Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 83
3
Water and Life
Water and life are inseparable and interacting. Ecosystems depend on and
drive hydrologic processes, giving rise to distinct patterns of biotic com-
munities, and a climate system strongly coupled through vegetation to
moisture in the ground.
INTRODUCTION
The evolution of life on Earth likely began with the formation of
liquid water and has been shaped by the flow of water ever since. Water
is essential for all living organisms, and, on land, the magnitude of water
supply and the timing of water delivery structures biological systems at all
spatial and temporal scales. Over geologic time scales, hydrologic change
has been a major force of natural selection. Across modern Earth, annual
precipitation and temperature explain much of the variation in the stature
and composition of vegetation, and the pattern of hydrologic connections
constrains the distribution of many organisms as they migrate to complete
their life cycles.
Although the amount and timing of water supply ultimately constrains
life on Earth, living organisms collectively influence the water cycle and
the global climate. Vegetation blankets the majority of Earth's land sur-
face, altering its albedo (reflection of solar energy), recycling its water, and
mediating its gaseous and aqueous chemistry. Biotic communities directly
alter landscape properties (such as topography, permeability, weathering
geochemistry, and erodability), fundamentally changing soil formation, ero-
sion processes, runoff paths, river morphology, and in turn the water and
nutrient availability that sustains biotic communities. The currency of these
interactions, which take place over molecular to global scales, is water.
Recently ecologists, geomorphologists, biogeochemists, soil scientists,
and hydrologic scientists have found that a common frontier of their fields
lies at the nexus of life and water. New cross-disciplinary research is emerg-
83
OCR for page 84
84 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
ing with a surge of literature already illuminating progress and ways for-
ward and applying disciplinary names such as ecohydrology, ecological
climatology, and hydroecology. One measure of this surge is the increased
frequency of published articles using the terms ecohydrology and hydro-
ecology, terms that weren't widely used prior to 1990. Today, a Web of
Science search for these terms shows the growth of these interdisciplinary
publications over the past 20 years, with the majority of that growth taking
place from 2000 to the present. The breadth, strength, depth, and impor-
tance of this topic--the co-action of life and water on Earth--ensures that
the great opportunities for discovery will be pursued.
RESEARCH OPPORTUNITIES
This chapter discusses six research topics and associated exemplary
questions. The topics are not meant to be exclusive but are among the most
important in the subject area. The central theme is the idea of bidirection-
ality (i.e., water affects life, which affects water). This interaction can be
over a short time period of, e.g., a growing season, or over geologic time in
which evolution occurs. Studies suggest that action at the finest scale, such
as the controls on moisture availability to root hairs, can have consequences
for the large scale, such as regional climate. Hence, local mechanistic un-
derstanding is needed, and therefore the significant challenge of upscaling
should be tackled. Interactions can lead to patterns, from repeating patches
of vegetation to the self-organized development of ridge-and-valley topog-
raphy. Such patterns invite theory, and these two patterns in particular have
driven much research. The coupling, bidirectionality, and internal dynamics
of these patterns can lead to a high sensitivity to change and to the potential
for irreversible change (e.g., see D'Odorico and Porporato, 2006).
3.1. Deep Time Landscapes
Landscapes, hydrologic processes, ecosystems, and climate have
co-evolved throughout Earth's history and across all spatial
scales.
Early Earth history is nearly as mysterious as the geologic evolution
of other planets. To inform understanding of early Earth, scientists have
limited preserved bedrock, some chemical records, and scant fossil evidence.
Nonetheless, early life clearly interacted with the physical system. The fossil
record suggests that as the evolution of life exploded and proceeded, the
fossil record points to evolutionary inventions that altered how the planet
works. Much is yet to be learned.
OCR for page 85
WATER AND LIFE 85
How have hydrologic processes affected the co-evolution of life and
planet Earth?
Although a finite set of hydrologic processes are involved in Earth's
water cycle, the relative rates and importance of the processes have changed
dramatically throughout Earth's history. These changes corresponded and
contributed to major evolutionary steps in emergence of landscapes and
their ecosystems and the climate system (including the oceans). Three ex-
amples illustrate this point.
Early Earth is a great puzzle. The early Sun was 70 percent as bright
as today (Kasting, 2010), and consequently the Earth should have been a
frozen sphere for nearly one-half of its history. Instead geologic evidence
makes it clear that oceans were present very early, and it is possible that
Earth was even warmer than present. But then, approximately coincident
with the rise of atmospheric oxygen from photosynthesis, the first known
global glaciation occurred approximately 2.4 billion years ago. Since then
Earth has periodically experienced "ice house" conditions of large-scale
glaciations. Numerous hypotheses explain the "faint young sun" puzzle,
but three-dimensional climate simulations that explicitly account for hy-
drologic processes of runoff, storage, and evaporation must be developed
to fully explore them.
Another puzzle for hydrologic science arises from the "snowball Earth"
event about 650 million years ago, during which it has been argued that
much of Earth's ocean and terrestrial surface was covered with ice. This
event is of particular interest because its termination was marked by an
explosion of multicellular life, known as the Cambrian explosion. Why did
Earth's water freeze and then thaw? There was also a significant rise in oxy-
gen after the snowball Earth period ended. Recently it has been proposed
that this rise in oxygen resulted from accelerated erosion of high mountains
that flushed nutrients to the sea, greatly increasing photosynthesis. These
events highlight the need for further understanding of the relationships
among climate, topography, hydrology, erosion, and ecosystems.
Perhaps the most radical change in hydrologic processes after the emer-
gence of continental landmasses was the evolution of land plants and the
subsequent diversification of terrestrial life. Terrestrial organisms perma-
nently changed hydrologic processes in at least two fundamental ways.
With the development of stomata, about 400 million years ago, plants
could lift water from deep soil reservoirs without desiccation and transfer it
back to the atmosphere, greatly increasing this mass flux (Figure 3-1). This
acceleration of the hydrologic cycle profoundly affected climate processes.
The spread of vegetation and fauna across landscapes led to more intense
weathering and the development of deeper conductive soils, which must
have systematically increased near-surface storage of water and brought
OCR for page 86
86 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
FIGURE 3-1 The evolution of roots, vascular tissue, and stomata through the De-
vonian Period permitted plants to gain access to water stored deeper in the soil and
to transfer it into the atmosphere. This increase in transpiration generated increases
in precipitation over continents, significantly influencing the water cycle and climate
processes. SOURCE: Reprinted, with permission, from Berry et al. (2010). © 2010
R02116
by Elsevier.
