Reply to Comment by Sookhak Lari, K. and Davis, G. B. on “ ‘Large Eddy Simulation of Turbulence and Solute Transport in a Forested Headwater Stream’: Invalid Representation of Scalar Transport by the Act of Diffusion”

Based on the molecular diffusion coefficient used in the large eddy simulation (LES) (equation (3) of the article), one can easily infer that a molecular Schmidt number of 700 was adopted for the solute. It should be acknowledged that in the published article we omitted to include the value used for the molecular diffusion and did not mention that is the reciprocal of the turbulent Schmidt number. Although these details should have been included, these omissions do not change well-established facts that in turbulent flows the mixing of the solute by the molecular diffusion is many orders of magnitude less important than that induced by the turbulence. As we mentioned in our article, the viscous sublayer in the Eagle Creek stream was not resolved in our simulations as the first node off the solid boundaries in wall units was (on average) equal to 45 (see Table 3 of the article). This implies that all of the computational nodes are located well outside of the viscous sublayer, where molecular diffusion becomes dominant. Moreover, since we carry out LES, we do not resolve the Kolmogorov scales in the flow the rate of dissipation of which is governed by molecular viscosity. Rather, the subgrid scale eddy viscosity of the LES acts as the model for the effect of the unresolved scales of motion. Even though all these are well-known facts from fundamental theory of turbulent mixing, we include in this response additional simulations to address the concerns raised in this commentary.