|
<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 originates as snowfall that has accumulated In the mountains
<br />during the winter and early spring. As the snO\vpack accumulates, hydrologists estimate the runoff that will occur when
<br />it melts. Measurements of snow wDtcr equivalent at selected manual snow courses and automated SNOTEL siles, along
<br />. with precipitation, antecedent streamflow, and indices of the El Nino I Southern Oscillation are used In computerized
<br />statistical and simulation models to prepare runoff forecasts. These forecasts are coordinated bC'tween hydrologIsts In the
<br />~ Natural Resources Conservation Ser'llce 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 />of occurrence. The middle of the range is expressed by the 50% exceedance probability forecnst, for which there is a
<br />50% chance thatlhe actual flow will be above, and a 50% chance that the actual flow will be below, this value. To
<br />descnbe the expecled range around this 50% 'lalue, 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 thaI the actualllow 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 reflected by a
<br />narrowing of the range around the 50% exceedance probability forecast. Users should take this uncertainty into
<br />consideration when making operational decisions by selecting forecasts corresponding to the level of risk they are willing
<br />to assume about the amount of water to be expected. If users anticipate receiving a lesser supply ofwaler, or if they Wish
<br />to increase their chances of having an adequate supply of water for their operations, they may want to basc thclr deCISIOns
<br />on the 90% or 70% exceednnce probability forecasts, or something in between. On the other hand, If users are concerned
<br />about receiving too much water (for example, threat of floodmg). they may want to base their decisions on the 30% or
<br />10% exceedancc probability forecasts, or something 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 remember that even if the 90%
<br />exceedance probability forecast is used, there is still a 10% chance of receiving less than Ihis amount.) By using the
<br />exceedance probability mfonnation, users can easily detennine the cmnces of receiving more or less water.
<br />
<br />The Uniled Stales Department 01 Agria.llture (USDA) prohibits discriminallon In lis programs and activities 00 !he basis of race, color, natiOnal origin,
<br />gender, religion, age, disability. political beliefs, S61l!S31 orientation and marital or family status. (Not all prohibiled bases apply 10 all programs.)
<br />Persons wHh disabililies who require alternative means for communication 01 program Inlormation (Braille, large prinl. audio tape, elc.) should contact
<br />USDA's TARGET CooISf at 202.720-2600 (voice or TOO).
<br />
<br />To file a complaint 01 dIscrimination, Wfll& USDA. Director, Office of Civil Rights, Room 326-W. Whiltsn Building, 14th & Independence Avenue, SW.
<br />Washinglon, DC, 202~9410, or call (202) 72()..59l)4 (voice or TOO). USDA is an equal opportunity provider and employer.
<br />
|