Figure 3-1
bitmapped, uneditable
about a dominance of subsurface flow to river channels for much of the
global runoff that reaches channels. Erosion processes and rates also must
have changed, altering landscape evolution. All current landscapes and
ecosystems emerged under this new, biologically mediated hydrologic sys-
tem. More focused research is needed to understand how the hydrologic
feedbacks between organisms and the physical environment shape the co-
evolution of landscapes, and the extent to which vegetation controls hydrol-
ogy at local and regional scales.
OCR for page 87
WATER AND LIFE 87
3.2. The Hydrology of Terrestrial Ecosystems
Hydrology plays a critical role in driving the environmental pat-
terns that exist and evolve on Earth.
Dominant vegetation type or biomes, standing biomass, and annual
primary production (energy fixation from sunlight by plants) vary with
mean annual precipitation. Within biomes, inter- and intra-annual varia-
tion in the timing and amount of precipitation can exert strong control on
primary productivity and vegetation structure. In systems where vegetation
growth is strongly water limited, small changes in the magnitude and timing
of precipitation can lead to dramatic changes in vegetation composition and
productivity. These interactions are less obvious, but no less important, in
more humid, nutrient-limited ecosystems, where the amount, timing, and
routing of water supply can substantially affect nutrient availability and
soil carbon storage.
Across the full gradient of annual precipitation, the key link between
the biosphere and the hydrosphere is soil moisture. Soil moisture fuels
bare-earth evaporation and plant transpiration. Yet, evapotranspiration and
soil moisture dynamics are the two primary unknowns in water budgets of
landscapes. Soil moisture dynamics drive soil respiration (the flux of carbon
dioxide, CO2, from the soil to the atmosphere) and are a significant con-
tributor to the global CO2 budget, but soil moisture has proven extremely
difficult to predict. Climate models that find agreement in predicting future
temperatures and rainfall may nonetheless generate widely different predic-
tions of soil moisture. Reducing this critical uncertainty in understanding
the mechanisms of feedbacks between vegetation and climate is central to
the ability to effectively predict future climate and vegetation dynamics at
local and global scales.
Among the barriers to building meaningful predictions of soil mois-
ture are the challenges to linking small-scale transport processes in plants
and soils with local atmospheric processes. In a review article, Katul et al.
(2007) suggest that two primary barriers limit the progress of soil-plant-
atmosphere interactions mediated primarily by hydrologic fluxes. First,
scientists have limited ability to describe water movement at the very
smallest scales where fine plant roots interact with soil water, where small
tubes (xylem) carry water in plants, and where water diffuses through plant
tissue. Second, once scientists achieve appropriate microscopic descriptions,
the appropriate methods to extrapolate them to larger spatial scales and
longer time scales are lacking (what is known in the field as an "upscaling"
problem). Understanding how water molecules move through soils and
plant tissues and developing the scaling laws necessary for extrapolating
this understanding to ecosystem scales is a challenge.
OCR for page 88
88 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
Hydrologists have made significant progress in understanding how
vegetation responds to and controls local and regional hydrology in arid
and semi-arid climates, where water limitation is a major determinant of
vegetation patterns. Here competition for limited water leads to spatially
structured vegetation, which may simply trace the topographic controls on
water paths. Alternatively, plants themselves may create spatially variable
conditions favorable to their survival by influencing soil characteristics
that alter surface water infiltration, moisture retention, and erosion. Much
remains to be discovered about controls on pattern and process.
Hydrologists, ecologists, and geomorphologists have found common
cause in studying the water mediated interactions among climate, vegeta-
tion, and landscapes within terrestrial ecosystems. These ecosystems include
the entire food web of animal life they support, but the dependency of ani-
mals on precipitation is often indirect and tied to precipitation variability
and seasonality. In water-limited landscapes, correlations between ungulate
populations (for example) and rainfall have been found, but even here
density-dependent interactions, such as competition for food, may obscure
simple climate dependencies (Owen-Smith, 2006). Understanding how the
loss of native species or the addition of invasive species to ecosystems may
alter vegetation and climate is equally important as understanding how
changing climates may affect terrestrial organisms.
How do soil and rock moisture vary across landscapes and in turn drive
biotic, geochemical, erosional, and climatic processes?
Water in unsaturated soils, typically referred to as soil moisture, is
returned to the atmosphere through bare soil evaporation and plant tran-
spiration. This evapotranspiration is approximately 57 percent of the total
land precipitation (Van der Ent et al., 2010), and it uses up about 50 per-
cent of the total solar energy absorbed by the land surface (e.g., Seneviratne
et al., 2010). Soil moisture influences climate as a source of moisture, and
in various ways it influences latent and sensible heat fluxes. Soil moisture
influences water potential gradients (and thus infiltration rates and unsatu-
rated subsurface flow rates), thereby influencing runoff paths and the re-
sulting erosion during storms. Geochemical reactions are partially paced by
water content and associated microbial activity. Soil moisture controls and
is regulated by vegetation; hence, understanding moisture dynamics is cen-
tral to related studies. Soil respiration varies with seasonal moisture in the
soil. Despite the central role that soil moisture plays in Earth surface pro-
cesses and ecosystems, the spatial and temporal dynamics of soil moisture
are poorly documented, and theory predictions have had limited success.
Hydrologists have played a leading role in mapping soil moisture and
developing theory about its distribution across landscapes. Remote sens-
OCR for page 89
WATER AND LIFE 89
ing technology for mapping moisture dynamics, while providing valuable
observations, only penetrates a few centimeters into the ground. Therefore,
field observations and theory remain essential to obtaining estimates of soil
moisture patterns and dynamics. Ground-based technology for mapping
soil moisture to significant depths is advancing and will be important in cre-
ating field data sets to test remote sensing and model-based predictions of
soil moisture dynamics. For example, naturally produced neutrons and their
thermalization are utilized in the Cosmic Soil Moisture Observing System1
(COSMOS) to provide larger footprint measures of soil moisture (Zreda et
al., 2008). Distributed temperature sensing (DTS), in which optical fibers
are "planted" across agricultural fields at varying depths, is now provid-
ing soil moisture estimates using either the natural diurnal heat flux within
the shallow soils, or by actively heating the cable over its entire length to
form an enormous heat dissipation sensor. New approaches for assessing
both the spatial and depth distribution of soil moisture are beginning to
fill the gap between point sensors and remote sensing, yet this major data
gap remains. Climate models are beginning to reach a resolution where
the topographic effects on moisture redistribution can be treated. Large
differences remain in how such models treat the water holding capacity
of soils and how soil moisture regulates evapotranspiration. What are the
effects of topography, geology, and land history on soil moisture patterns
and dynamics?
