The custom groundwater flow equation state variable “Theta” is not recognized by the engine. The engine looks for MS instead.

Work around:

Use “MS” instead of “Theta” in your custom groundwater flow equation.

Suggested resolution:

In the declarations for gwater.c:

// Names of GW variables that can be used in GW outflow expression
static char* GWVarWords[] = {"HGW", "HSW", "HCB", "HGS", "KS", "MC", NULL};    //(5.1.007)

“MC” and “KS” are used instead of “Theta” and “KSat” as indicated in the EPA SWMM5 GUI.

Interestingly, using “KSat” still works as the program only looks for the first two letters “KS”.