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1. If zero flow is a defined point (Ho, Qo) on the Rating Curve then:
If specified Hm is less than Ho then set Hm=Ho
If Ho>0.0 or Ho=min(Hf) then:
Sf=0.0 and Hs=Ho
Wiki Markup
Sf=0.01-Ho and Hs=min\[[min(Hf),Hp|min(Hf),Hp]\]
where Hp is the smallest positive stage value ]
where Hp is the smallest positive stage value defined
2. If the zero discharge point is not included in the Rating Curve definition then:
a. If Hm and the stage at the first defined point on the Rating Curve (H1) are both greater than zero then:
Sf=0.0 and Hs=Ho
where Ho is computed by using Qo=0.0001 CMS
b. If Hm or H1 <= 0.0 then:
Sf=0.01-minHm,H1 and Hs=minmin(Hf),Hp
c. If Hm is less than the computed Ho then:
Hm=Ho
The shift factor will never be applied to the same range of stages to which an offset is applied. The shift factor is also only applied to the portion of the Rating Curve that needs to be shifted to avoid zero or negative values in logarithmic calculations. The shifted portion of a Rating Curve most likely will not plot as a straight line on logarithmic paper. Therefore it is better off to specify offset factors to insure that reasonable results are obtained during
extrapolation at the low end of the Rating Curve rather than to rely on the automatic shift procedure.
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