Field studies of rooting depth of vegetation, direct observation of water
transport in roots, and results from climate modeling all point to the im-
portance of deep water sources (several meters below the ground surface).
Sufficiently deep roots can lift deeper water to near surface soils, increas-
ing moisture availability to shallow roots. In some places, soils are thick
and can provide this deep water, but in others, especially places underlain
by bedrock, the moisture available to plants may reside in the underlying
fractured rock. In seasonally dry, hilly landscapes with thin soils, vegetation
may be sustained by moisture extracted from weathered bedrock beneath
the soil. The importance of this so-called "rock moisture," and the ground-
water that lies beneath it, in providing water to vegetation is relatively
unexplored.
To improve hydrologic, climate, geochemical, and ecological models,
field observations and theory are needed to explain and predict the spatial
variation in the thickness and properties of the soil mantle and the underly-
ing conductivity of weathered rock across landscapes. Climate models rely
on compilations of soil thickness and texture properties extracted from
soil surveys, but as the finer-scale topography of hills and valleys enter
into climate models, soil data spatially mapped onto this topography will
1 See http://cosmos.hwr.arizona.edu/.
OCR for page 90
90 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
be needed. Very little is known quantitatively about this mostly invisible
mantle, especially with regard to the spatial variation in depth of weather-
ing of bedrock across landscapes. Although soil maps can provide guidance
to estimating soil properties, there are no comparable data to estimate the
depth of weathered rock that may serve as a moisture reservoir. Recently
the zone from the canopy top through the soil and down into the underlying
weathered bedrock and groundwater has been referred to as the "Critical
Zone" (Box 3-1). Six Critical Zone Observatory field projects are currently
BOX 3-1
Critical Zone Observatories
In 2001 the National Research Council report Basic Research Opportunities
in Earth Science (NRC, 2001) proposed the term Critical Zone to describe "the
heterogeneous, near-surface environment in which complex interactions involv-
ing rock, soil, water, air, and living organisms regulate the natural habitat and
determine the availability of life-sustaining resources." The report recommended
"integrative studies" of the Critical Zone as one of six "important problem areas
spanning a wide range of future activity in Earth science." This challenge was
embraced by the research community, and by 2007 the National Science Founda-
tion (NSF) funded three Critical Zone Observatories (CZOs) for an initial 5-year
investigation and subsequently added three more in 2009. This effort has inspired
a corresponding program in Europe referred to as SoilTrEC (Soil Transformation
in European Catchments)a and rapidly expanding international collaborations
across the globe.
The CZOs are envisioned as field environmental laboratories to explore
the chemical, physical, and biological processes that shape Earth. The three
main goals are (1) to develop a unifying theoretical framework of Critical Zone
evolution that integrates new understanding of coupled hydrological, geochemi-
cal, geomorphological, and biological processes; (2) to develop coupled systems
models to predict how the Critical Zone is driven by anthropogenic effects, climate,
and tectonics; and (3) to develop an integrated data-measurement framework to
document processes and test hypotheses.b
The six established CZOs are located in (1) the Southern Sierra (California),
(2) the Jemez River and SantaCatalina Mountains (New Mexico and Arizona),
(3) Boulder Creek (Colorado), (4) the Susquehanna Shale Hills (Pennsylvania),
(5) the Christina River basin (Delaware and Pennsylvania), and (6) the Luquillo
Mountains (Puerto Rico). Each of these CZOs is managed by different multidisci-
plinary teams, and the mix of sites offers opportunities to explore different aspects
of the Critical Zone. These CZOs form a national network and meet, coordinate,
and collaborate as a shared program.
a See http://www.soiltrec.eu/.
b See http://criticalzone.org/.
OCR for page 91
WATER AND LIFE 91
funded by the National Science Foundation (NSF) across the United States,
wherein hydrologic processes and development of the soil and weathered
bedrock zone are studied intensively in conjunction with biogeochemical
and geomorphic processes. How can results of local mechanistic studies
of soil and weathered bedrock be upscaled to watershed hydrologic and
regional climate models?
How have vegetation assemblages, landscapes, climate, and the
hydrologic systems that drive them co-evolved?
We live on a patterned Earth. Across the planet, vegetation is banded
into distinct bioclimatic zones of differing dominant vegetation. Within a
zone, topography and geology can drive moisture, slope stability, and soil
and mineralogical differences that structures vegetation assemblages. Sys-
tematic differences emerge with respect to orientation of hills (i.e., aspect),
with, for example, more forested, moisture-demanding vegetation facing
north in northern latitudes (Figure 3-2). The visible co-organization of
vegetation and topography, in which topographically structured moisture
availability drives dominant vegetation assemblages (most prominent in
water-limited environments), has attracted many researchers. These pat-
terns are being examined at least three ways: (1) how vegetation patterns
are driven by water stress, (2) how vegetation patterns may be used to docu-
ment water availability and transpiration, and (3) how vegetation patterns
may, in turn, affect hydrologic and erosional processes, thereby altering soil
and topographic evolution.
Highly structured vegetation patterns also occur that do not correspond
to strong topographic and soil control but, instead, are argued to be emer-
gent features that arise from competing effects of facilitative and competi-
tive processes within the vegetation community. In some cases remarkable
vegetation patterns of repeating bands, spots, and mosaics develop. In
water-limited environments where overland flow occurs, vegetation cluster-
ing can have such facilitative effects as inducing water infiltration, trapping
nutrients, providing shade, and protecting against herbivory. A considerable
body of theory has been advanced to predict these self-organized emer-
gent vegetation patterns. The challenge is to provide definitive tests of the
theory. The vegetation patterns themselves--without extensive field work
to confirm driving mechanisms--may reveal very little about their origin
and therefore provide an insufficient test of theories.
