Page "Hydrology" Paragraph 63

* Models based on data.

These models are black box systems, using mathematical and statistical concepts to link a certain input ( for instance rainfall ) to the model output ( for instance runoff ).

Commonly used techniques are regression, transfer functions, and system identification.

The simplest of these models may be linear models, but it is common to deploy non-linear components to represent some general aspects of a catchment's response without going deeply into the real physical processes involved.

An example of such an aspect is the well-known behaviour that a catchment will respond much more quickly and strongly when it is already wet than when it is dry ..