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<br />Basin Outlook Reports
<br />and
<br />Federal - State - Private
<br />Cooperative Snow Surveys
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<br />For more water supply and resource management information, contact:
<br />Michael A. Gillespie
<br />Data Collection Office Supervisor
<br />USDA, Natural Resources Conservation Service
<br />655 Parlet St., Rm E200C
<br />Lakewood, CO 80215-5517
<br />Phone (720) 544-2852
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<br />How forecasts are made
<br />
<br />Most of the annual streamflow in the western United Slales originates as snowfall that has accumulated in the mountains
<br />during the winter and early spring. As the snowpack accumulates, hydrologists estimate the runoff that will occur when
<br />it melts. Measurements of snow WOller equivalent at selected manual snow courses and automated SNOTEL siles, along
<br />with precipitation, antecedent streamflow, and indices of the EI Nino I Southem Oscillation are used in computerized
<br />statistical and simulation models to prepare runoff forecasts. These forecasts arc coordinated between hydrologists in the
<br />Natural Resources Conservation Service and the National Weather Service. Unless otherwise specified, all forecasts are
<br />for flows that would occur naturally without any upstream influences.
<br />
<br />Forecasts of any kind, of course, are not perfect. Streamflow forecast uncertainty arises from three primary sources: (I)
<br />uncertain knowledge of future weather conditions, (2) uncertainty in the forecasting procedure, and (3) errors in the data.
<br />The forecast, therefore, must be interpreted not as a single value but rather as a range of values with specific probabilities
<br />ofoecurrence. The middle of the range is expressed by the 50% exceedance probability forecast, for which there is a
<br />50% chance that the actual flow will be above, and a 50% chance that the actual now will be below, this value. To
<br />describe the expected range around this 50% value, four other forecasts are provided, two smaller values (90% and 70%
<br />exceedance probability) and two larger values (30%, and 10% exceedance probability). For example, there is a 90%
<br />chance that the actual flow will be more than the 90% exceedance probability forecast. The others can be interpreted
<br />similarly.
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<br />The widcr the spread among these values, the more uncertain the forecast. As the season progresses, forecasts become
<br />more accurate, primarily because a greater portion of lhe future weather conditions become known; this is reflected by a
<br />narrowing of lhe range around the 50% exceedance probability forecast. Users should lake this uncertainty into
<br />consideration whcn making operational decisions by selecting forecasts corresponding to the level of risk they are willing
<br />to assume about the amount of watcr to be expected. ffusers anticipate receiving a lesser supply of water, or if they wish
<br />to increase their chances of having an adequate supply of water for their operations, they may want to base their decisions
<br />on the 90% or 70% exceedance probability forecasts, or something in between. On the other hand, if users are concerned
<br />about receiving too much water (for example, threat of Oooding), they may want to base their decisions on the 30% or
<br />10% exceed:mce probability forecasts, or somcthing in between. Regardless of the forecast value users choose for
<br />operations, they should be prepared to deal with either more or less water. (Users should remcmber that evcn if the 90%
<br />exceedance probability forecast is used, there is still a 10% chance of receiving less th:1n this amount.) By using the
<br />exceedance probability information, users can easily determine the chances of receiving more or less water.
<br />
<br />The United Stales Oepartmenl of Agrlcullure (USDA) prohibits disatmination in its programs aocl actlvlties on the basis of race, color, national origin,
<br />gender, religion, age, disability, pollUcat beliefs. sexual orientation and marital or family status. (Nol all prohibited bases apply 10 all programs.)
<br />Persons with disabilities who require alternative means fOf communication of program Information (Braille. large print, audIo tape, ete,) should contact
<br />USDA's TARGET Center aI202-720-2600 (voice or TOOl.
<br />
<br />To file a complaint of discrimlnallon. 'Mite USDA, Director, Office of Civil Rights, Room 326-W, Whil1en Building, 14th & Independence Avenue. SW,
<br />Washington, DC. 20250-9410, or call (202) 720-5964 (voice or TOO). USDA is an equal opportunity prOVider and emptoyer.
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