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San Juan Water Supply Outlook, May 1, 2011

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


San Juan Summary

*Median of forecasts within each basin.

San Juan Basin Conditions

The following conditions influenced this month's forecasts:

Seasonal October through April precipitation was 95 percent of average, up 5 percent from last month. April precipitation was 140 percent of average, which was much above average.

Current snow water equivalent for the entire basin is 95 percent, which is average. The Animas basin is 100 percent and above Navajo is 90 percent. All three are up from the April 1st percentages.
--- Animas River Basin Snow Plot.
--- Above Navajo Basin Snow Plot.

Streamflow for the entire San Juan Basin was much below average for April at 60%. Seasonal streamflow since October continued to be much below average at 60%, due to the much below average fall precipitation.

Soil Moisture:
Modeled soil moisture as of November 1st, 2010 was average to below average across the entire San Juan Basin.

Climate Forecasts:
The correlation of La Nina and April through July water volumes in the San Juan basin does suggest slightly drier than average December through May precipitation, but it did not influence the April forecast process.

Forecast Summary:
The April-July forecast volumes changed little from last month and currently range between 60 and 90 percent of average with a median value of 75%.

* 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-July17419486220
Carracas, Nr April-July25531077365
Navajo Res, Archuleta, Nr April-July46554569630
Farmington April-July76587072995
Bluff, Nr April-July670875711110
Rio Blanco
Pagosa Springs, Nr, Blanco Dam, Blo April-July34438154
Chromo, Nr, Oso Div Dam, Blo April-July42558070
Arboles, Nr April-July13717074210
Los Pinos
Vallecito Res, Bayfield, Nr April-July13015375178
Durango April-July35040091450
Lemon Res, Durango, Nr April-July32417151
La Plata
Hesperus April-July14.4187222
Mancos, Nr April-July17.7247331
South Ck
Lloyds Rsvr Nr Monticello, Abv March-July0.370.64461.1

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 End of Month Reservoir Contents (kaf)

Last Year
Last Year
untitled Los Pinos
Vallecito Res, Bayfield, Nr 125.4 94.5 75 64.6 51
untitled San Juan
Navajo Res, Archuleta, Nr 1701.3 1357.4 80 1383.7 81
untitled Florida
Lemon Res, Durango, Nr 39.8 18.4 46 14.2 36
TOTAL 1866.5 1470.3 79 1462.4 78

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 1981-2010 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