Abstract
The theory of the continuous injection method of dilution gauging requires that the stream discharge remains constant during the gauging. In this paper the effect of changing discharge is studied using a model based on the residence time of fluid elements in the gauging reach, and digital simulation shows that the systematic error caused by changing discharge is greatest at the start of storm runoff where the error, which is negative in sign, may be in excess of 60 per cent of the discharge for steeply rising hydrographs. Dilution gauging is accurate when performed on the recession limb of a hydrograph, less so when performed late on the rising limb, and most unreliable immediately after the start of a storm hydrograph.

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