IGNACIO RODRÍGUEZ-ITURBE (1942-2022)

BY SIMON A. LEVIN AND ANDREA RINALDO

IGNACIO RODRÍGUEZ-ITURBE, the inspirational and highly influential hydrologist, suddenly passed away on Oct. 27, 2022. He was distinguished university professor and Wofford Cain I Chair Professor Emeritus at Texas A&M University, and James S. McDonnell Distinguished University Professor Emeritus of civil and environmental engineering at Princeton University. A master of contemporary thought and a pathbreaking scientist, Ignacio was at the center of the cultural process and intellectual ferment that transformed the field of hydrology from an empirical branch of applied engineering to a mainstream environmental science. He creatively and rigorously showed that the analysis, synthesis, and sampling of hydrological processes could pave the way for a new and deeper understanding of floods, droughts, and a “fair” distribution of water, including water controls on living communities — in a nutshell, how nature works through the inner workings of the water cycle. Rodríguez-Iturbe’s pioneering work has influenced generations of researchers across many fields and around the world and left a long-lasting legacy through many disciples and students, whose lives he changed by his intellectual depth and empathic maieutics. Ignacio made each student and collaborator alike feel special and mobilized entire communities to shift their foci to modern hydrologic research. Within hydrology, his wide-ranging and highly mathematical work blended theory from spatial point processes and fractal mathematics to the dynamics of river basins and other hydrological patterns. His contributions went well beyond hydrology to include ecophysiology and plant community ecology, essentially creating a new discipline at the interface between ecology and hydrology. He remained active in research until his death.

Ignacio Rodríguez-Iturbe was born in Maracaibo, Venezuela, on March 8, 1942, into a large family blessed with deep intellectual gifts. He received a C.E. with maximum honors in 1963 from the Universidad del Zulia in Maracaibo, a M.S. at the California Institute of Technology in 1965, and a Ph.D. from Colorado State University in 1967. In 1964, he married Mercedes Maiz, with whom he had five children: Oscar, Ignacio, Olimpia, Juan, and Luis. At his death, Ignacio had only recently retired from Texas A&M, with the intent to return with Mercedes to his native Venezuela, where much of their family lived.

Ignacio served as faculty in various universities: the Universidad del Zulia in Maracaibo (from which he graduated and where his father had long been dean of the faculty of engineering), MIT, Universidad Simon Bolivar in Caracas, University of Iowa, Texas A&M, and Princeton University. He held appointments as visiting professor in several countries, fostering the establishment of thriving research communities in various domains of hydrology through his ease in communicating, the empathy he emanated, and his infectious enthusiasm. These gifts were particularly instrumental in the transformation of the Italian hydrologic community. His influence spread even further through countless collaborations and the success of his students who now hold academic positions in major institutions worldwide.

Ignacio’s early research focused on the analysis, synthesis, and sampling of hydrological processes, particularly on the stochastic character of rainfall events and those of their measurement networks. His contributions stood out for their extraordinary mathematical and statistical insights. The theory of stochastic processes was a lifelong love affair that led to publication of a series of fundamental papers in collaboration with leading hydrologists and statisticians.¹

In the late 1970s, Ignacio’s interests turned to the connection of geomorphology and hydrology, especially on the role of form and function of river networks. He pioneered the so-called geomorphological theory of the hydrological response,² which radically changed the century-old empirical approaches used by engineers to predict floods. The theory remains a tour de force of insight, usefulness, and elegance. Later, he concentrated on the origins, dynamics, and stationary states of the geomorphology of natural hydraulic waterways, especially of river networks in runoff-producing areas.³ In the early ’90s, he led the development of the theory of optimal channel networks,⁴ a significant contribution to understanding the dynamic origins of the fractal geometry of nature. In parallel, Ignacio contributed to the gathering of key field evidence on the form of river networks, using the then cutting-edge technology of remotely acquired and objectively manipulated information from digital terrain maps and the extraction of properly channelized landforms.⁵ The proof of the scale-invariance of the master variables of river network hydrology over up to six orders of magnitude, from 1 meter to thousands of kilometers, placed hydrology and geomorphology firmly within the physics of fractals. The development and validation of the theory of optimal channel networks represented a major advance that revolutionized our understanding of the form and function of river networks and of the dynamic origins of fractal geometries in nature.

