Rien's Many Contributions to Numerical Modeling of Water Flow and Solute Transport in Soils.

In this presentation, I will review Dr. van Genuchten’s many contributions to numerical modeling of variably-saturated water flow and solute transport in soils. Rien’s contributions do not end with developing some of the most widely used models (e.g., SUMATRA-1, WORM, UNSAT1, HYDRUS) that involve many highly innovative approaches (e.g., Hermitian cubic finite elements schemes in the SUMATRA-1 code or higher-order temporal and spatial approximations), he has also always understood the importance of public availability of these codes, their long-term user support, and especially user training (teaching some fifty short courses on five continents, attended by almost a thousand students, researchers, or practitioners). He has also dedicated a lot of effort to developing tools needed to obtain parameters for these numerical models. Examples are RETC (earlier SOHYP) and CXTFIT programs for obtaining soil hydraulic and solute transport parameters, respectively. His long term support of colleagues working on these topics is highly appreciated.