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.