last fall. This was most likely due to limited monsoonal precipitation over the summer months. Observed streamflow for the month of April was near average in the upper portions of the basin, but below normal in the lower San Juan.

-Snowpack/Precipitation...Snow water equivalent for the entire San Juan Basin on May 1st was 75 percent of average. After three months of much below normal precipitation, the San Juan Basin recieved 95 percent of average precipitation during April. Due to the extremely wet December, seasonal precipitation has stayed near average and is currently 95 percent of average.

-Short Term Precipitation Forecast... As of May 6th, meteorological models were predicting increased chances for above normal temperatures and below normal precipitation through the middle of May.

-General Discussion...Due to the below average soil moisture in the upper basin this fall and the combination of current snowpack conditions and April observed flow, the median April-July forecast for the San Juan Basin is 85 percent of average. All forecasts are exactly the same as those issued April 1st. The median of the May-July forecasts is also 85 percent of average.

*Median of forecasts within each basin.

San Juan Basin Conditions

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

San Juan Specific Site Forecasts (kaf)

Click site name for graph.
San Juan
Pagosa Springs April-July17120089250
Carracas, Nr April-July29535588425
Navajo Res, Archuleta, Nr April-July56569088825
Farmington April-July7951030851220
Bluff, Nr April-July7651010821290
Rio Blanco
Pagosa Springs, Nr, Blanco Dam, Blo April-July40509462
Chromo, Nr, Oso Div Dam, Blo April-July50629077
Arboles, Nr April-July15219083235
Los Pinos
Vallecito Res, Bayfield, Nr April-July14217083200
Durango April-July29537084460
Lemon Res, Durango, Nr April-July36457855
La Plata
Hesperus April-July15.1197624
Mancos, Nr April-July10.7257639
South Ck
Lloyds Rsvr Nr Monticello, Abv March-July0.110.36260.85

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).

Reservoir Monthly Inflow Forecasts

San Juan Reservoir Contents (kaf)

Last Year
Last Year
untitled Los Pinos
Vallecito Res, Bayfield, Nr 125.4 93.5 75 42.8 34
untitled San Juan
Navajo Res, Archuleta, Nr 1701.3 1332.1 78 1360.2 80
untitled Florida
Lemon Res, Durango, Nr 39.8 24.0 60 21.4 54
TOTAL 1866.5 1449.6 78 1424.3 76

Monthly Streamflows

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: Tracy Cox