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Last modified
7/29/2009 1:55:42 PM
Creation date
3/27/2008 12:36:28 PM
Metadata
Fields
Template:
Water Supply Protection
File Number
8040.800
Description
Water Resources for Colorado
State
CO
Date
5/1/2003
Author
US Dept of Agriculture - Natural Resoruces Conservation Service
Title
Colorado Basin Outlook Report May 1, 2003
Water Supply Pro - Doc Type
Data
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<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 Parlet SI., Rm E200C <br />Lakewood, CO 80215-5517 <br />Phone (720) 544-2852 <br /> <br />How forecasts are made <br /> <br />Most oflhc annual streamflow in the western United Slates originates as snowfalllhat has accumulated In the mountains <br />during the winter nod early spring. As the sno\\1'3ck accumu]a.tcs. hydrologists estimate the runoff that will occur when <br />it melts. Measurements of snow water equivalent at selected manual snow courses and automated SNOTEL slIes, along <br />\\ith precipitation, antecedent streamflow, and indices orlhe El Nino I Southern OscillatIOn are used In computenzed <br />'statistical and simulation models to prepare runoff forecasts. These forecasts are coordmatcd between hydrologists III the <br />Natural Resources Conservation Service and Ihe National Weather Service. Unless otherwise speCified, all forecasts are <br />for flows that would occur naturally \I/1thout any upstream influences. <br /> <br />Forecasts of any kind, of course, are not perfect. Streamllow forecast uncertainty arises from three primary sources: (I) <br />uncertain knowledge of fUlUre 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 forecast, for which there IS a <br />50% chance that the actual flow will be above, and a 50% chance that the actual flow will be below, this value. To <br />describe the ex~cted range around this 50% value, four other forecasts are provided, two smaller values (90% and 70% <br />excecdance probability) and two larger values (30%, and 10% exccedance probability). For example, there is a 90% <br />chancc that the actual flow will be more than the 90% excecdance probability forecast. The others c~m 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 becomc 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 />10 assume about the amount of water to be expected. If users anticipate receIving a lesser supply of water, or If they wish <br />10 increase their chances of having an adequate supply of water for their operations, they may want 10 base their decisions <br />on the 90% or 70% cltceedance: probability forecasts, or something in between. On the other hand, if users arc concerned <br />about receiving too much water (for example, threat of llooding), they may want to base their decisions on the 30% or <br />10% exceedance probability forecasts, or something in between. Regardless of the: forecast value: use:rs choose for <br />operations. they should be prepared to deal with either more or less water. (Users should remember thaL even if the 90% <br />exceedance probability forecast is used, there is still a 10% chance of receiving less than this amount.) By usmg the <br />excccdance probability information, users can easily determine the chances of receiving more: or less waler. <br /> <br />The United States Departm8flt 01 Agriculture (USDA) prohibits discrimination in its programs and actiVIties on the basls 01 race, color. nabonal OligIn, <br />gender, re~gion. age, dlS<lbillly, political beHefs, sexual orientation and marital or family status. (Not all prohibiled bases apply to all PfDgfams.) <br />Persons with disabililles who require alternative means for communication of program inlormaUon (Braille. large print, audio tape, etc.) should conlact <br />USDA's TARGET Center at 202-72{l..2600 (voice Of TOO). <br /> <br />To file a complainl 01 discrimination. wrile USDA. Director, Office of Civil Rights. Room 326-W, Whitlen Building. 141h & Independence Avenue, SW, <br />Washington. DC, 20250-9410. or call (202) 72{l..S964 (voice Of TOO). USDA is an eQual opportunity provider and employer. <br />
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