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2
The Water Cycle: An Agent of Change
Water has helped shape our planet to produce the world in which we now
live. Knowing how water has acted throughout Earth's history and how
water cycles function on other planets will broaden our understanding of
how Earth's water cycle functions. This knowledge will allow us to better
predict how human and natural factors will combine to produce the world
we leave to our children and our children's children.
INTRODUCTION
Water plays multiple roles in the evolving physical system of the planet.
It shapes the landscape as rivers and glaciers flow over the surface, waves
break on shorelines, and freeze-thaw cycles crumble rocks. Water influences
the movement of energy through the climate system as a greenhouse gas
absorbing radiation and reemitting it to the surface; a reflector of sunlight
when condensed into clouds, snow, and ice; and a transporter of heat when
evaporating, circulating, and condensing; it influences the distribution of
Earth's gravity field. The distribution of water affects the location and char-
acter of life, the movement of Earth's crust, and even the rotation rate of
the planet. The flow, phase changes, accumulation, and dispersal of water
around the world--the water cycle--vary substantially with time and loca-
tion. Thus, water does not respond passively to physical processes govern-
ing Earth: it is a dynamic agent whose influence is central to processes that
produced today's world and that will affect its evolution into the future.
Human intervention in the water cycle alters water's dynamic role on
the planet. Humans have become major agents alongside nature in the
functioning of the water cycle, through water management and changes in
the land, atmosphere, and ocean that alter natural water processes. Humans
also are altering Earth's climate, which produces further changes in the
water cycle. Hydrologic and related sciences require credible accounting of
water to assess how the water cycle acts and will change. Such accounting
is needed for timely and accurate prediction of natural hazards, including
45
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46 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
"abrupt" or irreversible changes, and relies on basic understanding of in-
teracting processes, that is, understanding how the world works.
Scientific and technological advances now offer exceptional opportu-
nity to develop a comprehensive understanding of water's pervasive activity
throughout the Earth system over periods ranging from early epochs to
the present and future. Although many disciplines can contribute to this
opportunity, hydrologic science plays a central role, as highlighted in this
report. Over the past several decades, the knowledge of Earth as a system
has grown considerably through new observational techniques, analysis
methods, and computing tools, all of which have helped hydrology mature
as a discipline (see Chapter 1). This maturation has given hydrologic sci-
ence a leading role in advancing understanding of the water cycle and the
processes that affect and are affected by it over a range of scales and envi-
ronments. As part of this leadership, hydrologists and engineers have forged
links with closely related disciplines, especially the atmospheric, soil, plant,
and cryospheric sciences (which deal with snow, ice, and frozen ground)
to develop a more comprehensive and coherent view of water as a central
component in Earth's climate system.
Admittedly, how water acts varies significantly across time scales rang-
ing from seconds to decades and longer, and spatial scales from millimeters
to planetary, thus presenting a very complex dynamic picture and a monu-
mental task in monitoring all its storage and transport aspects. However,
advances in observing systems and computing allow use of computational
techniques such as data assimilation that could support development of
this comprehensive view by merging observation sources into a unified,
global portrayal of water with unprecedented temporal and spatial detail.
New observing systems such as space-based platforms coupled with global
networks of existing observing tools could produce global, real-time views
of where water is and where it is going, in all its phases (Gao et al., 2010;
Wong et al., 2011). The sensor revolution is in its nascent stages, but for
the first time the promise of closing the global and regional water budgets
with direct measurement of flux and storage components may be just within
reach. Opportunities also exist to extend this portrayal of water into the
future. Scientific advances have contributed to progress in understand-
ing the interactions between water and other Earth system components,
leading to modeling of the water cycle as part of a comprehensive Earth
system simulation system. Yet an opportunity exists for models providing
plausible scenarios of the impact of climate change and land use change on
the regional water cycle.
