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<br />
<br />Basin Outlook Reports
<br />and
<br />Federal - State - Private
<br />Cooperative Snow Surveys
<br />
<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 Partet St., Rm E200C
<br />lakewood, CO 80215-5517
<br />Phone (720) 544-2852
<br />
<br />How forecasts are made
<br />
<br />Mo!'t of the annual slrcamllow in the western United States 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 water equivalent at selected manual snow courses and automatcd SNOTEL sites, along
<br />with precipitation, antecedent streamflow, and indices of the EI Nino/ Southern Oscillation arc uscd 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 othcf\\fise 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: (1)
<br />uncertain knowlt:dge 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 />of occurrence. The middle of the rangc is expressed by the 50% cxcccdance probability forecast, for which there is a
<br />50% chance that the actual flow \vill be above, and a 50% chance that the actual flow 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 />cxcecdance probability) and two larger values (30%, and 10% exceedance probability). For example, there is a 90%
<br />chance that the actualllow will be morc than the 90010 exceedance probability forecast. Thc others can be interpreted
<br />similarly.
<br />
<br />The wider the spread among these values, the more uncertain the forecast. As the season progresses, forecast.. become
<br />more accurale, primarily because a greater portion of the future weather conditions become known; this is renectcd by a
<br />narrowing of the range around the 50% exceedance probability forecast. Users should take this uncertainty into
<br />cunsideration when making operational decisions by selecting forecasts corresponding to the level of risk they are willing
<br />10 assume about the amount of water to be expected. If users anticipate receiving a Icsser 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 Ihe other hand, if users are concerned
<br />aboul receiving too much water (for example, threat of flooding), they may want to base their decisions on the 30% or
<br />10% cxcecdanee probability forecasts, or something in bet'....ecn. Regardless of the forecast value uscrs choose for
<br />operations, (hey should be prepared to deal with either morc or less waler. (Users should remember that even if the 90%
<br />exceedance probability forecast is used, there is still a 10% chance of receiving less than this amount.) By using the
<br />exccedance probability information, users can easily determine the chances ofrecei....ing more or less water.
<br />
<br />TliD Unilod Stales Dupartmenl ot AgricurtufD (USDA) prohibrls dlsaimination In Us pfOgrams and activities on the basis 01 race, ooIor, na~1 Origin,
<br />gcndef, religion. age, disability, political beliefs, sexual orientation and marital Of family stalus. (Not all prohibiled bases apply to all programs.)
<br />Persons wilh disabilities who requlre alternaUve means lor communication of program InformaUon (Braille, large prinl. audiO tape, ele.) should contad
<br />USDA.s TARGET Cenler at 202.720-2600 (voice or TOO).
<br />
<br />To rile a complaint 01 discrlmination, wrtle USDA, Director, Office of Civil Rights, Room 326-W, Whi"en Building, 14th & Independence Avenue, SW,
<br />Washington, DC, 20250-9410, or can (202) 720-5964 (voice or TOO). USDA Is an equal opportunity provider and employer.
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