A frontier area of research is to expand these inquiries into humid
regions where spatially structured water availability is less apparent, but
vegetation patterns still form (Rodriguez-Iturbe et al., 2007). Fire pattern
and history may strongly dictate vegetation patterns in both arid and humid
landscapes, either emphasizing or obscuring water availability differences
OCR for page 92
92 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
FIGURE 3-2 Digital image of the Gabilan mesa area, south of San Francisco, Cali-
fornia, showing the strong aspect control on forest distribution. Image is derived
from airborne laser swath mapping data collected by the National Center for Air-
borne Laser Mapping (NCALM) and then colorized and shaded to reveal patterns
of vegetation and topography. The distance between each valley is approximately
R02116
160 m. SOURCE: Reprinted, with permission, from Ionut Iordache, NCALM, and
Whipple (2009). © 2009 by Nature Figure 3-2
Publishing Group.
bitmapped, uneditable
OCR for page 93
WATER AND LIFE 93
and other disturbances (e.g., extreme storms, grazing, insect outbreaks, and
invading species) can take significant roles. The inclusion of multispecies,
food-web, and disturbance-driven processes in coupled models of climate,
hydrologic, and vegetation pattern development is an area of expanding
and exciting research. What are the hydroecological interactions that rein-
force the spatial patterns across diverse landscapes and varying hydrocli-
mate regimes?
All landscapes and their ecosystems have experienced climate change.
Some systems may currently be legacies of a previous climate state under
which they became established and now exist with limited resilience to
further change. Ecosystems, hydrologic processes, and regional climate
can co-evolve to create a self-sustaining system, but one that if disturbed
may not recover. Perhaps the most important such system on Earth is the
Amazon rainforest. A significant fraction of all terrestrial evaporation (and
transpiration) is returned as precipitation over land (e.g., Van der Ent et
al., 2010). Some argue that wholesale cutting of the forest or the effects
of global warming could disrupt the self-sustaining hydrologic cycle of the
Amazon, leading to widespread soil drying and a shift from mesic (having
a moderate supply of water) forests to drier grasslands. Scientists need to
understand how modern hydrosphere-biosphere feedbacks, like those in the
Amazon, have evolved to maintain current vegetation patterns in order to
anticipate future states. How will vegetation communities and their regional
climate co-evolve with climate variability and change?
Landscapes evolve as channels erode down, steepening adjacent hill-
slopes, which, through this connection, may eventually develop a form that
erodes at a rate similar to that of the channel. The competition of advective
processes driven by runoff (which tend to predominate in channels) and
diffusive processes (which tend to predominate on hillslopes) can lead to a
regular ridge-and-valley topography with distinct wavelengths (Figure 3-2).
Nearly all landscapes evolve under a biota mantle, yet explicit account-
ing for the effects of vegetation (or the assemblages of biota in the soil)
in geomorphic models is just beginning. Do topography, vegetation (and
their animal ecosystems), and the hydrologic processes that connect them
co-organize over geomorphic time scales?
How can scientists predict abrupt change in terrestrial ecosystems?
Of great concern is the possibility that future state changes may be irre-
versible, and that the approach to state changes will be nonlinear or abrupt
and thus very difficult to predict. The tight feedbacks between vegetation
and climate set the stage for the potential for rapid transitions in vegeta-
tion dynamics. As global and regional climate changes, vegetation will both
respond to and affect the climatic regime. Global climate models predict
OCR for page 112
112 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
BOX 3-3
Mississippi Delta Restoration
The Mississippi River Delta has lost about one-third of its original wetland
since European settlement of North America. This wetland provides many ecosys-
tem services, most notably freshwater habitat and storm surge protection. At the
current rate of land loss and shoreline migration, it is estimated that New Orleans
will be exposed to the open sea by 2090 (Fischetti, 2001).
The reduction of sediment supply to the delta due to dam construction in the
Mississippi watershed has contributed to this land loss. But presently the main
cause of land loss is the confinement of the river to levees, which have created
an efficient "pipeline" of sediment (and nutrients) directly out to the Gulf of Mexico.
The levees prevent the river spilling into adjacent coastal basins, where sediment
deposition would provide the mineral base for wetlands vegetation growth and
thus sustain the delta level. Without this resupply the coastal wetlands drown
because of natural and anthropogenic (due to hydrocarbon extraction) subsid-
ence and sea-level rise. The final result is open water with no remaining wetlands
ecosystem services.
Engineered avulsions, in which river flood flow is directed into coastal basins,
had been proposed, but doubts were raised about sufficient sediment supply, the
high rates of subsidence, and the anticipated effects of climate change induced
sea-level rise. Figure 3-8 shows an example, however, where numerical model-
ing performed as part of the delta dynamics integrated research project of the
National Center for Earth Surface Dynamics demonstrates the possibility of land
building. This model suggests that effective land building can be done without
threatening navigation or demanding a large fraction of the Mississippi flood flow.
Basic research is needed to guide such a major restoration project. Recently
NSF funded a "Delta Dynamics Collaboratory" that will support an intensive field
observatory (the Wax Lake Delta, a recent, actively growing delta about 100
km west of the main Mississippi delta) and a modeling activities center associ-
ated with the NSF supported Community Surface Dynamics Modeling System
(CSDMS) at the University of Colorado. The interaction of sediment supply and
wetland ecosystems dynamics will be a primary focus.
hydrologic change (climate induced or managed) prevent or exacerbate the
spread of invasive species in wetlands? How will the critically important
role of wetlands in global carbon cycling change as a result of climate
change or direct hydrologic alteration?