Stepping away from the contemporary and future perspective, exami-
nation of water flow and storage during periods ranging over the past de-
cades, centuries, millennia, and into deep geologic time offers opportunity
to understand how Earth's water cycle evolved to its present state. Equally
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THE WATER CYCLE: AN AGENT OF CHANGE 47
important, understanding the range, frequency, and rate of change of past
behavior provides a baseline of natural variability as well as a means to
gauge the impact of humans on the water cycle, which can be helpful now
and in forecasting the future. New methods of data acquisition and refine-
ment of existing techniques yield an expanding set of paleoclimate data
that shed new light on past hydrology. Further advances could provide
longer views with yet finer temporal and spatial detail. Just as modeling has
shed new light on contemporary and potential future water cycle behavior,
modeling past climates guided by advances in paleoclimate reconstruction
can further test the limits of knowledge, as expressed by models, while also
revealing physical insight that complements proxy records.
Stepping away from considering the Earth alone, the understanding
of Earth's water history gains from comparison with alternative planetary
evolution pathways. Advances in planetary science have broadened un-
derstanding of where and how planets and moons form, both in the solar
system and beyond. Awareness of the emergence and evolution within
the solar system of "water cycles" based on water or other condensing
constituents (e.g., methane) provides unparalleled opportunity to reveal
cosmological principles that guided the formation of Earth and its water
cycle. The discovery of terrestrial extrasolar planets potentially broadens
that perspective even more.
RESEARCH OPPORTUNITIES
In this section, the committee discusses several research opportunities
for the hydrologic sciences and presents underlying research questions.
The research opportunities are ordered as in the Introduction above, fo-
cusing on challenges involving human influences and on contemporary
and near-future hydrology (i.e., water-process measurements and model-
ing), then considering challenges and opportunities involving hydroclimatic
variability, from abrupt changes to long-term variability, including the
paleoclimate perspective, and finally considering the opportunities posed
by exohydrology.
2.1. Human Influences on Water Availability and Distribution
The hydrologic cycle is being perturbed and "replumbed" through
human activities.
The hydrologic cycle has been described and depicted in a variety of
different ways, most frequently as a natural system, even though it has long
been altered by human activity. Alterations of the hydrologic cycle for ag-
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48 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
riculture, transportation, and domestic and industrial needs have amplified
dramatically over the past century, along with building of infrastructure in
the form of dams and canals (Figure 2-1). In particular, population growth
and development in arid and semi-arid regions where surface water is scarce
have led to large reservoirs, diversion projects, and intensive groundwater
pumping. These practices have had major impacts on surface and ground-
water supplies, which have in turn impacted the downstream systems reli-
ant on these supplies.
FIGURE 2-1 Alteration of the water cycle in the form of dams and canals is em-
bedded deeply within the modern global water cycle. In the United States, the geo-
graphical extent of dams and reservoirs has increased dramatically over the past 200
years (top). This trend extends to developed parts of the world, with river regulation
expanding rapidly in the 20th century (middle). As a result, human engineering
and water use distort hydrographs (bottom). The left-hand graph is a purposeful
interbasin transfer for hydroelectric production, the middle is the Aswan High Dam
impact, and the right-hand is flow depletion
R02116 due to cotton production in the Aral
Sea contributing basin. SOURCE: Reprinted, with permission, from Vörösmarty et
al. (2004). © 2004 by the American Figure 2-1 Union.
Geophysical
bitmapped, uneditable
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THE WATER CYCLE: AN AGENT OF CHANGE 49
The hydrologic cycle is altered by not only direct physical alteration,
but also anthropogenic climate change; the most obvious symptom is the
global redistribution of precipitation and the resulting change in surface
water flow (Figure 2-2). In turn, changing the terrestrial branch of the wa-
ter cycle impacts climate by altering the surface energy balance, changing
evapotranspiration and surface reflectance characteristics. The hydrology
of the land surface is affected directly by warming temperatures due to
changes in snow and ice cover and shifts in vegetation patterns. The causes
of hydrologic replumbing (land use change, hydrologic storage, climate
change, etc.) are not independent and can yield compounding and cascading
effects. For example, dam construction in arid regions impacts downstream
hydrology and ecosystems, and additional stresses due to climate change
challenge dam operations that strive to meet competing needs and further
impact conditions downstream.
