Upper Yellowstone and Madison River Basins (2)
February 2002

Snow Snowfall in these basins this year has been below average for this time of the year, but better than last year. Snow water equivalent (SWE) is about 92 percent of average (201 percent of last year) in the Madison drainage. SWE in the Yellowstone drainage is about 83 percent of average (165 percent of last year at this time). See the "Snow Course Basin Summary" at the beginning of this document for more details on specific sites.

Precipitation
Last month's precipitation in the Madison and Yellowstone drainage was about 90 percent of average for the 5 reporting stations -- percentage range was from 70 to 103 percent of average. Water-year-to-date precipitation is about 89 percent of average (145 percent of last year's amount). Year to date percentage ranges from 85 to 95 percent

Reservoir
Ennis Lake did not report this month, but last month was storing 30,700 acre-feet (75 percent of capacity) – 97 percent of average. Hebgen Lake is storing about 286,900 acre-feet of water (76 percent of capacity) – 108 percent of average. Hebgen Lake is storing about 96 percent and Ennis Lake, last month, was storing about 100 percent of last year's volume.

Streamflow
All the following forecasts are based on the 50 percent chance runoff for the April through September runoff period. Yellowstone at Lake Outlet is expected to yield about 630,000 acre feet (78 percent of normal). Yellowstone at Corwin Springs will yield about 1,510,000 acre-feet (77 percent of normal). Yellowstone near Livingston will yield about 1,740,000 acre feet (76 percent of normal). Hebgen lake inflow is estimated to be 410,000 acre feet (82 percent of normal). See the following page for detailed runoff volumes. 

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - February 1, 2002
==================================================================================================================================
                                              |  <<====== Drier ======  Future Conditions  =======  Wetter =====>>  |             
                                              |                                                                     |
  Forecast Point                     Forecast |  ==================== Chance Of Exceeding * ======================  |
                                      Period  |    90%       70%    |  50% (Most Probable)  |      30%       10%    |  30-Yr Avg.
                                              | (1000AF)  (1000AF)  |   (1000AF)  (% AVG.)  |    (1000AF)  (1000AF) |    (1000AF)
====================================================================|=======================|=====================================
YELLOWSTONE at Lake Outlet           APR-SEP       475       567    |      630        78    |       693       785            805
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Springs  APR-SEP      1171      1373    |     1510        77    |      1647      1849           1970
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    APR-SEP      1437      1618    |     1740        76    |      1862      2043           2280
                                                                    |                       |
HEBGEN Reservoir Inflow              APR-SEP       323       375    |      410        82    |       445       497            500
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of January               |        Watershed Snowpack Analysis - February 1, 2002
==================================================================================================================================
                                      Usable |  *** Usable Storage ***  |                            Number     This Year as % of
Reservoir                            Capacity|  This      Last          |   Watershed                  of       =================
                                             |  Year      Year     Avg  |                          Data Sites   Last Yr   Average
========================================================================|=========================================================
ENNIS LAKE                              41.0     28.9     29.6     31.3 |   MADISON RIVER in WY         9       201        92
                                                                        |
HEBGEN LAKE                            377.5    286.9    298.2    266.5 |   YELLOWSTONE RIVER in WY    12       165        83
                                                                        |
==================================================================================================================================
 * 90%, 70%, 30%, and 10% chances of exceeding are the probabilities that the actual volume will exceed the volumes in the table.

(1) - The values listed under the 10% and 90% Chance of Exceeding are actually 5% and 95% exceedance levels.                  
(2) - The value is natural volume - actual volume may be affected by upstream water management.
To February 2002 Basin Outlook Report Contents Page