Dispersion model for mountain streams

Major differences have often been observed between experimental measurement- s and solutions of the one-dimensional dispersion equation for the spreading of tracers in rivers. This paper compares calculated values of peak concentration decay rates, variances, and concentration distributio- ns for three different one-dimensional models with experimental results obtained by T. J. Day. The calculations show that the Fickian and dead-zone models have similar behaviors for peak decay rates and variances, but that these behaviors do not agree with Day's measurements. The third model, in which the dispersion coefficient increases linearly with distance downstream, gives results that agree closely with Day's measuremen- ts. It is suggested that the increase in the dispersion coefficient with distance downstream may be the result of a relatively large amount of dispersion from velocity shear near the leading and trailing edges of the tracer cloud.