If there is no negative discharge, the network is calculated in the upstream-downstream "topological" direction as given by Talweg.

At a node, the concentrations $C_{p}$ and $C_{b}$ of incoming offtakes and reaches are known and give $C_{nd}$ and $C_{b}$ for outgoing offtakes and reaches using the node mass balance equation :

$C_{b}=C_{\mathit{nd}}k_{b}$ if the distribution coefficient is defined as fix
$C_{b}=C_{\mathit{nd}}k_{b}k_{a}$ if it is defined as adjustable.

Negative discharge in reaches are not yet implemented in the quality module.