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Water Supply Outlook Published Jomtly by the Nation. al Weather Service NOAA and the Soil Conservation Service USDA followmg the principal snow survey dates from Janu ary 1 through May 1 0;' ,'. Copies of this publication may be obtained on request from National Weather Service, National Oceanic and Atmospheric Administration, Silver Spring, Maryland 20910, Attention: Office of Hydrology, and the Soil Conservation Service, West Technical Service Center, Room 510, 511 NW. Broadway, Portland. Oregon 97209_ Some Basic Data and Stream- flow Forecasts prepared by cooperating agencies are pre- sented In thiS bulletm. These agencies mclude the Bureau of Reclamation_ Corps of Engi- neers. Forest Service_ National Park Service_ Geological Sur- vey. British Columbia Ministry of the Environment, and the California Department of Water Resou rces. Water Supply Outlook reports prepared by other agencies mclude a report for Calitornia by the Snow Surveys Branch, California Department of Water Resources, P.O_ Box 388, Sacramento_ California 95802 - for British Columbia by the Mmlstry of the Environment, Water Investigations Branch, Parliament Buildmgs, Victoria, British Columbia V8V IX5 - for Yukon Temtory by the Department of Indian and Northern Affairs_ Northern Operations Branch, 200 Range Road_ White- horse. Yukon Territory YIA 3Vl - and for Alberta, Saskatche- wan. and NW.T. by the Water Survey of Canada, Inland Waters Branch. 110-12 Avenue SW, Calgary, Alberta BC lA6_ To Recipients of Water Supply Outlook Reports Most of the usable water m western states originates as mountain snowfall. This snowfall accumulates during the winter and sprmg_ several months before the snow melts and appears as streamflow_ Since the runott from precipitation as snow IS delayed, estimates of snowmelt runoff can be made well in advance of its occurrence Fall precipitation influences the soil moisture conditions prior to formation ot snowpack and explains, m part, the effectiveness of the 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 runoft are measured All forecasts assume that climatiC factors during the remainder of the snow accumulation and melt season Will interact 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 United 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 determined by analysis of past records are utilized in the preparation of probability runoff forecasts. The forecasts include an evaluation ofthe standard error of the prediction model. The forecasts are presented at three levels of probability as follows I. Most Probable - That runoff which is expected to occur If precipitation subsequent to the date of forecast IS median. 2. Reasonable Maximum - That runoff which is expected to occur if precipitation subsequent to the date ot torecast 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 nine out of ten years_ RUNOFF FORECASTS AT ALL POINTS ARE FOR FULL NATURAL OR UNIMPAIRED RUNOFF CORRECTED FOR EVAPORA TION. 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