Hydrologic models are simplified, conceptual representations of a part of the hydrologic cycle. They are primarily used for hydrologic prediction and for understanding hydrologic processes. Two major types of hydrologic models can be distinguished: Stochastic Models. These models are black box systems, based on data and using mathematical and statistical concepts to link a certain input to the model output. Commonly used techniques are regression, transfer functions, neural networks and system identification. These models are known as stochastic hydrology models.