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<br />OJ1327 <br /> <br />the present modified data. If the historical data include an error <br />or mistake, only that one piece of data at the given station and time <br />frame is in error. <br /> <br />In contrast, an error in the station data will be reflected in the <br />ungaged inputs in the adjacent reaches and will normally persist for <br />some time. For example, consider the three stations: <br /> <br />UAB <br /> <br />I <br /> <br />U BC ' <br />! <br /> <br />A <br /> <br />B <br /> <br />c <br /> <br />Let the true values at A, B, and C be TA' TB' and TC' respectively. <br /> <br />Assume that the measured values at A and C are the true ones, while <br />that at B has an error, E. Thus the measured values are: <br /> <br />M A = TA' ~I B = T B + E, M C = T C <br /> <br />If the reaches are river reaches, the true ungaged inputs are: <br /> <br />UAB = TB - TA' UBC= TC - TB <br /> <br />while the computed ones are: <br /> <br />or <br /> <br />UC A B = T B + E - T A' UC B C = T C - T B - E <br /> <br />UC A B = U A B + E, UC B C = U B C - E. <br /> <br />Thus, the errors in ungaged inputs are: <br /> <br />EAB=E, EBC--E. <br /> <br />Both errors are equal in magnitude, but are opposite in sign. The <br />net error for the system is 0, but a fluctuation of plus or minus E <br />about the true value is introduced. Thus, means for the system are <br />correct, but an error-induced variation exists. <br /> <br />In routing masses in the model, mass errors will eventually be stored <br />in the system's reservoirs. As a consequence, they will persist with <br />time. For example, if Reach AB were a reservoir reach, the error EAB <br />is stored within the reservoir and without releases will not affect <br />downstream stations during the time frame, while EBC has an immediate <br />effect on downstream stations. <br /> <br />17 <br />