  In this paper we present a massively parallel open source solver for Richards
equation, named the RichardsFOAM solver. This solver has been developed in the
framework of the open source generalist computational fluid dynamics tool box
OpenFOAM (R) and is capable to deal with large scale problems in both space and
time. The source code for RichardsFOAM may be downloaded from the CPC program
library website.It exhibits good parallel performances (up to $\sim$90%
parallel efficiency with 1024 processors both in strong and weak scaling), and
the conditions required for obtaining such performances are analysed and
discussed. These performances enable the mechanistic modelling of water fluxes
at the scale of experimental watersheds (up to few square kilometres of surface
area), and on time scales of decades to a century. Such a solver can be useful
in various applications, such as environmental engineering for long term
transport of pollutants in soils, water engineering for assessing the impact of
land settlement on water resources, or in the study of weathering processes on
the watersheds.
