Garrison Sposito

2004 Robert E. Horton Medal Winner

Garrison Sposito received the Horton Medal at the 2004 Fall Meeting Honors Ceremony on 15 December, in San Francisco, California. The medal is given for outstanding contributions to hydrology.


It is a great pleasure to introduce Professor Garrison Sposito of the University of California, Berkeley as the 2004 AGU Robert E. Horton Medalist. Those of us who know Gary well already recognize him as an undisputed leader in the field of hydrology. Gary’s extensive scientific contributions in geochemistry, thermodynamics, mathematics, and subsurface hydrology are legendary. He is largely responsible for bridging the areas of aqueous geochemistry and physical hydrology. Andrew Barry commented that “there is simply no other hydrologist who brings such a mastery of theoretical physics and chemistry to bear on hydrologic problems.”

Wilfried Brutsaert summarized Gary’s overall contribution as one of intellectual leadership and integrity in the field of hydrology, through his development of the physical and chemical bases of soil hydrology. In a field that often suffers from an overemphasis on operational methods, Gary is one of the very few who, some 40 years ago, took a fundamental approach to solving hydrologic problems. His work covers an incredibly broad range of frontline research areas and bears witness to his penetrating scientific insight. Gary’s writing style is extraordinarily lucid; he has a gift for making deep theoretical issues understandable to the broader scientific community.

Gary’s work on scaling is one of his many contributions to physical hydrology that has a thoroughness rarely achieved. Starting with the scaling of spatial variability in partly saturated soils, he advanced into the relevance of the fractal nature of soil water properties and more recently formulated a complete theory of the significance of ergodicity in the fundamental construct of the spatial scale. His theory for solute transport in heterogeneous formations generalized existing results and allowed for the prediction of macrodispersion coefficients. Gary’s continued emphasis on “putting the physics into soil physics” is an inspiration for all of us.

Gary helped establish another cornerstone in hydrology by linking physical chemistry with hydrology. James Morgan pointed to the clarity and rigor of Gary’s work on the ion exchange properties of soils and the characterization of the surface chemical properties of metal oxide particles in water. He has done pioneering research on adsorption isotherms, statistical mechanical foundations of surface equilibria, and electrical properties of solid/ aqueous interfaces. And William Casey notes that other geochemists often rely upon Gary to do the “heavy lifting” of interpreting their observations. Gary’s formidable breadth of work has led to new scientific insights that extend into microbiology and ecology.

Gary’s interests are also reflected in his important scholarly books ranging from quantum physics and classical dynamics to thermodynamics and the chemistry of soils. The incredible span of Gary’s contributions would make a small army of researchers internationally known—yet he is a single individual.

As many of us have experienced, Gary is a scholar with an enormous heart and an uncommon graciousness toward younger scientists, offering invaluable help in the form of mathematical derivations and detailed explanations. For his “spirit of helpfulness and friendliness in unselfish cooperative research,” we can think of no more deserving recipient of the Horton Medal.

—MARC B. PARLANGE, L’Ecole Polytech Federale Lausanne, Switzerland


Thank you, Marc, for your very kind words, and thank you, my colleagues, for this splendid honor.

One November afternoon, during the halcyon days of my tenure at our Riverside campus, I was paid a visit by the soil physicist, Lorenzo Richards, who called on me after reading the dissertation of my first doctoral student, Juan Giráldez, in which an attempt was made to find a covariant form of Richards’ celebrated partial differential equation describing water flow in an unsaturated porous medium. I learned that afternoon about the near-rejection of Richards’ paper in which his famous equation was introduced and of his continuing wonder over the 50 years since its publication that his work could be so influential. He was especially bemused at the naming of the equation by his colleagues and by its exegesis in the literature of hydrology. That there could be a deeper significance than operational in his equation had simply not occurred to him.

Richards’ preoccupation was in fact with the measurability of the hydraulic conductivity as a function of matric potential. In field soils, both properties are inexorably variable, thus reflecting the vicissitudes of heterogeneous texture and structure. But suppose now that a simplifying hypothesis is made: that the underlying cause of this spatial variability is not related to the mechanistic underpinnings of how water flows. If this is true, then the physical law governing flow must itself be uniform across a field, and this implies a similar uniformity in the functional relationship mentioned above, in the sense that it is quantified by the same parametric equation everywhere in the field. Once the physical law is expressed as a partial differential equation whose spatial uniformity is invoked, a strong constraint is placed on the mathematical forms permitted for its variable coefficients, making this conclusion inescapable.

Formally, one says that the Richards equation then exhibits scale invariance, with Lie group theory applied to show that the hydraulic conductivity can have only a power law dependence on matric potential. Physically, one says that the mechanism of water flow then has no intrinsic length scale, a matter which must be left for field experimentation to decide. The Gardner soil stands as a powerful reminder that nature need not conform to such perfect symmetry (think of the entrance to Notre Dame de Paris).

This necklace of thoughts is meant simply to convey a preoccupation with searching for a certain order in chaos, typified by scale invariance and, most recently, topological structures, in subsurface flows. I was so fortunate to have two early mentors, Duwayne Anderson and Kenneth Babcock, from whom to learn this abiding theme in the context of soil physics and chemistry. Tom Anderson, in a course we co-taught 35 years ago, introduced me to the broader realm of hydrology, and Vijay Gupta, in a collaboration begun 30 years ago, opened the gateway to unified theory. My wife, Mary, and my six children, Doug, Dina, Frank, Jennifer, Sara, and Cris, have sustained this journey with a joy beyond measure.

—GARRISON SPOSITO, University of California, Berkeley