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<br />.. <br />I <br /> <br />I <br /> <br />Instantaneous discharge can usually be related to correlative data, <br /> <br />such as the water-surface elevation, water-surface slope, or head and <br /> <br />gate openings in the case of co~~rol structures. The relation between <br /> <br />instantaneous discharge and the correlative data, known as a discharge <br /> <br />rating, is not necessarily constant throughout the life of a stream gaging <br /> <br />station. In standard stream gaging procedures, temporary shifts in the <br /> <br />discharge rating are handled by adjusting the correlative data, while <br /> <br />more persistent shifts are treated by redefinition of the discharge <br /> <br /> <br />rating. In the long run, shifts of either a temporary or relatively <br /> <br /> <br />persistent nature may fluctuate about a more or less steady-state dis- <br /> <br /> <br />charge relation or the discharge relation may have a definite non-stationarity <br /> <br />with respect to time. The latter case is frequently exhibited in alluvial <br /> <br />reaches of a stream downstream from a recently constructed reservoir. <br /> <br />Impoundment of the sediments ,carried by the natural stream causes degradation <br /> <br />of the stream's channel downstream from the reservoir and a concurrent shift <br /> <br />in the relation of discharge to water-surface elevation as the channel erodes. <br /> <br />In such a case the most stable discharge relation may be a function of both <br /> <br />the correlative data and time. <br /> <br />Even though the correlative data and time can be measured much more <br /> <br />precisely than can instantaneous discharge itself, discharge relations <br /> <br /> <br />are not exact. Let the difference between qT(t) and rated discharge, qR(t), <br /> <br /> <br />based on the correlative data and possibly on time, be denoted =2(t). <br /> <br /> <br />Thus measured discharge can be expressed <br /> <br />qM(tJ = qT(tJ + v(tJ = qR(tJ - =2(tJ + v(tJ (2) <br />A typical form of discharge rating is <br />~3 <br />qR(tJ = Co + cl (h(tJ + c2J (3) <br />18 <br /> <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br />I <br /> <br />I <br /> <br />I <br /> <br /> <br />I <br /> <br />I <br /> <br />L, <br /> <br />I <br />