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HydPy is an interactive framework for developing and applying different types ofhydrological models, originally developed for specific research purposes at theRuhr-University Bochum. Later, it was extended on behalf of theGerman FederalInstitute of Hydrology in order to apply it in practical applications like runoffforecasting in large river basins. Now, it is being maintained byBjörnsen ConsultingEngineers.

HydPy is intended to be a modern open-source software based on the programminglanguagePython, commonly used in many scientific fields. By using differentwell-establishedPython libraries and design principles, we target high standards ofquality and transparency. To avoid writing model cores in a more native programminglanguage,HydPy includes aCython-based automatism to translate Python code to Ccode and to compile it, which results in high computational efficiency. It also offersa multi-threading mode, which speeds up simulations even more by utilising a freelyselectable number of CPUs in parallel.

HydPy has no graphical user interface (so far). Instead, it is thought to be appliedby executing Python scripts. These scripts help to increase the reproducibility ofstudies performed withHydPy because they can be easily shared and repeatedlyexecuted. This approach facilitates discussing possible weaknesses ofHydPy and itsimplemented models and comparing different methodical approaches (e. g. differentstrategies to calibrate model parameters). However, if you are not an experiencedhydrologist with basic programming skills, you may need some help to become acquaintedwithHydPy.

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

HydPy offers many functionalities to make the implemented models as transparent andreliable as possible. For this reason, theonline documentation is automaticallyupdated for each newHydPy version and includes differentdocumentation testmechanisms, ensuring thatHydPy is working as expected and that the completedocumentation is up-to-date.