Initial conditions define the starting values of constituents within the water quality domain at the beginning of a simulation. ResSim provides an editor for creating and defining initial condition sets. These initial condition sets are linked to both a Geometry Dataset (Chapter 4) and a WQ Constituent Set (Chapter 5).

When an initial condition set is created, the selected geometry and constituent set determine the inflow locations such as reservoirs, junctions, and reservoir outflow junctions, where initial values for simulated constituents must be defined. For reservoirs, vertically varied initial conditions can be specified. The user must provide reservoir elevation values and initial constituent values for each inflow location. These elevation values must be entered in a monotonically decreasing order to ensure that a stratified view of constituent initial values is maintained.

The prerequisites for creating an Initial Condition Set include a Geometry Dataset (Chapter 4) and a WQ Constituent Set (Chapter 5).