This Special Issue (SI) is dedicated to Dr. Rien van Genuchten (Martinus Th. van Genuchten, hereafter called Rien) as a tribute to him for being awarded the very prestigious international Wolf Prize for Agriculture. The Wolf Foundation awards prizes annually to outstanding scientists and artists worldwide for their excellent achievements in the interest of humanity and friendly relations among peoples, regardless of their background (About the Wolf Foundation—Wolf Foundation [wolffund.org]). Many would state that receiving this Prize is analogous to being awarded a Nobel Prize, as both recognize scientists for their impacts on society. For one of our colleagues, to be selected for this award is especially rewarding as it recognizes the relevance of soil physics to the field of agriculture and environmental sciences globally. Rien created a much-needed scientific basis for understanding fluid flow and contaminant transport processes in unsaturated soils, reshaping the disciplines of soil physics and vadose zone hydrology. Vadose zone hydrology would be unthinkable without his many contributions, which established links between agriculture, soil science, geology, environmental sciences, and hydrologic engineering. The SI targets articles from authors who have either collaborated with Rien or built their research program based on his many contributions to the field of Soil Physics and Vadose Zone Hydrology. It also includes selected contributions by scientists from other disciplines who applied the concepts developed by Rien, addressing various relevant environmental issues. There have been very few contemporary colleagues in soil science who have made such a huge impact on the field of soil physics and the careers of so many students and early-career soil physicists than Rien. His influence was perhaps not so predictable early on when he was a graduate student in the early 70s under the mentorship of Pete Wierenga at New Mexico State, Las Cruces. As Rien explained in his autobiography (van Genuchten, 2024), Pete was not so impressed with his experimental skills, letting him instead tinker with mathematical models that can better represent physical transport processes in soils. His proficiency in modeling was already evident in his first peer-reviewed paper (van Genuchten et al., 1974), where he used the CSMP computer language (as he learned this very early software for solving differential equations while studying at Wageningen University with C. T. de Wit; Research of the Plant Production Systems Group—WUR) to simulate the nonequilibrium transport of pesticides in soils as well as their parametric estimations using column studies. Subsequently, under the supervision of Drs. Pinder and Gray at Princeton University, Rien developed a finite element model to solve multidimensional multiphase flow. He then went on to the USDA-ARS Soil Salinity Laboratory on the UC Riverside campus, further developing and advancing algorithms with numerous visiting scientists and lab staff to improve soil water and chemical transport solutions. Early on, at the US Salinity Lab, he published the parametric model for unsaturated soil hydraulic properties (van Genuchten, 1980). Rien continues to receive enormous recognition for this research paper as this model provided enormous computational time savings, thus allowing efficient numerical solutions to the highly nonlinear unsaturated water flow equation. Moreover, the so-called van Genuchten–Mualem (vG–M) parametric expressions have been shown to fit most experimental soil hydraulic data, thus making them attractive in many vadose zone applications. After being devoted to mostly theoretical research in the first half of his scientific career, Rien committed to applying his work to many different relevant problems that could be addressed with his modeling advancements, such as pesticide, pharmaceutical, nutrient, pathogen, and colloid transport processes, multicomponent geochemical and radionuclide transport, root water uptake and soil salinity, among others. He continued working on many of these applications after retiring from the US Salinity Laboratory at the Federal University of Rio de Janeiro in Brazil and Utrecht University in the Netherlands. An extensive summary of his research accomplishments is further written in Rien's autobiography (van Genuchten, 2024). In addition to the highly cited landmark article on the vG–M model (>32,000 citations), more than 180 of Rien's research papers and reports have been cited at least 100 times. To date, his research has led to an astounding total number of more than 100,000 citations (Google Scholar; a milestone surely to be reached before this introduction is published). While such numbers are not the sole determinant of the scientific impact of his work, the sheer magnitude of his citation numbers is mind-boggling. While Rien's scientific contributions go without question, his impact on people's professional careers is even more unprecedented. By recruiting the best and brightest students and scientists, he consistently cultivated their career development options and expertise through his encouragement, enthusiasm, and support. Especially remarkable has been his support of students and scientists from non-Western countries. In doing so, he provided opportunities that were otherwise not available to them at the time. Throughout his career, he hosted and inspired approximately 70 visiting scientists and served on many PhD committees across all continents, many of whom developed successful careers. Graduates of Rien's "Soil Physics Without Borders" movement hold faculty positions at about 10 US universities, and about twice as many have found similar positions abroad. Rien was elected a Fellow of AAAS, AGU, ASA, and SSSA, and has received many other major awards, such as the Don and Betty Kirkham Medal and the EGU Dalton Medal. In 2020, he