A procedure was developed to estimate the rate of longitudinal dispersion from the observed data and verified by applying it to both the narrow, partially-stratified estuary and the wide, well-mixed estuary. It was found that the dispersive flux is not proportional to the longitudinal salinity gradient (dS/dx) in estuaries and is most closely related to velocity, the advective flux and (dS/dx)2 rather than dS/dx. The results obtained from the procedure showed that negative longitudinal dispersion can occur especially in flood tide. It was investigated whether the dispersion coefficient estimated from observed salinity data can be applied to the prediction of a water quality, such as dissolved oxygen, BOD, ammonia or temperature. The observed dispersion coefficients were successfully applied in a water quality model.

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