# 4.3.3. Volumetric Sources

The Source line applies a model for a volumetric source term for a given equation. See Governing Equations for more information on the role of sources in Aria. The general format of a source line command is as follows:

    Source for [EQUATION] on [PART] = [MODEL] [MODEL ARGS...]

Here the `EQUATION` must be one declared with an `EQ` line including a `SRC` term defined at the same scope (i.e. Equation System or Region scope, see Equation Specification).

The `PART` corresponds to a mesh entity, and can include both single parts (e.g. block_1) as well as assemblies or mesh groups (see Assemblies and Mesh Groups).

Finally the `MODEL` defines how the source term is calculated. Note that multiple source lines may be defined for a given `EQUATION` / `PART`.

For example, a constant source for the energy equation can be defined as follows

    # Scope: Sierra > Procedure > Aria Region
    EQ Energy for Temperature on block_2 using Q1 with DIFF SRC
    Source for energy on block_2 = constant value = 0.2

The reader is referred to the command reference for a comprehensive list of all possible equations and model options.