ECOHYDROLOGY: KEYSTONE FOR SUSTAINABLE DEVELOPMENT
Ignacio Rodriguez-Iturbe, Department of Civil and Environmental Engineering, Princeton University
Introduction
It is widely acknowledged that water is fundamental for human life and the keystone of environmentally
responsible development. Poverty, disease, and political collapse will follow the failure of environmental
support systems (Brown, 2006). The dynamics of the hydrologic cycle is at the heart of these environmental
systems.
A crucial component of ecosystem functioning is the hydrologic dynamics responsible for many fundamental
ecological patterns and processes. Ecohydrology is precisely the study of such dynamics and lies at the
frontier of the environmental sciences where it takes place the natural convergence of the biological and the
physical sciences. This convergence will transform our understanding of the processes that control the
stability and sustainability of natural environmental systems. Such knowledge is critical for a safe and
sustainable future and will play a key role in problems related to food production, land degradation,
preservation of biodiversity, ecological services,
desertification and many others.
Sustainable Development and Climate Change
“Although it is natural to think first of temperature when we think of global warming, the impact of climate
change in precipitation may be even more important in the long run for
many places and many people” (Henson, 2006). The ecohydrological impacts of climate change are related
mainly to precipitation and streamflow and their consequences are both in terms of quantity and quality of
the water resources as well as in the ecosystem functions which are crucial to conserve biodiversity and
secure the important ecosystem services they provide.
It has been clearly demonstrated that changes in the frequency of rainfall events, even when the total rainfall
in the region remains unchanged, will frequently lead to dramatic shifts in the water balance of ecosystems
which are then accompanied by serious impacts on the net assimilation of plants, changes in vegetation type
and structure, soil conditions, etc. (Porporato et al., 2004). When the changes on the frequency of the
rainfall events is accompanied by a change in the total rainfall the impacts become even more dramatic.
Accounting for changes only in mean responses to climatic variability is far from sufficient for a realistic
evaluation of the impact of climate change in ecosystems. Soil moisture dynamics, photosynthesis, and
plant assimilation are greatly dependent on the temporal dynamics of precipitation. Changes in the frequency
of rainfall events as well as in the precipitation depth of such events are being detected throughout the planet
with consequences which extend from floods and droughts, to food production and biodiversity.
Climate-Vegetation Interaction
Defining the dynamic interactions between vegetation and climate is a research priority for the quantification
of criteria for sustainable development. Vegetation responds to climatic fluctuations over a range from
diurnal to multidecadal time scales as well as from local to regional spatial scales. This response is
especially significant in savanna ecosystems which cover 40% of the earth’s terrestrial surface and are vital
to the survival of a large part of humanity. They are commonly water-limited ecosystems, highly coupled to
rainfall variability which show a relatively robust coexistence between trees and grasses. The hydrological
aspects of these ecosystems are crucial to why savannas exist and persist. In particular the dynamical
nature of the vegetation in savannas, very much linked to the rainfall characteristics, is a key factor which
promotes robustness in an environment that has a fluctuating limiting resource.
There exist thresholds in the composition of vegetation-e.g., trees and grasses- beyond which the ecosystem
will not recover and will shift to a different structure or disappear completely. These thresholds could be
surpassed as a result of changes in climate-e.g., rainfall dynamics-or through man induced consequencese.g., overgrazing-or both.
Some key questions related to the functioning and sustainability of savannas are thus:
1-What are the controls on the variable wet season grass fractional cover?
2-What are the effects of the dynamic grass cover on the overall ecosystem water
use?
3-Are vegetation assemblages with a variable growth component more favorably
suited to persist in a fluctuating rainfall environment?
It has been found that the dynamic character of the grass cover enhances the degree to which the savanna
vegetation is optimized with respect to water use and helps to close the water cycle at the land surface
(Scanlon et al., 2005). Also, the frequently found
transition from the nutrient-limited to the water-limited savanna ecosystems is related to the shift from the
open to closed hydrologic cycle at the land surface.
Impacts of Large-Scale Land-Atmosphere Feedbacks
Fluctuations in the recycling component of precipitation over large spatial scales leads to dramatic shifts in
the probabilistic behavior of the soil moisture balance equation (Rodriguez-Iturbe et al., 1991). Thus, an
increase in the coefficient of variation of the rainfall component resulting from the local evaporation (rather
than from external or advective sources) will lead to important changes in the probability distribution of the
soil moisture over the region. With a not too large increase in these fluctuations of precipitation, the
probability distribution of soil moisture shifts to a bimodal regime where the moisture stays preferably in
either a wet mode or a dry mode, the second one frequently being the most likely one.
The appearance of a multiple mode regime in soil moisture is of key importance for the ecosystem
sustainability. Long periods of unusually severe droughts become much more likely with the soil moisture
exhibiting a strong persistence to remain for long periods below or above its statistical average. Such a
dynamics in the moisture content of the soil
induces drastic changes in the vegetation of the ecosystem. This is the case, specially, in savannas where it
may lead to irreversible transformations that will have profound and lasting consequences in the
sustainability of the region.
References
Brown, L.R., Plan B 2.0-Rescuing a Planet Under Stress and a Civilization in Trouble,
W.W. Norton and Co., 2006.
Henson, R., The Rough Guide to Climate Change, Penguin Books, 2006.
Porporato, A., E. Daly, and I. Rodriguez-Iturbe, Soil water balance and ecosystem response to climate
change, American Naturalist, vol.164, n.5, 625-632-2004.
Scanlon, T.M.,K.K. Caylor, S. Manfreda, S.A. Levin, and I. Rodriguez-Iturbe, Dynamic response of grass
cover to rainfall variability: implications for the function and persistence of savanna ecosystems, Advances in
Water Resources,Vol.28,291-302,2005.
Rodriguez-Iturbe, I.,D. Entekhabi, and R .L. Bras, Non-linear dynamics of soil moisture at climate scales,1Stochastic analysis, Water Resources Research,Vol.27,1899-1906,1991.