In addition to scientific issues, sociopolitical issues often take center
stage. As an example, picture a semi-arid area of urban growth with limited
water supply. Historically, water in many of these regions has largely sup-
ported agriculture, but in recent decades, the water needs of urban centers
have become more dominant. When this shift in water demand involves
transfer of water rights, tension between urban and rural areas can impact
FIGURE 2-2 Average aggregate (based on seven upper Rio Grande basin tributar-
ies) streamflow by month for six climate change scenarios. Three climate change
models represent the range of possible climate outcomes for New Mexico (wet,
dry, and middle) in 2030 and 2080. These projections illustrate that peak flow
and total streamflow decline for all climate scenarios in this basin. In 2080, there
is a pronounced shift in the peak runoff month, by about 30 days. SOURCE: Re-
printed, with permission, from Hurd and Coonrod (2008). © 2008 New Mexico
State University.
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50 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
the amount of water available for agricultural communities. Infrastructure
put into place to supply water for growing urban centers can impact sur-
rounding ecosystems, creating an additional layer of tension. Finally, the
impact of climate change introduces additional stresses on the hydrologic
resources of the area.
In recognition of the human influence on the Earth system and in
the context of Earth history, many scientists now call present times the
"Anthropocene." The traditional concept of the hydrologic cycle should
perhaps by revised to consider humans as a significant and recognizable
component. Research to further understand the human component of the
water cycle, i.e., human-induced replumbing from both climate change and
land use change, is recommended. This understanding is critical to provid-
ing and maintaining water supplies for humans and ecosystems.
How will water distribution and availability change because of
hydrologic replumbing?
Water-related infrastructure has allowed many parts of the world to
flourish, but at a cost to the natural environment and with growing and
unintended impacts on society and ecosystems (Box 2-1). The relationship
between human consumptive use and available water is not fully under-
stood, yet this information is essential to understand how distribution
and availability will change in the near and distant future. The hydrologic
community can shed light on this relationship by probing how replumbing
perturbs hydrologic fluxes. What are the impacts of groundwater over-
draft on the surrounding hydrologic regime? Finding answers to questions
such as this one will further the larger, societal goals of encouraging the
best conservation behaviors and pursuing water-efficient products, both of
which should be accomplished with the best possible scientific information
to assist fair, legal, and scientifically sound decision making.
Conservation measures have been increasingly applied as water avail-
ability has become more limited, but even measures designed to conserve
water can have downstream impacts. In some cases, agricultural return
flows have become an important source of water, as exemplified well in the
Cienega de Santa Clara, an open-water wetland in northern Mexico, which
is supported by the return flows from the lower Colorado River basin. The
Cienega is threatened by the Yuma Desalting Plant through which return
flows would be diverted to the plant for desalination. Thus, actions that
change the natural hydrologic system often have complex and interacting
impacts and can create competition for highly limited available water. These
include actions with direct impacts on the water cycle, such as changes in
water use, as well as those with indirect impacts, such as forest clearing.
What are the downstream consequences of replumbing in terms of the
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THE WATER CYCLE: AN AGENT OF CHANGE 51
amount and rate of flow and seasonality? What are the repercussions on
both society and natural environments?
One recent focus has been on what is commonly called the "water-
energy nexus" in which the human need for water is linked to energy just as
tightly is the human need for energy linked to water. The procurement and
delivery of water often requires energy for pumping, transport, desalina-
tion, and treatment. Likewise, almost all sources of energy, except perhaps
wind, require water for some aspect of production (e.g., extraction, cool-
ing, or conveyance), and many energy sources use considerable amounts
of water. Of the freshwater used by the United States, 39 percent is used
for electricity from fossil fuels and nuclear energy, and of that, as of 1995,
71 percent of that amount was used solely for the generation of fossil-fuel
electricity (Solley et al., 1998). Oil shale and gas production, along with
mining, have used a smaller portion of the freshwater in the United States,
at 5 percent of the total withdrawn from surface and groundwater supplies.
The increased use in the United States of biofuels, often touted as "green"
energy sources (Box 2-2), provides another example. These uses highlight
how increasing demands for energy correspond to increasing demands for
water. The age of "separate but equal" resource planning for water and
energy resources has passed--water is withdrawn and consumed during the
life cycle of almost every energy source. What are the impacts of energy-
related replumbing on water distribution and availability?
