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<br />002191
<br />
<br />Water Supply
<br />Outlook
<br />
<br />Published jOintly by the Nation,
<br />al Weather Service NOAA and
<br />the Soil Conservation Service
<br />USDA tolloWlllg the principal
<br />snow survey dates from Janu,
<br />ary 1 through May I
<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 />Oft ice of Hydrology, and the Sod Conservation Service, West
<br />Technical Service Center, Room 510, 511 N,W, 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 ICclude the Bureau of
<br />Reclamation, Corps of Engi-
<br />neers, Forest Service, National
<br />Park Service, Geological Sur
<br />vey, British Columbia Ministry
<br />01 the Environment. and the
<br />Calltornla Department at Water
<br />ResoLl rces.
<br />
<br />Water Supply Outlook reports prepared by other agencies
<br />Include a report for Callforma by the Snow Surveys Branch,
<br />Calitornla Department of Water Resources, P,O, 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 IX5 - for
<br />Yukon Temtory by the Department of Indian and Northern
<br />Aftalrs, Northern Operations Branch, 200 Range Road White'
<br />horse, Yukon Territory Y1A 3Vl - and for Alberta, Saskatche-
<br />wan, and N W,T, by the Water Survey of Canada, Inland Waters
<br />Branch, 110-12 Avenue S W" Calgary, Alberta nc IA6,
<br />
<br />To Recipients
<br />of Water Supply
<br />Outlook Reports
<br />
<br />Most of the usable water In western states originates 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 tram precipitation as snow IS delayed, estimates of snowmelt runoff can be made well In
<br />advance of ItS occurrence, Fall preCipitation Influences the sOil mOisture conditions prior to formation
<br />of snow pack and explains, in part, the effectiveness of the snowpack in 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 accurafe 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 will Interact with a resultant average effect on runoff, Early season fore-
<br />casts are thretore subject to a greater change than those made on later dates, The report tor 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 reserVOllS,
<br />
<br />Probability Forecasts
<br />
<br />PreCipitation and snowfall accumulation ot known probability as determined by analysis of past records
<br />are utili zed in t he 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 ot 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 runolf which is expected to occur If precipitation subsequent to the
<br />date of forecast IS equal 10 Ihe 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 WA TER SUPPLY PAPERS FOR DETAILED INFORMATION CONCERNING DIVERSIONS AND
<br />ADJUSTMENTS AT THE VARIOUS FORECAST POINTS,
<br />
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