Predefined functions perform a wide range of calculations common to water management policy and its expression in a rule context. Some predefined functions provide rules with access to the same algorithms used in RiverWare simulation for mass balance calculations, flow/volume conversions, and table lookups. Other predefined functions are available for common mathematical operations, date/time manipulations, topographical evaluations, and some specialized river basin management calculations.

Predefined functions are used in RPL sets in the same way as user-defined custom functions. They are selected from the palette, and inserted into a rule, internal function, or other expression. Each predefined function is an expression that evaluates to one of the seven rules data types:

Predefined functions may or may not have arguments. The computational algorithms and arguments to predefined functions may not be modified.