How will climate change influence the delivery of moisture (i.e., the
severity, duration, and extent of droughts and floods)?
Climate change is expected to impact key hydrologic fluxes, most nota-
bly precipitation, which translates to impacts on the severity, duration, and
extent of droughts and floods. A clear picture of the manifestation of cli-
mate change in floods and droughts has yet to emerge. Of course, increased
vulnerability to hydroclimate extremes may be exacerbated by social and
political factors, and having better scientific information may be of only
limited value. For example, encroachment of construction in floodplains
is a primary cause of increased damages (e.g., Pinter, 2005). Research op-
portunities exist, challenging the hydrologic community to provide better
scientific information upon which social-political action will be required.
Flooding in the United States is linked to diverse regional climatologies
of heavy rainfall. Extratropical cyclones, "atmospheric rivers" (Leung and
Qian, 2009), rain on snow events, and convective storms are some of the
most important flood agents in the western United States. Tropical cyclones,
warm-season thunderstorm systems, and cold-season extratropical cyclones
play important roles in the flood hydrology of the eastern United States
(Smith et al., 2011), with their relative importance strongly dependent on
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52 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
BOX 2-1
Is the North China Plain Running Out of Groundwater?
Groundwater overexploitation or persistent aquifer storage depletion is a
worldwide phenomenon (Konikow and Kendy, 2005). Restricting their analysis
to subhumid to arid areas, Wada et al. (2010) estimated that the total global
groundwater depletion more than doubled from 1960 to 2000. The confluence
of a multitude of factors, including rapid economic development, high population
density, and climate change, makes the North China Plain (NCP) a compelling
case study of a groundwater resource in peril (Zheng et al., 2010). The NCP
refers to a relatively flat, low-lying alluvial plain in eastern China with a total area
of 140,000 km2. It is home to the capital city Beijing and several other large cities
including Tianjin and Shijiazhuang. Approximately 130 million people now live
within the administrative borders of the NCP. With a population density of about
900 people per km2, the NCP is among the most densely populated regions in
the world. The NCP is also critically important to Chinese economy, contributing
about 12 percent of China's gross domestic product and producing more than 10
percent of China's total grain output.
The amount of exploitable water resource per capita in the NCP is in the
"absolute water scarcity" category according to the "water stress index" (Falken-
mark et al., 1989). Meanwhile, annual precipitation has steadily decreased by
approximately 100 mm since the 1950s. In recent years, with dwindling surface
water supplies, groundwater has become a primary source of water supply for
the NCP. According to Zheng et al. (2010), more than 70 percent of the NCP's
total water supply comes from groundwater to support the region's agricultural ir-
rigation, industrial expansion, and population growth. The question is, how much
longer can this be sustained?
The NCP sits on an expansive Quaternary aquifer system. The thickness of
the NCP aquifer is tens of meters on the western piedmont areas but increases
to hundreds of meters toward the eastern coastal areas. The NCP aquifer is com-
monly divided into two hydraulically connected units, referred to as the "shallow"
aquifer and "deep" aquifer. During the "predevelopment" period until the 1950s, the
water table of the shallow aquifer was usually no more than 3 m below the land
surface in most of the NCP. Since the 1960s to 1970s, however, ever-increasing
groundwater pumping has caused massive and continuing depletion in the NCP
aquifer. According to the latest data from the China Geological Survey, the maxi-
mum depths to water in the shallow and deep aquifers exceeded 65 m and 110 m,
respectively, in the shallow and deep parts of the NCP aquifer. Since the 1970s,
groundwater levels in many parts of the NCP aquifer have declined at a rate of
more than 1 m annually (Figure 2-3).
More than mere depletion of an invaluable natural resource, the overexploita-
tion of the groundwater resource in the NCP has other severe environmental and
ecological consequences, including dried-up rivers, land subsidence, seawater
intrusion, and water quality deterioration (Figure 2-4). For the NCP's main river,
the Haihe River, the annual runoff to the Bohai Sea has decreased by threefold
since the 1950s. Moreover, much of the surface water has disappeared. More than
4,000 km of various river channels in the NCP have dried up and the total size of
wetlands has decreased to 20 percent of their level in the 1970s. Land subsidence
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THE WATER CYCLE: AN AGENT OF CHANGE 53
has also exerted a heavy toll on the region's economic development, especially
near the major industrial and coastal city of Tianjin where the maximum cumula-
tive amount of land subsidence exceeded 3 m. For the NCP as a whole, a total
area of 60,000 km2 has experienced a cumulative subsidence of 0.2 m or more.