Although many restoration efforts are under way, there is considerable
controversy about their effectiveness. It is argued that constructed wetlands
are not a substitute for naturally formed ones, because they fail to perform
OCR for page 113
WATER AND LIFE 113
FIGURE 3-8 View of the delta of the lower Mississippi River below New Orleans, showing
predictions of the new land (delta surface) that could be built over 100 years starting from
2010. Two diversions are considered: Barataria Bay and Breton Sound. The calculation is
based on a "base case" scenario: a subsidence rate of 5 mm per year and sea-level rise rate
of 2 mm per year. The inset shows results for a "best case," subsidence rate of 1 mm per year
and sea level rise rate of zero mm per year, and a "worst case," with corresponding values of
10 and 4 mm per year. For the sake of clarity, land losses in the part of the deltaic wetlands
not subject to diversion are not estimated or shown. SOURCE: Reprinted, with permission,
from Kim et al. (2009). © 2009 by the American Geophysical Union.
the same hydrological, ecological, and biogeochemical functions. Monitor-
ing the effectiveness of restoration (and redesign) of wetlands is typically
not included in projects, so the opportunity to learn and make adaptive
management decisions is limited. Redesign may be the more common op-
tion for large wetlands. In this case, the original wetlands system cannot
be reconstituted because of permanent changes in land use and hydrologic
routing (e.g., through dams, diversions, and drains). California's San Fran-
OCR for page 114
114 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
cisco Bay Delta Estuary, the Florida Everglades, and the lower Mississippi
River wetland systems are prime examples of systems with strong con-
straints, multiple goals, and a complex community of users. Although
exceptionally to produce challenging, coupled models of hydrological, eco-
logical, geochemical, and geomorphological processes are needed to enable
communities to make reasonable comparisons about the costs and benefits
of alternative management decisions.
Wetlands research presents a great opportunity to explore and discover
the subtle linkages among water level (flow, duration, and frequency),
vegetation establishment and growth, soil chemical evolution, aquatic ver-
tebrate and invertebrate dynamics, and morphologic evolution of subdued
and emergence landforms. Wetlands produce distinct landforms, vegetation
patterns, and ecological interactions that invite model development--and
much work has been done to explore essential controls on wetlands at-
tributes. Nonetheless, wetlands, especially large lowland systems, remain
challenging to study because of their size, access difficulties, and the dis-
persed nature of ecologic and hydrologic processes (Harvey et al., 2009).
Remote sensing data (including current and future satellites for mapping
water level and storage) and high-resolution topographic data (of ground
and water surfaces), coupled with process oriented field studies, can reveal
underlying mechanisms and processes. What are the key controls on large
wetland system function and services? What are the minimum features re-
quired to build mechanistic models linking ecosystem, biogeochemical, and
hydrologic processes that will increase the efficacy of wetlands restoration?
Can wetlands be constructed, restored, or redesigned to provide resilience
to the consequences of global change?
What will make river restoration work?
In the United States and throughout the world, restoration of rivers and
streams is an increasingly common approach to managing freshwaters. This
trend reflects a growing awareness of river degradation and societal desires
for waterways that provide beneficial human uses while sustaining biodi-
versity and ecosystem goods and services. Rivers drain landscapes and thus
their form and dynamics are linked to the cumulative land use activities
across their watersheds. Land use alters the water runoff rate and stream
temperature, sediment supply (size and amount), and water chemistry that
a river receives and passes downstream. Rivers are straightened, dredged,
leveed (preventing access to adjacent floodplains), confined in hardened
banks, stripped of in-channel woody debris and riparian vegetation, di-
verted, and covered. The combined effects of altered flow, sediment, and
nutrients regimes together with altered physical states have led to significant
degradation of river biodiversity and to significant increases in the delivery
OCR for page 115
WATER AND LIFE 115
of pollutants to estuarine and off shore environments. Non-point-source
contamination, channel degradation, and aquatic species loss or decline
are among the most common motivations for undertaking stream restora-
tion (Bernhardt et al., 2007). River restoration projects typically attempt to
reestablish the water and habitat quality of degraded streams using one of
two overarching approaches, either focusing on reestablishing hydrographs
and hydrologic connectivity (hydrologic restoration) or focusing on restor-
ing habitat form (hydromorphological restoration).
Hydrologic restoration includes "reestablishing part of the historic flow
regime, removing levees to recover floodplain functionality, scheduling wa-
ter releases from reservoirs to restore native vegetation and riparian func-
tions, and in some cases even removing flow blockages or reconnecting river
reaches that have been fragmented" (Bernhardt and Palmer, 2011). Dam
removal, in particular, has become a widespread practice, often resulting
from relicensing assessments. A central goal of hydrologic restoration is to
reestablish natural processes such that the river system will create flow and
morphologic dynamics critical to maintaining ecosystems. Hydromorpho-
logical restoration places greater emphasis on increasing channel stability
and in-stream habitat by altering channel form and structure along a river
reach in order to restore biodiversity and ecological function. Commonly
this approach employs introduction of in-channel structures (using boulders
or large woody debris). Whole reaches of channels may be redesigned to
a fixed channel pattern to meet some desired form and assumed function.
But restoration projects have also been designed with the goal of chan-
nels recovering their morphologic dynamics, such as shifting laterally and
thereby creating increased habitat complexity. Reports monitoring the ef-
fectiveness (pre- and post-restoration quantitative sampling) of restoration
outcomes are rare (Bernhardt et al., 2005), but some hydrologic restoration
efforts have been success stories (e.g., Hall et al., 2010). In contrast, there
is limited evidence of demonstrable ecological improvements resulting from
hydromorphological restoration projects (Bernhardt and Palmer, 2011), de-
spite the fact that these types of projects are extremely common worldwide.
Restoration projects are mostly implemented without detailed under-
standing of the watershed context and potential channel morphodynamics.
Typically, limited, if any, ecological field studies precede project imple-
mentation. This is especially true in hydromorphological studies, which
commonly rely on the assumption that adding structures to channels or
redesigning a channel to a particular form will create an ecological benefit.
Restoration projects commonly lack two critical components: (1) an eco-
logical assessment to determine limiting factors to species success in order
to define what restoration would be most effective and (2) analysis of sedi-
ment supply and consequences for restoration strategy (e.g., Rosenfeld et
al., 2010).
OCR for page 116
116 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
Forty-five percent of the nation's assessed rivers are classified as en-
dangered or impaired (EPA, 2000); thus, considerable work is still ahead.
The restoration measures taken will vary greatly and depend on the eco-
system, the river and its watershed, and practical constraints. There is a
need for development of quantitative models that explicitly link hydrologic,
geomorphic, geochemical, and ecologic processes and that enable users
to ask "what if" questions regarding restoration measures in a changing
environment. The large number of river restoration projects offers many
opportunities for testing basic hydrological, biogeochemical, and ecological
understanding. If river sediments and their contaminants are responsible for
downstream ecosystem decline, can restoration measures increase upstream
retention? In a watershed where a large portion of its network of rivers
is degraded and water quality is poor, what are the most effective proce-
dures to restore habitats and aquatic biodiversity? How can key barriers
(physical, chemical, or temperature) to stream biota migration be efficiently
identified and eliminated? What is necessary to restore or redesign a chan-
nel such that it becomes a self-maintaining, morphologically dynamic river
that passes water and sediment and supports a diverse ecosystem? Can
scientists predict fish population dynamics for various restoration measures
and anticipated effects of changing climate and land use? Such questions
will naturally be best answered by interdisciplinary teams of scientists and
engineers; and high-quality hydrologic research should be at the center of
these inquiries.