Toward the turn of the century, Ignacio jump-started the study of the probabilistic structure of the interactions among climate, soil, and vegetation.⁶ In the process, he became the main architect of a new discipline, ecohydrology.⁷ A crasis of hydrology and ecology, ecohydrology is now accepted as the science that revolutionized the study and management of ecosystems through the probabilistic and realistic characterization of natural forcings linked to the waters of the hydrological cycle. The last decade was devoted mainly to studies in water controls of the distribution of species, populations, and pathogens in natural ecosystems, especially as applied to river networks seen as ecological corridors.

Ignacio liked to recall that the fundamental unity of the seemingly disparate problems of water-controlled population ecology, waterborne disease spread, and biological invasions in fluvial systems was solidified during a walk around the pond of the Institute for Advanced Studies in Princeton, in the best tradition of the peripatetic philosophers.⁸ Although he could work on them only sporadically, he also made deep contributions in other research fields, including studies on large-scale soil moisture dynamics and theoretical work on evapotranspiration patterns due to spatial heterogeneity of plant species. Patterns in nature constituted a lifelong theme in his work. Throughout his career he also contributed important work on virtual water trade networks, whose understanding is crucial in the development of a rigorous definition of a fair distribution of water.

Ignacio’s scientific production was characterized by methodological depth, insatiable curiosity, and pathbreaking originality, often giving rise to the establishment of new fields of research or even new disciplines. He explored the limits of current knowledge in many fields and attacked them with methods and models borrowed from other contexts and disciplines, most importantly mathematics and especially the theories of probability and stochastic processes.

Ignacio coauthored four research monographs,⁹ each of which offers a valuable collection of years of reflections. They deal with random functions and hydrology, fractal river basins, ecohydrology of water-controlled ecosystems, and river networks as ecological corridors, each written as he concluded a chapter of his research. His books have been instrumental in communicating the emergence of hydrology as an earth science to an audience extending well beyond a small community of specialists.

Ignacio was an exceptional mentor to students, postdoctoral associates, and collaborators. His boundless generosity and enthusiasm in disseminating ideas and fostering collaborations, ability to enchant audiences, especially those made up of young students, and fierce dedication to his research right up to the end are essential components of his legacy. In the messages he sent to his collaborators from the hospital bed during what turned out to be his last weekend, he consistently referred to the things that needed to be done as soon as possible, always indicating new objectives, and the “where is the gold medal?” question he so often asked to rally his troops.

Ignacio was a member or fellow of multiple international honorary societies beyond the National Academy of Engineering, including the U.S. National Academy of Sciences; the American Academy of Arts and Sciences; the Istituto Veneto di Scienze, Lettere ed Arti; the Pontifical Academy of Sciences; and the Third World Academy of Sciences. He received multiple honorary degrees, and was especially touched by the ones from his alma mater, the University of Zulia in Maracaibo; the degree in environmental engineering from the University of Genoa on the occasion of the Colombian celebrations; and an honorary Ph.D. from the University of Cantabria.

Ignacio was the recipient of an extraordinary number of awards and prizes for his research work, a testament to his exceptional academic standards. Among these, the 2002 Stockholm Water Prize stands out, as it is recognized as the Nobel Prize equivalent for water studies. Others include the Prince Sultan Bin Abdulaziz International Water Prize (for creativity) in 2010, and the multiple awards bestowed on him by the American Geophysical Union for both his science and his unselfish cooperation in science. Two awards are named for him: one for outstanding scientific achievements in any field of science given by the Universidad del Zulia; and the other for the best article published each year in the international journal Ecohydrology.

Ignacio Rodríguez-Iturbe was a visionary and demanding academician, who believed in the hierarchy of knowledge, the primacy of merit and reason, the value of speculation, and the specificities of the laboratories of modern knowledge. He believed in the universities and academies as free factories of culture and knowledge, and in the veritable mission of professors, centered on the production and the dissemination of knowledge for the betterment of society. He took this mission to heart, and often quoted Ernst Kantorowicz’s statement about the only professions worthy of wearing the gown: the judge, the priest, and the academician—the gown is the epitome of the maturity of the mind, independence of judgment, and one’s direct responsibility only to his conscience.

Brilliant and generous of his time and insight, he always and immediately revealed a special empathy with others and a world of sensitivity and attention without distinctions. Legions of collaborators experienced his unsparing generosity in advising, suggesting new and relevant research directions, and in pointing out relevant documents and studies in the most disparate fields. We mourn his passing but celebrate a most fulfilling life, rich in intellectual rewards and recognition, jointly with treasures of personal relations and family ties. We consider it a privilege to have known and worked with him.