was presented with the very prestigious AGU Horton Medal, and most recently, he was elected to the US National Academy of Engineering. Due to his landmark contributions and leadership in the agricultural sciences, specifically on theory development and applications in agriculture and soil hydrology, Rien was awarded the 2023 Wolf Prize for reshaping the disciplines of soil physics and vadose zone hydrology through "his groundbreaking work in understanding water flow and predicting contaminant transport in soils." In part because of the advanced model applications developed by Rien, knowledge of dynamic soil processes has rapidly evolved in the past decades, thus enhancing research collaborations between disciplines of soils and hydrology, engineering, biology (plant and microbial sciences), climate science, among others. Most illustrative has been the movement since the 1990s for soil physics to transcend its disciplinary field to vadose zone research, leading to the publication of the Vadose Zone Journal (VZJ) in 2002, strongly advocated by Rien and colleagues (van Genuchten, 2002). His efforts in reaching the broader scientific community were achieved by serving as the founding editor of VZJ, an interdisciplinary journal of the Tri-Societies (American Society of Agronomy, Crop Science Society of American, and Soil Science Society of America), broadly linking soil science with agriculture, hydrology, engineering, geology, ecology, and environmental sciences. The name "Vadose Zone Journal" was selected to clearly show the intent of VZJ to seek contributions and readership from across a broad range of disciplines, in addition to the soil and agricultural communities. The rapid acceptance of VZJ within the scientific community has been a recognition that vadose zone processes play a prominent role in many interdisciplinary environmental and resource management issues. As articulated in the previous section, the continued development of multidisciplinary collaborations has paved the way to further disciplinary contributions to broader societal issues, potentially leading to transdisciplinary applications (Hopmans, 2020). In the spirit of the scope of VZJ and the need to increase multidisciplinary soil investigations, this SI is an excellent example of the broad range of applications, thereby reflecting the impact of Rien's career. The desire for one's research to impact society is also well articulated by him in the closing remarks of his autobiography (van Genuchten, 2024), by stating that "I have become more concerned about all the unfortunate troubles we face on this planet, such as the environmental problems and climate change, and the enormous political and socio-economic problems all over the world. Because our planet is faced with so many environmental problems, it is not easy to choose the best area in one's research." Unsurprisingly, many SI contributions discuss the treatment of soil hydraulic properties and their advanced characterization and/or application for a wide range of soils, including those ill-behaved or nonideal that the van Genuchten vG–M model cannot describe well. Other papers contribute to further illuminating soil physical flow and chemical transport processes at various scales, further advancing his early computer modeling frameworks and applications beyond what one could have surmised at the time. Some SI papers address the need for further scientific investigations of the transport of specific chemical species, such as radioactive isotopes and heavy metals in soils and the vadose zone. In contrast, other contributors focused on soil salinity impacts on agricultural production, thus illustrating Rien's broad research at the US Salinity Laboratory. In summary, reflecting on the many published works herein, the Tribute is an excellent example of the enormous impact Rien has had on the direction of soil science research, specifically by introducing mathematical modeling of unsaturated soil water flow and chemical transport in the soil science literature, thereby making computer solutions widely accessible and allowing the wider professional community to significantly advance the scientific field. To much of Rien's credit, his model solutions accelerated the rate by which new theories and applications of unsaturated water flow and chemical transport were adopted in the agricultural, hydrological, and environmental sciences. During his 50-year career, Rien transformed the broad fields of soil physics and vadose zone hydrology, which are central to modern agricultural operations, environmental assessment, and climate science. Rien's contributions transcend his scientific achievements. He embodies genuine mentorship and collaborative growth. His remarkable ability to elevate the work of early-career researchers and his support for their career advancement exemplify a dedication to nurturing the next generation of vadose zone scientists. Apart from research, Rien is also passionate about teaching. He continues to make himself available by delivering guest lectures that enrich the education of students, extending his already broad impact. Rien has instilled in researchers across all stages of their careers the values of collegiality, the importance of fostering opportunities for others, and the power of a positive spirit. His commitment to research and teaching, unrelenting collegial support, and unusual humility inspire us all. Jan Hopmans: Conceptualization; writing—original draft. Binayak Mohanty: Conceptualization; writing—review and editing. Jirka Simunek: Conceptualization; writing—review and editing. The co-editors of this SI accredit Dr. Jaivime Evaristo of Utrecht University for his editing of the manuscript, especially the last paragraph related to Rien's contributions to the mentorship of students and other early-career scientists.
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