CONCLUDING REMARKS
In the past 20 years a major shift has occurred in the hydrologic com-
munity to embrace and explore all the ways water and life interact on
Earth. Hydrologists are investigating how interactions that link subsurface,
terrestrial, and freshwater environments are controlled by and contribute
to ecosystem dynamics and the planet's evolving climate. Theory is rapidly
developing, with mathematical models and process-rich numerical schemes
being proposed. Opportunities for basic discovery occur at all scales, from
the micrometer-scale processes surrounding root hairs to the regional and
global scale of water exchange with the atmosphere. These opportunities
require deep understanding of not only physics, chemistry, and biology of
processes, but also natural history and modeling.
If there is one common goal, then it is to gain enough understanding
of these interactions, so that models, whether conceptual, analytical, or
numerical, can provide reliable predictions of the future states of the Earth
system. The more scientists are able to anticipate future states, the better
society will be able to prepare for them or alter what might otherwise
occur. Models are needed to predict consequences of local changes (e.g.,
OCR for page 117
WATER AND LIFE 117
vegetation conversion for energy crops) as well as global changes (e.g.,
increasing CO2 in the atmosphere). It is vital that biotic processes be made
explicit in such models. This advance, however, will reveal the uncertain
future. Ecosystems are dynamic, time dependent, nonlinear, and complex.
Biotic interactions, disease, behavior, and other attributes far different from
purely physical or chemical processes challenge the goal of predictability.
The pursuit of answers to the kinds of questions raised in this report, which
explicitly seek to include life-water interactions, should contribute toward
reaching this goal.
Collaborative field studies are essential, because many questions of
process remain and can only be understood at the scale and richness of
dynamics found in the natural world. Ecological processes introduce dif-
ferent time scales than do hydrologic processes. Successive rainstorms can
provide repeated data sets about how a particular landscape delivers water
as runoff, for example. But that same landscape may undergo progres-
sive changes in vegetation composition, when, for example exotic species
invade, recovery occurs after a fire, or subtle climate shifts change the
competitive edge from some species to others. Such shifts may take years
to decades or longer to occur and feed back on the hydrologic and climate
system. These slow dynamics call for commitments to long term collabora-
tive studies. The development of inexpensive, easily deployed monitoring
devices for ecosystem and hydrologic studies holds the promise of revealing
processes over space and time.
One of the greatest challenges faced by Earth scientists is the poor
knowledge and inaccessibility of the hydrologically active ground beneath
society's feet. In many environments most runoff occurs as subsurface flow.
All moisture that gets pulled back to the atmosphere as transpiration as it
sustains life is extracted from this near-surface region. Documenting sub-
surface processes involves the technical challenges of finding ways of "see-
ing" into the ground and of quantifying the properties of the subsurface,
explored further in the following chapter. This documentation should also
include analysis of subsurface ecologic processes. Observational tools and
models are needed.
Restoration, conservation, and redesign projects to create or regain
desired ecosystem services are occurring across a wide range of scale from
small seasonal ponds to the entire Mississippi River Delta. Done for practi-
cal reasons, these projects are nonetheless invaluable experiments that test
the ability to make predictions. Post project monitoring is a critical tool
to for learning from these projects and making progressive improvements
(NRC, 2006, 2011).
As scientists look ahead, interdisciplinary approaches and perspectives
will be needed to gain enough understanding of the interactions between
water and life to predict the future states of the Earth system. Many of the
OCR for page 118
118 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
research challenges discussed in this chapter exist in the nexus between
hydrologic science, ecology, geomorphology, and soil science. Fostering
the interdisciplinary scientists and science teams best able to address these
research challenges can be ensured by making strategic investments in
providing access to inter- and cross-disciplinary research and training op-
portunities at all stages of professional careers. The magnitude and timing
of water delivery shape biological systems but the quality of water is essen-
tial to all organisms. The next chapter argues that the ability to have clean
water for ecosystems and humans requires understanding, predicting, and
managing water quality.
REFERENCES
ACIA (Arctic Climate Impact Assessment). 2005. Arctic Climate Impact Assessment. New
York: Cambridge University Press. Available online at: http://www.amap.no/acia/ [ac-
cessed September 1, 2011].
Anderson, C. B., G. M. Pastun, M. V. Lencinas, P. K. Wallem, M. C. Moorman, and A. D.
Rosemond. 2009. Do introduced North American beavers Castor canadensis engineer
differently in southern South America? An overview with implications for restoration.
Mammal Review 39(1):33-52.
Bernhardt, E. S., and M. A. Palmer. 2011. River restorationthe fuzzy logic of repairing
reaches to reverse watershed scale degradation. Ecological Applications 21(6):1926-
1931. doi: 10.1890/10-1574.1.
Bernhardt, E. S., M. A. Palmer, J. D. Allan , R. Abell, G. Alexander, S. Brooks, J. Carr, S.
Clayton, C. Dahm, J. Follstad Shah, D. L. Galat, S. Gloss, P. Goodwin, D. H. Hart, B.
Hassett, R. Jenkinson, S. Katz, G. M. Kondolf, P.S. Lake, R. Lave, J. L. Meyer, T. K.
O'Donnell, L. Pagano, B. Powell, and E. Sudduth. 2005. River restoration in the United
States: A national synthesis. Science 308(5722):636-637. doi: 10.1126/science.1109769.
Bernhardt, E. S., E. B. Sudduth, M. A. Palmer, J. D. Allan, J. L. Meyer, G. Alexander, J.
Follastad-Shah, B. Hassett, R. Jenkinson, R. Lave, J. Rumps, and L. Pagano. 2007.
Restoring rivers one reach at a time: Results from a survey of U.S. river restoration prac-
titioners. Restoration Ecology 15(3):482-493. doi: 10.1111/j.1526-100X.2007.00244.x.
Berry, J. A., D. J. Beerling, and P. J. Franks. 2010. Stomata: Key players in the earth
system, past and present. Current Opinions in Plant Biology 13(3):233-240. doi:
10.1016/j.pbi.2010.04.013.
