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<br />Water Supply
<br />Outlook
<br />
<br />Published 10lntly by the Nation-
<br />al Weather Service NOAA and
<br />the Sod Conservat Ion Service
<br />USDA fOllowing the principal
<br />snow survey dates from Janu-
<br />ary 1 through May 1,
<br />
<br />Copies of this publication may be obtained on request from
<br />National Weather Service, National Oceanic and Atmospheric
<br />Administration, Silver Spring. Maryland 20910, Attention:
<br />Office of Hydrology, and fhe Soil Conservation Service. West
<br />Technical Service Center, Room 510. 511 NW, Broadway.
<br />Portland. Oregon 97209,
<br />
<br />Some Basic Data and Stream,
<br />flow Forecasts prepared by
<br />cooperating agencies are pre-
<br />sented In this bulletin, These
<br />agencies Include the Bureau of
<br />Reclamation, Corps of Engi-
<br />neers, Forest Service. National
<br />Park Service, Geological Sur-
<br />vey, British Columbia Ministry
<br />of the Environment. and the
<br />California Department of Water
<br />Resources,
<br />
<br />Water Supply Outlook reports prepared by other agencies
<br />Include a report for California by the Snow Surveys Branch.
<br />California Department of Water Resources, P,D, Box 388.
<br />Sacramento, California 95802 - for British Columbia by the
<br />Ministry of the EnVironment, Water Investigations Branch.
<br />Parliament BUildings, Victoria. British Columbia V8V lX5 - for
<br />Yukon Territory by the Department of Indian and Northern
<br />Affairs, Northern Operations Branch, 200 Range Road. White-
<br />horse, Yukon Terntory YlA 3Vl - and for Alberta. Saskatche-
<br />wan, and N W,T, by the Water Survey of Canada, Inland Waters
<br />Branch, 110,12 Avenue SW, Calgary, Alberta nc lA6
<br />
<br />To Recipients
<br />of Water Supply
<br />Outlook Reports
<br />
<br />Most of the usable water In western states orlgrnates as mountain snowfall. This snowfall accumulates
<br />during the winter and spring. several months before the snow melts and appears as streamflow, Since
<br />the runoff from precipitation as snow is delayed. estimates of snowmelt runoff can be made well in
<br />advance of its occurrence Fall precipitation Influences the sod moisture conditions prior to formation
<br />of snowpack and explains, In part, the effectiveness of the snowpack rn producing runoff, The forecasts
<br />of natural runoff in this outlook are based principally on measurements of precipitation, snow water
<br />equivalent. and antecedent runoff, Forecasts become more accurate as more of the data affecting
<br />runoff are measured All forecasts assume that climatiC factors dUring the remainder of the snow
<br />accumulation and melt season wdl interact With a resultant average effect on runoff, Early season fore,
<br />casts are threfore subject to a greater change than those made on later dates, The report for Western
<br />United States presents a broad picture of water supply outlook conditions, including selected stream-
<br />flow forecasts, summary of snow accumulation to date. and storage In larger reservoirs,
<br />
<br />Probability Forecasts
<br />
<br />PreCipitation and snowfall accumulation of known probability as determined by analysis of past records
<br />are utilized In the preparatIOn of probability runoff forecasts, The forecasts Include an evaluation of the
<br />standard error of the predictIOn model. The forecasts are presented at three levels of probability as
<br />follows,
<br />
<br />1, Most Probable - That runoff which is expected to occur If precIpitation subsequent to the date of
<br />forecast IS median,
<br />
<br />2 Reasonable Maximum -That runoff which is expected to occur if precipitation subsequent to the
<br />date of forecast IS equal to the amount which is exceeded on the average once in ten years,
<br />
<br />3, Reasonable Minimum - That runoff which IS expected to occur if precipitation subsequent to the
<br />dates of forecast IS equal to the amount which IS exceeded on the average nine out of ten years,
<br />
<br />RUNOFF FORECASTS AT ALL POINTS ARE FOR FULL NATURAL OR UNIMPAIRED RUNOFF
<br />CORRECTED FOR EVAPORATION. UPSTREAM DIVERSIONS, AND ADJUSTED FOR OTHER HYDROLOG-
<br />IC CHANGES AS THEY ARE DEVELOPED, REFERENCE SHOULD BE MADE TO THE U,S, GEOLOGICAL
<br />SURVEY WATER SUPPLY PAPERS FOR DETAILED INFORMATION CONCERNING DIVERSIONS AND
<br />ADJUSTMENTS AT THE VARIOUS FORECAST POINTS
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