Initial conditions specify water quality concentrations used at the start of the simulation. Enough information must be provided to set concentration values for each simulated constituent in all water quality cells.
Reservoir Initial Conditions
For reservoirs, table data or DSS paired data records are used to specify constituent concentrations at given elevations or depths. These profiles are typically derived from field measurements taken with a water quality sonde, grab samples at discrete depths, or continuous monitoring sensors at fixed elevations.
Reservoir initial condition data is linearly interpolated using the given points to the elevation of the center of each water quality layer. Elevations outside the range of the user-provided data are extrapolated using the value of the closest point.
Reach Initial Conditions
Initial condition data for reaches are provided at the junction locations. As with reservoir initial conditions, linear interpolation is used to determine initial cell concentrations from junction values, based on distance from the junctions. When a junction is also an inflowing boundary condition location, the initial condition value is auto-populated based on the boundary condition value at the start time for the simulation; this value can be overridden by the user, if necessary.
For the typical time and space scales that are modeled with HEC-ResSim, reach initial conditions typically wash out within the first few days of the simulation. Reservoir initial conditions—particularly for very large reservoirs with low flushing times—may persist and influence the water quality profile for months or even years.