Boyer, S., and S. D. Wratten. 2010. The potential of earthworms to restore ecosystem
services after opencast mining--A review. Basic Applied Ecology 11(3):196-203. doi:
10.1016/j.baae.2009.12.005.
Copeland, C. 2010. Wetlands: An Overview of Issues. Congressional Research Service Re-
ports. Paper 37. Available online at http://digitalcommons.unl.edu/crsdocs/37 [accessed
September 1, 2011].
D'Odorico, P., and A. Porporato (eds.). 2006. Dryland Ecohydrology. Dordrecht, The Neth-
erlands: Springer.
EPA (Environmental Protection Agency). 2000. National Water Quality Inventory. U.S. En-
vironmental Protection Agency Office of Water. EPA-841-R-02-001. Available online at
http://www.epa.gov/305b [accessed January 9, 2012].
Farley, K. A., E. G. Jobbágy, and R. B. Jackson. 2005. Effects of afforestation on water yield: A
global synthesis with implications for policy. Global Change Biology 11(10):1565-1576.
doi: 10.1111/j.1365-2486.2005.01011.x.
OCR for page 119
WATER AND LIFE 119
Fierer, N., M. S. Strickland, D. Liptzin, M. A. Bradford, and C. C. Cleveland. 2009. Global
patterns in belowground communities. Ecology Letters 12(11):1-238-1249. doi:
10.1111/j.1461-0248.2009.01360.x.
Fischetti, M. 2001. Drowning New Orleans. Scientific American 285(4):76-85. doi:10.1038/
scientificamerican1001-76.
Gorski, K., H. V. Winter, J. J. De Leeuw, A. E. Minin, and A. J. Nagelkerke. 2010. Fish spawn-
ing in a large temperate floodplain: The role of flooding and temperature. Freshwater
Biology 55(7):1509-1519.
Hall, A. A., S. B. Rood, and P. S. Higgins. 2010. Resizing a river: A downscaled, seasonal
flow regime promotes riparian restoration. Restoration Ecology 19(3):351-359, doi:
10.1111/j.1526-100X.2009.00581.x.
Harvey, J. W., R. W. Schaffranek, G. B. Noe, L. G. Larsen, D. J. Nowacki, and B. L. O'Conner.
2009. Hydroecological factors governing surface water flow on a low-gradient flood-
plain. Water Resources Research 45:W03421. doi: 10.1029/2008WR007129.
Hinsinger, P., A. G. Bengough, D. Vetterlein, and I. M. Young. 2009. Rhizosphere: Biophysics,
biogeochemistry and ecological relevance. Plant Soil 321(1-2):117-152. doi: 10.1007/
s11104-008-9885-9.
Houghton, R. A. 2007. Balancing the global carbon budget. Annual Review Earth Planetary.
Science 35:313-347. doi: 10.1146/annurev.earth.35.031306.140057.
IPCC (Intergovernmental Panel on Climate Change). 2007. Climate Change 2007: The Physi-
cal Science Basis. Contribution of Working Group I to the Fourth Assessment Report
of the IPCC. S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M.
Tignor, and H. L. Millers, (eds.). Cambridge, UK: Cambridge University Press. 996 pp.
Junk, W. J. 1984. Ecology of the varzea floodplain on the Amazonian whitewater rivers. Pp.
215-244 in The Amazon: Limnology and Landscape Ecology. (H. Sioli, ed.). Dordrecht:
Dr W. Junk Publishers.
Kasting, J. F. 2010. Faint young Sun redux. Nature 464:687-689. doi: 10.1038/464687a.
Katul, G., A. Porporato, and R. Oren. 2007. Stochastic dynamics of plant-water interac-
tions. Annual Review of Ecology, Evolution, and Systematics 38:767-791. doi: 10.1146/
annurev.ecolsys.38.091206.095748.
Kim, W., D. Mohrig, R. Twilley, C. Paola, and G. Parker. 2009. Is it feasible to build new land
in the Mississippi River Delta? Eos 90(42):373-374.
Lavelle, P., T. Decaëns, M. Aubert, S. Barot, M. Blouin, F. Bureau, P. Margerie, P. Moraa, and
J. P. Rossi. 2006. Soil invertebrates and ecosystem services. European Journal of Soil
Biology 42(S1):S3-S15. doi: 10.1016/j.ejsobi.2006.10.002.
Leopold, L. B., and W. B. Langbein. 1962. The Concept of Entropy in Landscape Evolution.
U.S. Geological Survey Professional Paper 500-A. Washington, DC: U.S. Government
Printing Office. 20 pp.
Mack, M. C., M. S. Bret-Harte, T. N. Hollingsworth, R. R. Jandt, E. A. G. Schuur, G. R.
Shaver, and D. L. Verbyla. 2011. Carbon loss from an unprecedented Arctic tundra
wildfire. Nature 475(7357):489-492. doi: 10.1038/nature10283.
Milly, P. C. D., K. A. Dunne, and A.V. Vecchia. 2005. Global pattern of trends in streamflow
and water availability in a changing climate. Nature 438(17):347-350. doi: 10.1038/
nature04312.
Mitsch, W. J., and J. G. Gosselink. 2000. Wetlands, 3rd Ed. Hoboken, NJ: John Wiley and
Sons.
Nicholls, N. 2004. The changing nature of Australian droughts. Climatic Change 63(3):323-
326. doi: 10.1023/B:CLIM.0000018515.46344.6d.
Nielsen, U. N., E. Ayresa, D. H. Wall, and R. D. Bardgett. 2011. Soil biodiversity and carbon
cycling: A review and synthesis of studies examining diversityfunction relationships. Eu-
ropean Journal of Soil Science 62(1):105-116. doi: 10.1111/j.1365-2389.2010.01314.x.
OCR for page 120
120 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
NRC (National Research Council). 2001. Basic Research Opportunities in Earth Science.
Washington, DC: The National Academy Press.
NRC. 2006. Progress Toward Restoring the Everglades: The First Biennial Review. Washing-
ton, DC: The National Academies Press.
NRC. 2009. Summary of a Workshop on Water Issues in the Apalachicola-Chattahoochee-
Flint and Alabama-Coosa-Tallapoosa (ACF-ACT) River Basins. Washington, DC: The
National Academies Press.
