Fitting functions of soil hydraulic properties
SWRC Fit is a web interface which uses unsatfit and determines parameters for water retention function. As it is easy to use, it has been used in many papers.
Source code is in the repository.
You can run SWRC Fit locally on your machine by using Docker, which might be useful when the server is down or you have unstable network connection. Follow the instructions here.
GNU Octave version of SWRC Fit is no longer maintained but the code is available.
hystfit is a Python library for calculating hysteresis in water retention curve. It is implemented as a subclass of unsatfit.