5. CONCLUSIONS
The hyporheic zone RTD associated with bedform pumping across dunes is well represented by one or more analytical distributions, depending on the dimensionless streambed depth. From Kolmogorov-Smirnov ranking of the distributions we find that the empirical RTD is well represented by a Gamma distribution for shallow streambeds (\(d_{b}^{*}\leq 1.2\)), a Log Normal distribution for intermediate streambed depths (\({1.2<d}_{b}^{*}\leq 3.1\)); and the Fréchet distribution for very deep streambeds (\(d_{b}^{*}>3.1\)). Values of the distribution parameters are a function of the dimensionless streambed depth\(d_{b}^{*}\), and can be predicted to a high degree of accuracy using simple regression formulae. The results presented in this study should prove useful for modelling hyporheic exchange and assessing nutrient processing in the hyporheic zone. Even though the FR cannot capture the whole shape of the RTD in shallow beds, it still accurately represents the median residence time, \(t_{50}\). Therefore, the optimized FR distribution is best able to represent Damköhler Numbers estimated from the empirical RTD over the > 2 order of magnitude range of streambed depths evaluated in this study.