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== Hydrological Achievements ==

Don was a giant within the field of soil physics as such, as well as far beyond. He was internationally renowned for his innovative achievements in the areas of chemical transport in soils, in spatial variability of field soils, determining nitrogen pollution levels from agricultural operations, quantifying the hydrological properties of soils, and studying how soil management affects microbial transformations. He was instrumental in designing alternative agronomic experiments using ‘non-aggie’ statistics to derive management decisions that are meaningful to farmers. He authored more than 300 publications and was an invited speaker at numerous national and international conferences. Very significant early in the 1960s was his work with Jim Biggar on formalizing the theory of miscible displacement in unsaturated soils, which did lead to some 100 joint publications. Continued collaboration with Jim in the 1970s involved their pioneering work on the characterization of field soil variability and implications for water movement and solute transport.

Don has been the advisor for many outstanding graduate students and has been the host to more than 90 international scientists from nearly 40 different countries. Through his teaching, research and administrative efforts, as well as his service on countless national and international organizations, he has had a tremendous impact not only in the soil and hydrological sciences, but also on agricultural and environmental sciences in general. He was President of the Soil Science Society of America (SSSA) and the American Agronomy Society (ASA), as well as President of the Hydrology Section of the American Geophysical Union (AGU). He also served on the National Research Council’s (NRC) Board on Agriculture and chaired the U.S. National Committee of Soil Science, thereby representing the National Academy of Science (NAS) and U.S. soil scientists to the International Union of Soil Science (IUSS). Don held many contributing positions at the IUSS, being President of the Soil Physics Commission from 1978-82, member of its Budget and Finance Committee, and organizing or chairing several of its symposia. Many other roles can be cited, such as being a long-time consultant for the International Atomic Energy Agency, or his work for other national and international agencies. For example, he served on panels of Remote Sensing for Soil and Water Resources of the Space Applications Board of NRC and the Geophysics Board of the Water Resources Review Committee of the Food and Agriculture Board of the NAS. He further served on numerous journal editorial boards and review panels of universities and research organizations, as well as consulted for national and international government agencies. Particularly noteworthy was his service on the editorial board of Water Resources Research and becoming its editor-in-chief in 1965.

Because of his unique accomplishments in research, student education and professional service, Don Nielsen was honored with numerous awards. He became a Fellow of ASA, SSSA and AGU. He received the M. King Hubbert Award of the National Ground Water Association, was made a Honorary member of IUSS and the European Geophysical Society. He further received the very prestigious Horton Medal of AGU’s Hydrology section “for his fundamental and pioneering work in hydrology, combined with his uncanny love for the profession”. In 2008 he was awarded the Don and Betty Kirkham International Soil Physics Medal, which is awarded once every 8 years.

Entry adapted from obituary written by Jan W. Hopmans, Martinus Th. van Genuchten, and Ole Wendroth