River basin geomorphology resembles circulatory system
Professor Rodríguez-Iturbe discovers rules that control structure of river basins, regardless of size or location


The father of ecohydrology doesn’t smoke a pipe. Not anymore.

He confesses rather sadly to having some of his best, biggest ideas while smoking, and even keeps a few pipes as mementos of his salad days. Yet, post-quitting he is still coming up with big ideas and big discoveries.

During his career, Professor Ignacio Rodríguez-Iturbe, called the father of ecohydrology by some, has given the field of hydrology several big discoveries. Now a professor of civil and environmental engineering (CEE) at Princeton, he spent most of his career at Simon Bolivar University in Caracas, Venezuela.

He is a passionate man and an effervescent conversationalist, who will make a listener giggly and fascinated as long as they can keep up with him. He exudes a youthfulness that matches the nascent field he has helped to define.

New field of study

Ecohydrology, which combines hydrology and ecology to answer questions in each, is now the focus of most of Professor Rodríguez-Iturbe’s current research.

His research over the past 30 years has revolutionized his field by bringing more accurate methods of quantification to hydrological research. He is known for creating a model that clarified the Hurst Phenomenon of floods and droughts of long duration.

He created the Geomorphological Instantaneous Unit Hydrograph, a model that describes how river basins will respond to precipitation. In 2002 he was awarded the Stockholm Water Prize, known as the Water Nobel Prize in academic circles.

But, Professor Rodríguez-Iturbe said, the accomplishment that he is most proud of is the “spectacular, truly stellar, academic and professional careers of my many students.” He works closely with his graduate students and postdocs.

“Their successes make me very proud,” he added.

Forging ahead

Yet he is not resting on his laurels. He now seeks more grand discoveries at the intersection of hydrology and ecology.

“What are the hydrological dynamics that control vegetation and biological processes?” Professor Rodríguez-Iturbe asked, naming one his major research questions. “Hydrodynamics are very controlling over some living patterns and processes, specifically those of vegetation.”

When Professor Rodríguez-Iturbe visits a river basin and wanders through its savanna or forest, he wonders if there is more to the wildly beautiful scene than he knows. Is there actually some order directing the randomness of a natural landscape?

He began seriously contemplating this question after answering a similar question about the river basins themselves. He discovered that the geomorphology of a river basin—any river basin—looks rather like a circulatory system, or a tree. It has a central trunk with branches extending off of it, smaller branches twisting off of that, and delicate twigs bending off that.

“River basins, in their infinite variety of shapes and forms have a unifying structure,” Professor Rodríguez-Iturbe said. “I don’t care whether you’re dealing with the Amazon, the Mississippi, or the Stonybrook. We discovered that there are rules that control the structure of river basins, regardless of their size, regardless of where they are located, and so on.”

Further, the small pieces of the structure are the same as the whole. Thus, a blown-up diagram of a fraction of the Stonybrook would be essentially indistinguishable from a full diagram of the Nile.

“In fact, what the river does is organize itself to transport all the rain that falls in an effective manner, with very little work,” Professor Rodríguez-Iturbe said. “That’s why nature uses so many tree-like structures.”

The principle guiding the distribution of vegetation in a
water basin is an ecohydrology mystery. Below are
comparisons of ideal, random, and actual vegetation
distribution patterns. The “ideal” distribution was
created based upon the criterion of minimal water
stress at each location.

A real breakthrough

This discovery was a true breakthrough in hydrology. Suddenly it was possible to study hydrological flux in a more quantitative manner than ever before possible. By linking the geomorphology with hydrology Professor Rodríguez-Iturbe inspired greater discoveries in each.

He is now looking for similar links with ecology.

“A big question for me nowadays is: Is there an optimality principle controlling or guiding the organization of vegetation in the river basin?” he asked. “Does the drainage network act as a template for the organization of vegetation? Trees, grasses, and shrubs organize themselves based upon the composition of the soil, the climate, the topography. Can you see the signature of the organization in a river basin? Are there some laws or principles defining it? This is one of the things in which I am very interested.”

As usual, Professor Rodríguez-Iturbe has chosen a very complicated question. Many of the variables are highly random and inadequately quantified.

The most important factors governing the behavior of a community of plants are the cycles of water and energy, which are inextricably woven together.

Back to the basics

As everyone learned in elementary school science class, plants require plenty of water and sunlight to drive photosynthesis, and thus they constantly compete for each of these things.

These cycles are interdependent, because light causes evapotranspiration, which is the loss of water through the plant leaves. Depending upon how scarce or plentiful these resources are—particularly water, which has a much more variable and therefore uncertain cycle—plants may organize themselves in a wide variety of ways.

Adding to the already staggering complexity is the fact that different plants respond differently to fluxes n water. Professor Rodríguez-Iturbe likes to compare the average grass to the average tree.

“Grasses are very like Latin guys and trees are very Anglo-Saxon,” he joked. “Trees say ‘Let’s save.’ Grasses don’t give a damn, you know? Water comes, they use it.”

Trees have wide, deep root systems that can store a great volume of water, while grasses have shallow, dense root systems that are much better suited to quickly sucking up surface water.

“If you’re a tree with big roots or a person with a water tank in your house, you can say, ‘It pays for me to store as much as I can and then be parsimonious and be careful with how I use it,’” he said. “But if you don’t have big roots or a tank to store the water in, then why should you care? When the water comes, you’d wash all your clothes, drink it up, throw it away, give it to the neighbor!”

This difference in water use explains why the matrix of grasses in a savanna expands and contracts very quickly, while a copse of trees is less visibly changeable.

Hydrologically alone it seems that it would make good sense for these plants to give each other plenty of room to limit the competition for resources. The evapotranspiration factor, however, complicates matters.

If water is especially scarce, it may be safer for a grass to live under the canopy of a tree. Although the grass will have to share more water with the big, thirsty tree, the shade will protect its moisture from being lapped up by the heat of full sunlight. Depending upon the needs of each species and the climactic conditions, a variety of situations may be beneficial.

“No plant has the absolute advantage,” Professor Rodríguez-Iturbe said. “The intertwining of the energy balance and the water balance determines if the relationship between the plants is going to be mutualistic or aggressively competitive.

“This intertwine can be put into equations, of course. There is still a lot to be learned, though.”

Scientists trying to make sense of these relationships are ensnared in an imbroglio of spatial and temporal dynamics, highly random soil composition, hydrology, and energy cycles, all of which are highly variable. Scientists and engineers still have a long way to go in understanding their intertwined complexity.

A greater unanswered question that automatically turns hypothesis into conjecture is “what is the primary objective of an ecosystem?”

“Does it want to produce higher biomass? Does it want to produce more seeds so species can proliferate? Does it want higher biodiversity? What does it want most? And does one ecosystem want one thing while another one across the world wants something else?” Professor Rodríguez-Iturbe asked.

“These are the questions at the frontier of science, and many of them are unsolved.”

Professor Rodríguez-Iturbe recently conducted fieldwork in the Eel River basin in California with researchers from Berkeley University. They gathered empirical data about the river basins geomorphology, biodiversity, biomass, climate, and soil geochemistry. The hope is that, when pieced together, this information will sketch some fundamental law of the relation between vegetation, hydrology, and geomorphology.

Cross-disciplinary

Professor Rodríguez-Iturbe works closely not only with researchers from his home CEE department, but also with the faculty in the Department of Ecology and Evolutionary Biology.

With their help, he hopes to discover more of the elusive yet essential principles that govern nature.

“They are beautiful laws—laws that are fulfilling in a wonderful manner,” he said. “The beauty of finding unity in an infinite variety of things is what science is all about.”