42
40
38
36
Average monthly ground-water elevation
34
(meters above mean sea level)
32
30
28
26
24
22
20
18
16
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
FIGURE 2-3 The long-term decrease in groundwater elevation at an observation well near
Shijiazhuang City, Hebei Province, northern China. SOURCE: China Geological Survey (2009).
R02116
Figure 2-3
vector, editable
FIGURE 2-4 Eco-environmental consequences of groundwater depletion in China: a bridge
over a dried-up river (left); former riverbed used for farming (right). SOURCE: Photos courtesy
of Chunmiao Zheng, University of Alabama.
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54 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
BOX 2-2
Water and Energy: Biofuels in the United States
Motivated by an increasing national interest in energy independence as well
as concerns about the impact of greenhouse gas producing fuels, the United
States has taken legislative steps to encourage the development of technolo-
gies that reduce dependence on these types of fuels. These steps include a new
bioenergy program, which set the goal of developing technologies to generate
biofuels that are price competitive with gasoline or diesel fuels. Although ethanol
production could help achieve the national long-term goal of weaning the nation
off greenhouse gas producing fuels, it comes at a cost to water resources, with
regard to both water availability and water quality. Water is consumed not only in
the production of crops to generate biofuels but also in the refineries that produce
ethanol. Much of the water needed to cultivate biofuels relies on irrigation that
taps limited surface and groundwater supplies, notably the Ogallala aquifer, and
that compete with water supplies already used to support food production (NRC,
2007). An increase in the use of fertilizers to support additional crop yields for
biofuel results in nutrient and pesticide pollution with corresponding impacts on
water quality, including hypoxia, that endanger aquatic ecosystems (NRC, 2007).
The average water consumed in ethanol production, based on data from 19
states that produced ethanol in 2007, is 142 million liters of water for each million
liters of ethanol, but this number is highly variable from region to region, ranging
from 5 to 2,139 million liters (Chiu et al., 2009). The toll on regions where irriga-
tion is necessary is evident. In 2003, a U.S. General Accountability Office survey
indicated that many of the states that currently produce ethanol will experience
water shortages over the next decade. Some of the states most likely to undergo
shortages include those that consume the largest amount of water in the cultiva-
tion of corn and processing of ethanol, i.e., Colorado, Kansas, Oklahoma, and
Wyoming (Chiu et al., 2009).
As of 2008, corn was used to produce more than 95 percent of the U.S. sup-
ply of bioethanol (EPA, 2008). Corn genetics research, water-conserving irrigation
practices, and water pricing could help alleviate water stress in the production of
corn-based ethanol (Chiu et al., 2009; NRC, 2007). Also, alternative sources
of biofuels, such as sugar cane, oil seeds, the nonstarch parts of the corn plant,
grasses, trees, and municipal wastes, may be less water intensive than corn and
are being explored to determine their potential in terms of energy conversion
efficiency and water quality impacts (NRC, 2007). Regardless of the source, the
likely expansion of future production of biofuels has the potential to increase
the demand for water in many parts of the United States (NRC, 2007).
drainage basin scale (Miller, 1990). Mesoscale convective systems have
been the agents of extreme flooding in the central United States, notably
during the Great Flood of 1993 and the Iowa flood of June 2008 (Cole-
man and Budikova, 2010). The Iowa floods were made worse by a heavy
snow year resulting in greater than normal antecedent soil moisture for that
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THE WATER CYCLE: AN AGENT OF CHANGE 55
time of year which prevented the mesoscale precipitation from infiltrating.
How heavy rainfall translates into flood frequency and magnitude requires
substantial hydrologic insight. However, all of these processes leading to
flooding may change as climate change alters the water cycle, so insight
built from years of experience in watersheds is undermined by climate
change and associated changes in heavy rainfall. Furthermore, changes to
the landscape by humans will add to the challenge. Research is needed to
assess how changing rainfall patterns coupled with changing land use can
affect floods and their impact in the future.
