An engineer developing application specific simulation and optimization systems needs appropriate and handy tools for creating the right models. The toolbox “WaterLib” for the block-oriented, dynamic simulation system Simulink of the numerical computing environment Matlab is such a tool. It consists of the most important and commonly used modules for the construction of simulation models suitable for controller design and decision support systems. These modules where developed using the algorithms described in Chaps. 2 and 3. The first toolbox version started with simulation objects for run-of-river reservoirs and river sections , the second edition was enhanced with models for complex surface water systems and an interface to the finite element groundwater simulator FeFlow\(\circledR \).