Belle Fourche and Cheyenne River Basins (7)
May 1998

Snow
As of May 1, snow water equivalent (SWE) in the Black Hills is 51 percent of average. SWE in the Belle Fourche and Cheyenne River basins is 27 percent of last year. Measured snow depths ranged from 0 inches at Bearlodge Divide to 10 inches at Reuter Canyon. See Basin summary of Snow Course Data at the beginning of this report for a detailed listing.

Precipitation
Precipitation, for the month of April was 51 percent of average in the Black Hills; April precipitation was 47 percent of the same month last year. Of the four reporting sites in the basin, Warren Peak had the highest percentage of average for April (78 percent of average). The site with the lowest percentage of average was Blind Park (32 percent of average for the month). Year-to-date precipitation is 98 percent of average and 68 percent of last year's amount.

Reservoir
Reservoir storage is in good shape. Angostura is currently storing 106 percent of average (120,700 acre feet). Belle Fourche reservoir is storing 126 percent of average (184,100 acre feet). Deerfield reservoir is storing 107 percent of average (14,600 acre feet). Keyhole reservoir is storing 165 percent of average (181,000 acre feet). Pactola reservoir is storing 112 percent of average (53,800 acre feet), and Shadehill reservoir is storing 111 percent of average (72,300 acre feet).

Streamflow
Black Hills water users should see below average runoff during the May through July forecast period. The 50 percent chance runoff for Castle Creek below Deerfield Dam is 90 percent of average (2,700 acre feet). The 50 percent chance runoff for Rapid Creek below Pactola Dam should be 11,500 acre feet (76 percent of average). Drainage's relying on direct diversion for their water supply could be experience irrigation water shortages 

==================================================================================================================================
                                              BELLE FOURCHE & CHEYENNE RIVER BASINS
                                               Streamflow Forecasts - May 1, 1998
==================================================================================================================================
                                              |  <<====== 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)
====================================================================|=======================|=====================================
CASTLE CREEK blw Deerfield Dam (2)   MAY-JUL      1.68      2.29    |     2.70        90    |      3.11      3.72           3.00
                                                                    |                       |
RAPID CREEK blw Pactola Dam (2)      MAY-JUL       5.0       8.8    |     11.4        76    |      14.0      17.8           15.1
                                                                    |                       |
==================================================================================================================================
                  BELLE FOURCHE & CHEYENNE RIVER BASINS                 |          BELLE FOURCHE & CHEYENNE RIVER BASINS
             Reservoir Storage (1000 AF) - End of April                 |        Watershed Snowpack Analysis - May 1, 1998
==================================================================================================================================
                                      Usable |  *** Usable Storage ***  |                            Number     This Year as % of
Reservoir                            Capacity|  This      Last          |   Watershed                  of       =================
                                             |  Year      Year     Avg  |                          Data Sites   Last Yr   Average
========================================================================|=========================================================
ANGOSTURA                              122.1    120.7    121.0    113.7 |   BELLE FOURCHE               5        25        51
                                                                        |
BELLE FOURCHE                          178.4    184.1    178.4    145.7 |
                                                                        |
DEERFIELD                               15.2     14.6     14.7     13.6 |
                                                                        |
KEYHOLE                                193.8    181.0    186.6    109.6 |
                                                                        |
PACTOLA                                 55.0     53.8     55.6     47.9 |
                                                                        |
SHADEHILL                               81.4     72.3     77.6     65.2 |
                                                                        |
==================================================================================================================================
 * 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.
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