Another consequence of climate change is a potential expansion and
further drying of a semicontinuous band of aridity in subtropical latitudes.
Temperatures in all regions are projected to increase, but an intensification
of the equatorial to subtropical circulation, called the Hadley cell circula-
tion, is expected to result in a poleward expansion of the global latitudinal
bands of aridity (Held and Soden, 2006; Lu et al., 2007) and with further
reductions in precipitation. The impacts of this increased aridity will be
felt in regions such as the U.S. Southwest and the Mediterranean region
of Europe, with related impacts on water supplies for human and natural
systems. On a more regional scale, research suggests that the winter-spring
storm track over North America may be retracting poleward earlier in the
season, leading to reduced spring precipitation in the western United States,
a shift consistent with climate change projections under warmer condi-
tions. Also, the moisture variability and occurrence of drought in regional
climates in many parts of the world are strongly influenced by El Nińo/
Southern Oscillation (ENSO), a coupled ocean-atmosphere pattern of cir-
culation in the tropical Pacific Ocean associated with climate in many parts
of the world. In fact, the relationship between precipitation and ENSO is
chaotic in some regions. In the California Sierra Nevada, for example, El
Nińo years are either wet or dry but generally not near the median. But
research results do not agree on the impact of climate change on ENSO,
and therefore this issue remains to be resolved. Planning for future water
resources in these regions should evolve in the face of an anticipated reduc-
tion in precipitation. Research is needed, for example, to determine optimal
measures for water management as precipitation declines.
Additional factors can amplify the impacts of climate change. For
example, depending on location and climate regime, glacier meltwater pro-
vides essential water resources throughout the year, and in some locations
it acts as a supplementary water source in the summer. Glacier loss due to
climate warming and other factors could impact water resources in these
areas, especially semi-arid and arid regions. Although many glaciers have
exhibited recession over recent decades, with the highest retreat exhibited in
glaciers at lower elevations, the rates of retreat can vary widely as regional
factors (precipitation, aerosol concentration, etc.) affect glaciers in addition
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72 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
FIGURE 2-10Reconstructions of snowpack in the northern and central Rocky
Mountains from tree-ring data. The graphs of the April 1 snow water equivalent
(SWE) reconstructions show multiple watershed reconstructions (gray lines) within
each larger region, with the regional average (orange line), smoothed (dark blue
line). For the Northern Rockies and Greater Yellowstone region, the reconstructions
are most reliable after 1376 (dotted vertical line). The 20th-century records of ob-
served April 1 SWE are also plotted for each large region (black lines) and smoothed
R02116 SWE periods highlighted in the full
(cyan line). Shaded intervals show decadal-scale
Figure
paper. The observed and reconstructed SWE2-10
records are plotted as anomalies from
the long-term average. These records indicate that wet conditions in the northern
bitmapped, uneditable
Rockies and Yellowstone region tended to coincide with dry conditions in the Colo-
rado River basin over the past centuries. However, this natural variability at 20- to
50-year time scales has been more synchronous because of late-20th-century warm-
ing, resulting in a decline in snowpack across all three major watersheds. SOURCE:
Reprinted, with permission, from Pederson et al. (2011). ©2011 by the American
Association for the Advancement of Science.
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THE WATER CYCLE: AN AGENT OF CHANGE 73
circulation use land-based proxies that imply moisture transport from remote
locations and have some utility. What types of paleoclimate data can be better
exploited to indicate behavior for regions, such as the oceans, that are rela-
tively data scarce? Notable features of past climate, such as the droughts of
the medieval period, contain information that may be useful for anticipating
the future, but understanding the global climate context for these droughts
would be even more valuable. Can multiproxy records (records from mul-
tiple sources that document climate, such as tree rings, lake sediments, and
ice cores) of past climate, along with climate modeling, be used to simulate
global climatic variability and its drivers during anomalous periods, such
as the medieval period? Much focus has been placed on the tropical Pacific
Ocean variability, and with good reason, but is it possible to develop robust
ensembles of paleoclimate reconstructions of modes of climatic variability in
other parts of the world as well? Finally, how can paleoclimate data be best
used to disentangle the low-frequency natural variability from trends due
to climate change, and can this information be used to inform the range of
hydroclimatic conditions that can be expected in the future?