NRC. 2011. A Review of the Use of Science and Adaptive Management in California's Draft
Bay Delta Conservation Plan. Washington, DC: The National Academies Press.
Odum, E. P. 1959. Fundamentals of Ecology, 2nd Ed. Philadelphia and London: W.B. Saunders
Co. 546 pp.
Oechel, W. C., S. J. Hasting, G. Vourlitis, M. Jenkins, G. Riechers, and N. Grulke. 1993.
Recent change of Arctic tundra ecosystems from a net carbon dioxide sink to a source.
Nature 361:520-523.
Owen-Smith, N. 2006. Demographic determination of the shape of density dependence
for three African ungulate populations. Ecological Monographs 76(1):93-109. doi:
10.1890/05-0765.
Reddy, K. R., and R. D. DeLaune. 2008. Biogeochemistry of Wetlands: Science and Applica-
tions. Boca Raton, FL: CRC Press.
Rodriguez-Iturbe, I., P. D'Odorico, F. Laio, L. Ridolfi, and S. Tamea. 2007. Challenges
in humid land ecohydrology: Interactions of water table and unsaturated zone with
climate, soil, and vegetation. Water Resources Research 43:W09301. doi: 10.1029/
2007WR006073.
Rosenfeld, J. S., D. Hogan, D. Palm, H. Lundqvist, C. Nilsson, and T. J. Beechie. 2010. Con-
trasting Landscape Influences on Sediment Supply and Stream Restoration Priorities in
Northern Fennoscandia (Sweden and Finland) and Coastal British Columbia. Environ-
mental Management 47(1):28-39. doi:10.1007/s00267-010-9585-0.
Seager, R., A. Tzanova, and J. Nakamura. 2009. Drought in the Southeastern United States:
Causes, variability over the last millennium, and the potential for future hydroclimate
change. Journal of Climate 22:5021-5045. doi: 10.1175/2009JCLI2683.1.
Seneviratne, S. I., T. Corti, E. L. Davin, M. Hirschi, E. B. Jaeger, I. Lehner, B. Orlowsky, and
A. J. Teuling. 2010. Investigating soil moistureclimate interactions in a changing climate:
A review. Earth-Science Reviews 99(3-4):125-161. doi: 10.1016/j.earscirev.2010.02.004.
Smakhtin, V. U. 2001. Low flow hydrology: A review. Journal of Hydrology 240(3-4):147-
186. doi: 10.1016/S0022-1694(00)00340-1.
U.S. Fish and Wildlife Service (USFWS). 2009. Fire Management--North Carolina: L argest
Refuge Fire of 2008 Declared Out. Available online at http://fws.gov/fire/news/nc/evans_
road.shtml.
Van der Ent, R. J., H. H. G. Savenije, B. Schaefli, and S. C. Steele-Dunne. 2010. Origin and
fate of atmospheric moisture over continents. Water Resources Research 46:W09525.
doi: 10.1029/2010WR009127.
Wardle, D. A., R. D. Bardgett, J. N. Klironom, H. Setala, W. H. van der Putten, and D. H.
Wall. 2004. Ecological linkages between aboveground and belowground biota. Science
304(5677):1629-1633. doi: 10.1126/science.1094875.
Whipple, K. X. 2009. Geomorphology: Landscape texture set to scale. Nature 460(7254):
468-469. doi: 10.1038/460468a.
Zhang, Z., M.-H. Wang, J. Hu, and Y.-S. Ho. 2010. A review of published wetland research,
19912008: Ecological engineering and ecosystem restoration. Ecological Engineering
36(8):973-980. doi: 10.1016/j.ecoleng.2010.04.029.
OCR for page 121
WATER AND LIFE 121
Zreda, M., D. Desilets, T. P. A. Ferré, and R. L. Scott. 2008. Measuring soil moisture content
non-invasively at intermediate spatial scale using cosmic-ray neutrons. Geophysical Re-
search Letters 35:L21402. doi:10.1029/2008GL035655.
SUGGESTED READING
Borgogno, F., P. D'Odorico, F. Laio, and L. Ridolfi. 2009. Mathematical models of veg-
etation pattern formation in ecohydrology. Review of Geophysics 47:RG1005. doi:
10.1029/2007RG000256.
Falk, D. A., M. A. Palmer, and J. B. Zedler. 2006. Foundations of Restoration Ecology. Wash-
ington, DC: Island Press.
Jefferson, A., G. E. Grant, S. L. Lewis, and S. T. Lancaster. 2010. Coevolution of hydrology
and topography on a basalt landscape in the Oregon Cascade Range, USA. Earth Surface
Processes and Landforms 35(7):803-816. doi: 10.1002/esp.1976.
Karlsson, J. M., A. Bring, G. D. Peterson, L. J. Gordon, and G. Destouni. 2011. Oppor-
tunities and limitations to detect climate-related regime shifts in inland Arctic ecosys-
tems through eco-hydrological monitoring. Environmental Research Letters 6(1). doi:
10.1088/1748-9326/6/1/014015.
Mori, A. S. 2011. Ecosystem management based on natural disturbances: Hierarchical con-
text and non-equilibrium paradigm. Journal of Applied Ecology 48(2):280-292. doi:
10.1111/j.1365-2664.2010.01956.x.
NRC. 2010. Landscapes on the Edge: New Horizons for Research on Earth's Surface. Wash-
ington, DC: The National Academies Press.
Poff, N. L., J. D. Allan, M. B. Bain, J. R. Karr, K. L. Prestegaard, B. D. Richter, R. E. Sparks,
and J. C. Stromberg. 1997. The natural flow regime: A paradigm for river conservation
and restoration. Bioscience 47(11):769-784.
Reith, F. 2011. Life in the deep subsurface. Geology 39(3):287-288. doi: 10.1130/
focus032011.1.
Schiermeier, Q. 2001. Fears grow over melting permafrost. Nature 409(6822):751. doi:
10.1038/35057477.
Schwinning, S. 2010. The ecohydrology of roots in rocks. Ecohydrology 3(2):238-245. doi:
10.1002/eco.134.
Zedler, J. B., and S. Kercher. 2005. Wetland resources: Status, trends, ecosystem services, and
restorability. Annual Review of Environment and Resources 30:39-74. doi: 10.1146/
annurev.energy.30.050504.144248.
OCR for page 122
