River basin geomorphology resembles
discovers rules that control structure of river basins,
regardless of size or location
father of ecohydrology doesn’t smoke a pipe. Not
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,
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.
by Frank Wojciechowski
Professor Ignacio Rodríguez-Iturbe
is world-renown for his research on river basins.
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
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
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
“Their successes make me very
proud,” he added.
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
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
“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,
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
“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.
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
With their help, he hopes to discover
more of the elusive yet essential principles that govern
“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.”
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