Hydrologic systems, particularly the large reservoirs that contain water
storage that equals several years of accumulated flow, may have different
time scales of response compared to the combined effects of seasonal,
annual, and multiyear climate variability. Low-frequency variation in hy-
droclimate superimposed on trends in temperature may affect the impact
of drought in such managed hydrologic systems. Understanding the low-
frequency variability and its impacts is critical for managing water supplies
under a variety of scenarios. Can modeling be used to produce ensemble
projections of hydroclimatic variables for use in water resource decision
making? Some use has been made of paleoclimate data in assessing long-
term reservoir operations under a broader range of conditions than that
provided by the gauge records (e.g., Lower Colorado region, Bureau of
Reclamation). How can these applications be expanded to use the low-
frequency information in paleoclimate records, along with projections for
future climate change, to present a plausible range of conditions?
What causes "tipping point" transitions of the climate and what are the
hydrologic implications?
A tipping point with global implications is a transition from what is
now occurring to an entirely new climate state or a point where abrupt
climate change occurs. Earth's climate system has shown some evidence of
regime behavior, most notably the potential to exist in at least two differ-
ent states even with the same solar forcing, such as states with or without
a thermohaline circulation. A shift in the climate regime from one state to
another would have huge implications for many aspects of the hydrologic
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74 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
cycle, including the behavior of the cryosphere, surpluses and deficits in
surface water reservoirs, and the frequency of extreme hydrologic events
such as floods and droughts.
As climate models have become more complex, the spread of their pro-
jections of future climate has tended to widen, suggesting that increasing
the number of processes included in the models reduces the predictability
of the climate system. This also suggests that positive feedback processes
not yet studied or modeled may amplify some fluctuations in the climate
system. If the amplifications are large enough, they could push the climate
system into new modes of behavior or induce an abrupt change. Further-
more, paleoclimate records have revealed past episodes of rapid change to
new climate regimes. There is value in understanding the past frequency
of such "abrupt" episodes and the processes that caused them, but this
requires relatively long records. Refinement of records extending back
hundreds to many thousands of years is needed to provide clarity on how
natural processes, acting alone or in conjunction with human-caused fac-
tors, may yield rapid climate change in the future. Paleoclimate analyses
that document abrupt climate change under natural climate variability
(Overpeck and Cole, 2006) coupled with improved hydroclimate modeling
will provide insights into causes and consequences of climate transitions
and their hydrologic implications. Because models likely do not contain the
feedbacks that trigger tipping points that are documented as abrupt changes
in the paleoclimate record, scientists still lack the information needed to
understand and anticipate tipping point transitions. What modeling im-
provements and paleoclimate data are needed to understand and project
potentially catastrophic abrupt changes in a warming climate?
2.5.Exohydrology
The presence of water in and on planets changes everything--
from deep interior dynamics to the surface evolution of land-
scapes to the potential for life.
The recent exploration of Mars has popularized the idea of "fol-
lowing the water" to look for life on other planets. Life occurs nearly
everywhere on Earth's surface and, surprisingly, microbial life may occur
even deep in Earth's wet underlying bedrock, perhaps as much as 5 km
into the igneous rock underlying the oceans. There may even be more
biomass in this deep rock reservoir than in the overlying oceans and on
the entire land surface.
Of course Earth is a water planet, with 71 percent of its surface area
in oceans. Earth is blanketed in a watery atmosphere and washed by rain-
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THE WATER CYCLE: AN AGENT OF CHANGE 75
storms and snowmelt. What is less appreciated is that the dynamics and
composition of the entire solid planet are affected by the distribution and
abundance of water. There is a deep water cycle. The motion of mobile
plates of crust that collide, creating mountains, and separate, making ocean
basins, depends on water in the underlying mantle (with some of that wa-
ter coming from subduction of wet ocean slabs). Water deep in the planet
affects its chemical evolution and its internal dynamics. The abundance
of granite on Earth records the extensive mixing of water with basalt and
other rocks. The nature of volcanic eruptions and the movement of faults
are strongly influenced by water. Ultimately, water weathers bedrock, and
erodes, transports, and deposits mass, some of which is subducted with the
ocean floors and enters the mantle. Hence, the water cycle, including the
deep water cycle, strongly influences the composition and dynamics of the
planet and likely the same is true on other water-rich planets (Marcy, 2009).
