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Water Supply Outlook Published lointly by the Nation al Weather Service NOAA and the Soli Conservation Service USDA followlllg the principal snow survey dates from Janu. ary 1 through May I. Caples of this publication may be obtallled on request from National Weather SerVice, National Oceanic and Atmospheric Administration, Silver Spring, Maryland 20910, Attention: Of lice of Hydrology, and the Soli Conservation SerVice, West Technical Service Center, Room 510, 511 N W. Broadway, Portland, Oregon 97209. Some BasIc Data and Stream. flow Forecasts prepared by cooperating agencies are pre sented In thiS bulletin. These agencies include the Bureau of Reclamation, Corps of Engi- neers. Forest Service, National Park SerVice, Geological Sur- vey. British Columbia MlIlIstry of the Environment. and the California Department of Water Resources. Water Supply Outlook reports prepared by other agencies IIlclude a report for California by the Snow Surveys Branch, Calilornla Department of Water Resources, P.O. Box 388, Sacrilmento, California 95802 - for British Columbia by the Ministry of the Environment. Water Investigations Branch, Parliament Buildlllgs, VictOria, British Columbia V8V lX5 - for Yukon Territory by the Department of Indian and Northern Affairs. i~orthern Operations Branch, 200 Range Road, White- horse. Yukon Territory YlA-3VI - and for Alberta, Saskatche- wan, and NW.T. by the,Water Survey of Canada, Inland Waters Branch. 110-12 Avenue SW., Calgary, Alberta BC IA6 To Recipients of Water Supply Outlook Reports Most of the usable water in western states originates as mountalll snowfall. ThiS snowfall accumulates dUring the winter and spring, several months before the snow melts and appears as streamflow. SlIlce the runoff from precipitation as snow IS delayed, estimates of snowmelt runoH can be made well in advance of ItS occurrence. Fall preCipitation influences the soil moisture conditions prior to Im-mation of snowpack and explains, In part, the effectiveness of.t~e snowpack in producing runoff. The forecasts of natural runoff In this outlook are based principally on measurements of precipitation, snow water eqUivalent, and antecedent runoff. Forecasts become more accurate as more of the data affecting runoff are measured. All forecasts assume that climatic factors during the remalllder of the snow accumulation and melt season willlllteract with a resultant average effect on runoff. Early season fore- casts are threfore subject to a greater change than those made on later dates_ The report for Western UllIted States presents a broad picture of water supply outlook conditions, Including selected stream- flow forecasts, summary of snow accumulation to date, and storage in larger reservoirs. Probability Forecasts PrecIpitation and snowfall accumulation of known probability as determllled by analysisof past records are utilIZed in the preparation of probability runoff forecasts. The forecasts include an evaluation of the standard error of the prediction model The forecasts are presented at three levels of probability as fallows' 1. Most Probable - That runoff which IS expected to occur If precipitation subsequent to the date of forecast 1$ median. 2 Reasonable Maximum -That runoff which IS expected to occur il precipitation subsequent to the date of forecast IS equal to the amount which IS exceeded on the average once in ten years. 3. Reasonable Minimum - That runoff which IS expected to occur if preCipitation subsequent to the dates of forecast is equal to the amount which IS exceeded on the average IlIne out of ten years_ RUNOFF FORECASTS AT ALL POINTS ARE FOR FULL NATURAL OR UNIMPAIRED RUNOFF CORRECTED FOR EVAPORATION, UPSTREAM DIVERSIONS, AND ADJUSTED FOR OTHER HYDROLOG- iC CHANGES AS THEY ARE DEVELOPED. REFERENCE SHOULD BE MADE TO THE U_S. GEOLOGICAL SURVEY WATER SUPPLY PAPERS FOR DETAILED INFORMATION CONCERNING DIVERSIONS AND ADJUSTMENTS AT THE VARIOUS FORECAST POINTS.