Diffuse sources correspond to inflows to and outflows from river reaches, including inflows from groundwater and surface water (not tributaries), and outflows to groundwater and evaporation. The upstream and downstream boundaries of a QUAL2K diffuse source are the upstream and downstream distance markers of the corresponding WEAP reach. For inflows, QUAL2K needs to know the concentration of each constituent in the inflow; for outflows, the concentration is irrelevant.