Martinus Theodorus van Genuchten

Early career in the United States

Near the completion of his doctorate, van Genuchten joined the Department of Civil Engineering at Princeton University, where he worked from 1975 to 1978. His research there included numerical solutions for water flow and contaminant transport in variably saturated porous media.^[6]

At Princeton he worked with researchers including George F. Pinder and William G. Gray. Among the problems he investigated was how to represent unsaturated soil hydraulic properties using continuous analytical functions rather than interpolated experimental data.^[6]

In 1978 van Genuchten moved to Riverside, California. He subsequently spent about three decades associated with the U.S. Salinity Laboratory of the Agricultural Research Service of the United States Department of Agriculture, while also having an academic association with the University of California, Riverside.^[2][1]

His research in Riverside included unsaturated water flow, salinity, contaminant transport, soil hydraulic properties, inverse parameter estimation and mathematical modeling. He later served as a supervisory soil scientist and research leader at the Salinity Laboratory.^[7]

The Riverside period produced much of van Genuchten's best-known theoretical and computational work, including his soil hydraulic equations, studies of nonequilibrium transport and collaborations leading to a series of analytical and numerical computer programs.^[2]

International appointments and later career

Van Genuchten's career also involved research appointments and collaborations outside the United States. He spent sabbatical periods in Avignon, France, in 1985–1986 and in Zurich, Switzerland, in 1995–1996, as well as shorter periods working with researchers in other countries.^[6]

He left the U.S. Salinity Laboratory in 2008 and subsequently developed long-term affiliations with the Federal University of Rio de Janeiro (UFRJ) in Brazil and Utrecht University in the Netherlands.^[6]

At UFRJ, van Genuchten began collaborating with COPPE in 2008, initially through the Mechanical Engineering Program. In 2017 he became associated with the Nuclear Engineering Program, working on applications of porous-media hydrology to environmental and engineering problems.^[8]

His research collaborations at UFRJ have included studies of radioactive materials, contaminant transport, waste disposal and flow through carbonate rocks.^[6][7]

Van Genuchten also became affiliated with the Department of Earth Sciences at Utrecht University. His work there has included collaborations in hydrogeology, pore-scale processes, fluid flow and solute transport. Utrecht University lists him as a visiting researcher in its Environmental Hydrogeology group.^[9]

At the time he received the 2023 Wolf Prize in Agriculture, van Genuchten was serving as a professor in UFRJ's Department of Nuclear Engineering. FAPERJ identified him in February 2023 as a professor of that department, while the Wolf Foundation formally listed "The Federal University of Rio de Janeiro, Brazil" as his "affiliation at the time of the award".^[10][5]

His later publications have continued to list affiliations with both UFRJ's Department of Nuclear Engineering and Utrecht University's Department of Earth Sciences.^[6]

Vadose-zone hydrology

A major focus of van Genuchten's research has been the vadose zone, the part of the subsurface between the land surface and the groundwater table in which soil and rock pores contain both air and water.^[5]

Understanding water and chemical movement through this region is important to agriculture, groundwater recharge, contaminant migration, irrigation, flood processes and water quality. Van Genuchten developed analytical and numerical methods for describing these processes in variably saturated soils and rocks.^[2][1]

His research has included equilibrium and nonequilibrium solute transport, preferential flow, contaminant sorption, soil-water retention, unsaturated hydraulic conductivity, parameter estimation, salinity, root-water uptake and transport through structured soils and fractured materials.^[2]

Van Genuchten model

Van Genuchten's most widely used theoretical contribution is the soil hydraulic formulation published in his 1980 paper A Closed-form Equation for Predicting the Hydraulic Conductivity of Unsaturated Soils.^[3]

Effective saturation, ${\displaystyle S_{e}}$, can be defined as

${\displaystyle S_{e}={\frac {\theta -\theta _{r}}{\theta _{s}-\theta _{r}}},}$

where ${\displaystyle \theta }$ is volumetric water content, ${\displaystyle \theta _{r}}$ is residual water content and ${\displaystyle \theta _{s}}$ is saturated water content.

A commonly used form of the van Genuchten soil-water retention function is

${\displaystyle S_{e}(h)=\left[1+(\alpha |h|)^{n}\right]^{-m},}$

where ${\displaystyle h}$ is pressure head and ${\displaystyle \alpha }$, ${\displaystyle n}$ and ${\displaystyle m}$ determine the shape of the retention curve. In the formulation commonly combined with Mualem's conductivity model,

${\displaystyle m=1-{\frac {1}{n}}.}$

The corresponding unsaturated hydraulic conductivity can be expressed as

${\displaystyle K(S_{e})=K_{s}S_{e}^{\ell }\left[1-\left(1-S_{e}^{1/m}\right)^{m}\right]^{2},}$

where ${\displaystyle K_{s}}$ is saturated hydraulic conductivity and ${\displaystyle \ell }$ is a pore-connectivity parameter.^[3]

The combination became known as the Mualem–van Genuchten model. Its relatively simple mathematical form made it suitable for incorporation into numerical solutions of the nonlinear equations governing unsaturated flow.^[2]

The European Geosciences Union cited the widespread use of van Genuchten's parametric soil hydraulic models when awarding him the John Dalton Medal in 2010.^[2]

Nonequilibrium and preferential transport

Another major component of van Genuchten's work concerns situations in which water and dissolved substances do not move uniformly through porous materials.

His doctoral research included models describing physical and chemical nonequilibrium in soil columns. These included representations of porous media as containing mobile and relatively immobile water regions.^[6]

Van Genuchten subsequently helped develop and apply dual-porosity and dual-permeability approaches to water and solute transport. Such models are used to represent preferential flow and exchange between different pore regions in aggregated soils, macroporous media and fractured rocks.^[5]

The Wolf Foundation highlighted his work on dual-porosity and dual-permeability models as one of his principal contributions to soil physics and vadose-zone hydrology.^[5]

Inverse modeling

Van Genuchten was also among the researchers who introduced inverse parameter-estimation approaches into soil hydrology.

Inverse modeling determines hydraulic or transport parameters by adjusting a mathematical model until its calculated results reproduce laboratory or field measurements. Van Genuchten and collaborators applied these methods to water-flow and solute-transport problems beginning in the 1980s.^[2]

His later work incorporated parameter-estimation procedures into numerical flow and transport models. These methods became important in the interpretation of experimental measurements and in estimating soil hydraulic and solute-transport properties.^[2]

HYDRUS and other computer models

Van Genuchten contributed to the development and application of several generations of analytical and numerical software for water flow and solute transport.

Among the programs associated with his work were RETC, used for describing soil hydraulic properties, and CXTFIT, used in the analysis of solute-transport experiments. Analytical transport models were later incorporated into STANMOD.^[2][4]

Van Genuchten also collaborated extensively with Jiří Šimůnek, Miroslav Šejna and other researchers on the development and application of the HYDRUS family of numerical models.^[4]

HYDRUS is used to simulate water flow, heat transport and solute movement in variably saturated porous media. Its development involved several generations of programs and multiple research groups rather than a single software package developed by one individual.^[4]

The Prince Sultan Bin Abdulaziz International Prize for Water cited RETC, CXTFIT, STANMOD and especially HYDRUS among the theoretical and software tools associated with van Genuchten's contributions to hydrogeology when it awarded him its Groundwater Prize in 2018.^[11]

Later versions of HYDRUS have incorporated processes including root-water uptake, chemical reactions, nonequilibrium transport and multicomponent solute systems.^[12]