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This tribute is an edited version of the following retrospective published under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND): Levin SA, Rinaldo A. 2022. Ignacio Rodríguez-Iturbe (1942–2022): A review of a pathbreaking academic career combining chance and self-organization. PNAS 119(49):e2217606119. https://doi.org/10.1073/pnas.221760611. The retrospective includes additional references and information.
¹Rodríguez-Iturbe I, Cox DR, Isham V. 1987. Some models for rainfall based on stochastic point processes. Proc. R. Soc. London A 410:269-288; Rodríguez-Iturbe I, Cox DR, Eagleson PS. 1986. Spatial modeling of total storm rainfall. Proc. R. Soc. London A 403:27-50; Rodríguez-Iturbe I, Gupta VK, Waymire E. 1984. Scale considerations in the modeling of temporal rainfall. Water Resour. Res. 20:1611-1619; Waymire E, Gupta VK, Rodríguez-Iturbe I. 1984. A spectral theory of rainfall intensity at the meso-β scale. Water Resour. Res. 20:1453-1455; Rodríguez-Iturbe I. 1986. Scale of fluctuation of rainfall models. Water Resour. Res. 22:15S-37S; Rodríguez-Iturbe I, Eagleson PS. 1987. Mathematical models of rainstorm events in space and time. Water Resour. Res. 23:181-190.
²Rodríguez-Iturbe I, Valdés JB. 1979. The geomorphologic structure of hydrologic response. Water Resour. Res. 15:1409-1420; Rodríguez-Iturbe I, González M, Bras RL. 1982. A geomorphoclimatic theory of the instantaneous unit hydrograph. Water Resour. Res. 18:877-886.
³Tarboton DE, Bras RL, Rodríguez-Iturbe I. 1988. The fractal nature of river networks. Water Resour. Res. 24:1317-1322; Rinaldo A, Rigon R, Ijjasz-Vasquez E, Bras RL, Rodríguez-Iturbe I. 1993. Self-organized fractal river networks. Phys. Rev. Lett. 70:822-825; Rinaldo A, Dietrich WE, Rigon R, Vogel GK, Rodríguez-Iturbe I. 1995. Geomorphological signatures of varying climate. Nature 374:632-635.
⁴Rodríguez-Iturbe I et al. 1992. Energy dissipation, runoff production, and the 3-dimensional structure of river basins. Water Resour. Res. 28:1095-1103; Rinaldo A, Rigon R, Banavar JR, Maritan A, Rodríguez-Iturbe I. 2014. Evolution and selection of river networks; statics, dynamics, and complexity. Proc. Natl. Acad. Sci. U.S.A. 111:2417-2424.
⁵Tarboton DG, Bras RL, Rodríguez-Iturbe I. 1991. On the extraction of channel networks from digital elevation data. Hydrol. Processes 5:81-100.
⁶Rodríguez-Iturbe I, Porporato A, Ridolfi L, Isham V, Cox DR. 1999. Probabilistic modeling of water balance at a point: The role of climate, soil and vegetation. Proc. R. Soc. A 455:3789-3805.
⁷Rodriguez-Iturbe I. 2000. Ecohydrology: A hydrologic perspective of climate-soil-vegetation dynamics. Water Resour. Res. 36:3-9; Porporato A, D’Odorico P, Laio F, Rodríguez-Iturbe I. 2003. Hydrologic controls on soil carbon and nitrogen cycles. I. Modeling scheme. Adv. Water Res. 26:45-58; D’Odorico P, Laio F, Porporato A, Rodríguez-Iturbe I. 2003. Hydrologic controls on soil carbon and nitrogen cycles. II A case study. Adv. Water Res. 26:59-70; Rodriguez-Iturbe I, Porporato A. 2005. Ecohydrology of Water-Controlled Ecosystems: Soil Moisture and Plant Dynamics. Cambridge University Press.
⁸Rinaldo A, Gatto M, Rodriguez-Iturbe I. 2020. River Networks as Ecological Corridors. Species, Populations, Pathogens. Cambridge University Press.
⁹Bras RL, Rodríguez-Iturbe I. 1985. Random Functions and Hydrology. Addison-Wesley; Rodríguez-Iturbe I, Rinaldo A. 2001. Fractal River Basins: Chance and Self-Organization. Cambridge University Press; Rodriguez-Iturbe I, Porporato A. 2005. Ecohydrology of Water-Controlled Ecosystems: Soil Moisture and Plant Dynamics. Cambridge University Press; Rinaldo A, Gatto M, Rodriguez-Iturbe I. 2020. River Networks as Ecological Corridors. Species, Populations, Pathogens. Cambridge University Press.