Water and Climate Program
Products and Services Briefing Book

Water Supply Forecasting

Water supply forecasting is the science and art of predicting the volume of water that will flow past a given point on a stream during a specific period of time. The Natural Resources Conservation Service (NRCS), in cooperation with the National Weather Service (NWS), provides seasonal water supply forecasts for over 600 points in the Western United States. Wise use of these forecasts can provide a critical advantage to irrigators and water managers in the West.

Seasonal water supply forecasts are generated monthly from January through June for most areas. At the current time, data availability and user service generally limit the update interval to one month. In some basins, mid-month (or more frequent) forecasts are generated at the request of specific water users. The main reason forecasts are updated during the month is to account for changes in weather and hydrologic conditions.

 
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                              LOWER COLUMBIA BASIN 
                      Streamflow Forecasts - April 1, 1997
================================================================================ 
            |  <=== Drier ===  Future  Conditions  === Wetter ===>  | 
            |                                                       | 
Forecast Pt |  ============== Chance of Exceeding * ==============  | 
   Forecast |  90%       70%   | 50% (Most Prob) |  30%       10%   | 30 Yr Avg 
   Period   |(1000AF)  (1000AF)|(1000AF) (% AVG.)|(1000AF)  (1000AF)| (1000AF) 
================================================================================ 
COLUMBIA R. at The Dalles (2) 
   APR-JUL   101763   108454    113000      133    117546   124237     84760   
   APR-SEP   119900   127700    133000      134    138300   146100     98982   
 
SANDY RIVER near Marmot 
   APR-JUL      378      417       443      138       469      508       321   
   APR-SEP      446      487       515      138       543      584       373   
 
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   * 90%, 70%, 30%, and 10% chances of exceeding are the probabilities that the 
     actual flow will exceed the volumes in the table.
     

1. (1) - The values listed under the 10% and 90% Chance of Exceeding are actually 5% and 95% exceedance levels.
2. (2) - The value is natural flow - actual flow may be affected by upstream water management.

                           B A S I N    W I D E 
                      R E S E R V O I R    S U M M A R Y 
 
                        FOR THE END OF  MARCH   1997 
 
 
 BASIN AREA         CURRENT AS  LAST YR AS  AVERAGE AS  CURRENT AS  CURRENT AS 
  RESERVIOR         % CAPACITY  % CAPACITY  % CAPACITY   % AVERAGE   % LAST YR 
-------------------------------------------------------------------------------- 
 
 ARIZONA RESERVOIR STORAGE (8 RESERVOIRS)                                       
  LAKE PLEASANT             52          52           8         659         100 
  SAN CARLOS                22          50          44          51          45 
  SALT RIVER RES SYSTEM     57          68          79          72          84 
  VERDE RIVER RES SYSTEM    46          35          59          79         132 
 TOTAL OF  4 RESERVOIRS     47          57          50          93          82 
 Raw AF Totals: Current=1901.7 Last Year=2306.2 Average=2035.1 Capacity= 4055.0