Upper Yellowstone and Madison River Basins (2)
January 2000

Snow
Snowfall in the basin this year has been well below average. For this time of the year, snow water equivalent (SWE) is about 71 percent of average (56 percent of last year) in the Madison drainage. SWE in the Yellowstone drainage is about 72 percent of average (61 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
December precipitation in the Madison and Yellowstone drainage was about 97 percent of average (96 percent of previous year) for the 7 reporting stations -- percentage range was from 53 percent of average at Two Ocean Plateau to 161 percent of average at Tower falls. Water-year-to-date precipitation is about 67 percent of average (59 percent of last year's amount). Year to date percentage ranges from 46 to 88 for the 7 reporting stations.

Reservoir
Ennis Lake is storing 28,900 acre-feet (86 percent of average and 70 percent of capacity). Hebgen Lake is storing about 344,800 acre-feet of water (140 percent of average and 91 percent of capacity). Hebgen Lake is storing about 82 percent and Ennis Lake is storing about 88 percent of last year's volume.

Streamflow
All the following forecasts are the 50 percent chance runoff for the April through September runoff period. Yellowstone at Lake Outlet is expected to yield about 52,000 acre feet (66 percent of normal). Yellowstone at Corwin Springs will yield about 1,315,000 acre-feet (68 percent of normal). Yellowstone near Livingston will yield about 1,515,000 acre feet (68 percent of normal). Hebgen lake inflow is estimated to be 415,000 acre feet (85 percent of normal). See the following page for detailed runoff volumes.

==================================================================================================================================
                                             UPPER YELLOWSTONE & MADISON RIVER BASINS
                                               Streamflow Forecasts - January 1, 2000
==================================================================================================================================
                                              |  <<====== 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       340       447    |      520        66    |       593       700            792
                                                                    |                       |
YELLOWSTONE RIVER at Corwin Spgs.    APR-SEP       948      1166    |     1315        68    |      1464      1682           1937
                                                                    |                       |
YELLOWSTONE RIVER near Livingston    APR-SEP      1041      1323    |     1515        68    |      1707      1989           2241
                                                                    |                       |
HEBGEN Reservoir Inflow              APR-SEP       315       374    |      415        85    |       456       515            486
                                                                    |                       |
==================================================================================================================================
                UPPER YELLOWSTONE & MADISON RIVER BASINS                |        UPPER YELLOWSTONE & MADISON RIVER BASINS
             Reservoir Storage (1000 AF) - End of December              |        Watershed Snowpack Analysis - January 1, 2000
==================================================================================================================================
                                      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     33.6     33.7 |   MADISON RIVER in WY         9        56        73
                                                                        |
HEBGEN LAKE                            377.5    344.8    331.5    246.8 |   YELLOWSTONE RIVER in WY    11        62        75
                                                                        |
==================================================================================================================================
 * 90%, 70%, 30%, and 10% chances of exceeding are the probabilities that the actual volume will exceed the volumes in the 
table.

The average is computed for the 1961-1990 base period.

(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.
(3) - Forecasts produced in cooperation with Alberta Forecasting Staff Canada


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