|
<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 />Most of the annual streamflow in the western United States originales as snowfalllhal has accumulated in the mountains
<br />during the winter and early spring. As the snowpack accumulates, hydrologists estimate the runoff thnt will occur when
<br />it melts. Measurements of snow water equivalent at selected manu31 snow courses and automated SNOTEL sites, along
<br />with precipitation, antecedent sueamllow, and indices of the El Nino I Southern Oscillation arc 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 /lows that would occur naturally without any upstream influences.
<br />
<br />Forecasts of any kind, of course, are not perfect. Streamllow forecast uncertainty arises from three primary sources: (1)
<br />uncertain knowledge of future weather conditions, (2) uncertainty m the forecastmg procedure, and (3) errors In the data.
<br />The forecast, therefore, must be interpreted not as a single value but rather as a mnge of values with specific probabilities
<br />of occurrence. The middle of the range is expressed by the 50% exccedance 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 .....ill be below, this value. To
<br />describe the expected range around this 50% value, four other forecasts arc 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.
<br />
<br />The wider 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 the future weather conditions become known; this is re/lected by a
<br />narrowing of the range around the 50% excecdancc probability forecast. Users should take this uncertainty mto
<br />consideration when making operational decisions by selecting forecasts corresponding to the level of risk they arc wil1mg
<br />10 assume about the amount of water to be expected. Ifusers anticipate receiving a lesser supply ofwster, or if they wish
<br />to increase their chances of having an adequate supply ofwaler for their operations, they may \\lant to base their decisions
<br />on the 90% or 70% exceedance probabiIJty forecasts, or something in between. On the other hand, if users are concerned
<br />about receiving too much water (for example, threat of lloodingl, they may want to base their dccisions on the 30% or
<br />10% exceedance probability forecasts, or something in between. Regardless orlhe forecast value users choose for
<br />operalions, they should be prepared to deal with either more or less water. (Users should remember that even if the 90%
<br />exccedance probability forecast is used, there is still a 10% chance of receiving less than this amount.) By using the
<br />excecdance probability information, users can easily determine the chances of receiving more or less water.
<br />
<br />The Untied States Oepartm(lflt of Agriculture (USDA) prohibits dlscrimlnalion In ils programs and actlvilies on the basis of race, color, national origin,
<br />gender, religion, age, disabllity, political beliefs, sexual orientation and marital or family status. (Not all prohibited bases apply to all programs.)
<br />Persons with disabllities who require alternative means for communication of program information (Braille, laroe print, audio tape, elc.) should contact
<br />USDA's TARGET Center at 202.720-2600 (volce or TOOl.
<br />
<br />To tile a complaint of discrimination, wnte USDA, Director, Office of Civil Righls, Room 326-W, Whitten Building. 14th & Independence Avenue. SW.
<br />Washington. DC, 20250-9410, or call (202) 720-5964 (voice 01'100). USDA Is an equal opportunity provider and employer.
<br />
|