The systematic effect of streambed conductivity heterogeneity on hyporheic flux and residence time.

Abstract A systematic understanding of hyporheic flux (HF) and residence times (RT) is important as they are a major control of biogeochemical processing in streambeds. Previous studies addressing the effect of heterogeneity in streambed hydraulic conductivity (K) on HF and RT have come to deviating conclusions depending on the specific study design and the selection of heterogeneity cases being investigated. To more systematically evaluate the effects of conductivity heterogeneity on HF and RT, we simulated hyporheic exchange induced by idealized streambed ripples over a large range of heterogeneities. Conductivity heterogeneity was represented in the simulations in terms of 10,000 different heterogeneity realizations from a geostatistical model based on continuous Gaussian and discrete indicator random fields. We demonstrate that any isotropic homogeneous K-field, as an average of a heterogeneous K-field, can only match RT or HF of the respective heterogeneous K-field, but never both. We found exponential correlations of RT and HF with the variance of heterogeneous conductivity. Based on these correlations, an equivalent anisotropic homogeneous conductivity tensor K can be derived. This equivalent anisotropic K efficiently accounts for the effects of small scale heterogeneity on HF and RT. It can be calculated from the median and variance of the hydraulic conductivity distribution of the targeted heterogeneous sediment, without explicitly characterizing the sediment texture. [ABSTRACT FROM AUTHOR]