HydPy is an interactive framework for developing and applying different types of hydrological models, originally developed at the Ruhr-University Bochum for specific research purposes. Later it was extended on behalf of the German Federal Institute of Hydrology in order to be applicable in practical applications like runoff forecasting in large river basins. Now it is being maintained by Björnsen Consulting Engineers.

HydPy is intended to be a modern open source software, based on the programming language Python, commonly used in many scientific fields. Through using different well-established Python libraries and design principles, we target high quality and transparency standards. To avoid writing model cores in a more native programming language, HydPy includes a Cython based mechanism to automatically translate Python code to C code and to compile it.

HydPy has no graphical user interface (so far). Instead, it is thought to be applied by executing Python scripts. These scripts help to increase the reproducibility of studies performed with HydPy because they can be easily shared and repeatedly executed. This approach facilitates discussing possible weaknesses of HydPy and its implemented models and comparing different methodical approaches (e. g. different strategies to calibrate model parameters). However, if you are not an experienced hydrologist with basic programming skills, you may need some help to become acquainted with HydPy.

We host HydPy in a GitHub repository and everyone is allowed to download, modify, and use it. However, when passing the (possibly modified) code to third parties, you have to be aware you cannot change the selected GNU Lesser General Public License 3 to a "less open source" license. If you, for example, implement a model into HydPy, you can be sure that all possible further developments of your model code are still open source and the mentioned third parties are allowed to pass this modified source code to you.

HydPy offers many functionalities to make the implemented models as transparent and reliable as possible. For this reason, the online documentation is automatically updated for each new HydPy version and includes different documentation test mechanisms ensuring that HydPy is working as expected and that the complete documentation is up-to-date with it.