When society explores other planets, then, a key goal is to quantify the
abundance and dynamics of water, not only to determine the possibility of
life elsewhere but also to understand how the entire planet operates. The
recent discovery of planets beyond the solar system has led to new models
of the positioning of planets and their size and composition relative to
their sun. The presence of water is central to predicting the composition
and dynamics of these planets, as well as to the potential for "habitability."
The study of hydrologic processes on other planets could be termed
"exohydrology" and it is only just beginning. As a sign of the current im-
portance of this topic, the American Geophysical Union convened a session
at the fall 2011 meeting titled "Follow the Water: Insights into the Hydrol-
ogy of Solar System Bodies." There is also a literature developing on exo-
hydrology (e.g., Andrews-Hanna and Lewis, 2011). Although exohydrology
is necessary to understand the evolution and climate of other planets, it
also offers a test of the understanding of how Earth works. Abundant new
imagery has presented startling observations of river channels, alluvial fans,
gullies, and deltas on a planet where surface liquid water is currently absent
(Mars) and channels, lakes, and rain driven by condensing and evapo-
rating methane (Titan). The committee suggests two important research
questions that focus on surface water processes. These questions present
challenges that are ripe for interdisciplinary research between hydrologic
scientists, paleohydrologists, planetary scientists, and geomorphologists in
exohydrology.
What are the definitive signatures of rain and surface water transport on
a planetary surface?
Is there is a unique relationship between surface morphologic features
and the mechanism that formed them, and if so can hydrologists specifi-
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76 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
cally assign a role to water? On Mars, channels that originate near drainage
divides (the tops of ridges) suggest that, in the past, the planet had an atmo-
sphere capable of producing precipitation (rain or snow). Can geomorphic
features give us such unique interpretations? For example, how do hydrolo-
gists distinguish gullies formed by dry avalanches versus wet debris flows
versus bedload transport in surface water flows? Early in the manned ex-
ploration of Earth's moon, satellite imagery revealed sinuous channels (also
called rills). These channels are broadly distributed across the Moon and
show morphologic features quite similar to river channels found on Earth.
The first papers on these new observations proposed that they were possibly
formed by meltwater from permafrost. Subsequent landings on the moon
revealed that channels were formed by flowing lava. Sinuous rills, which
possess morphology similar to river channels, have been mapped on Venus,
likely formed there by flowing lava. On Titan sinuous channels and valley
networks show great resemblance to features formed by flowing water, but
in this case scientists can be certain that the fluid is not water but most
likely methane. Despite the abundance of features on Mars that bear very
strong resemblance to terrestrial water-driven landforms such as outburst
channels, alluvial fans, and deltas, debate continues about the abundance,
origin (rain, snowmelt, or spring flow), and necessity of surface water.
What is the role of water in creating specific landforms? Although
scientists have the advantage at times of seeing geomorphic processes oc-
curring on Earth, they more often have only the resulting morphology to
interpret. Hence this research, while motivated by planetary questions, also
has bearing on the understanding of landforms and what they reveal about
processes. What are the distinguishing metrics and mechanistic theory that
can yield these insights?
Is it possible to estimate the magnitude, duration, and frequency of
surface waters (river channel flow, springs, lakes, and oceans) from
morphologic evidence alone?
This question, which emerged with regard to Mars after the discovery
of so many compelling, potentially water-driven features, applies equally
to Earth and other planets. Perhaps the clearest example is the problem of
how to extract such information about flow from a simple river channel.
If scientists had data on channel plan form, cross-section, slope, and even
bed material grain size (much more difficult to obtain on other planets),
then what can be said about the flows that the channel experiences? Sedi-
mentologists viewing preserved channels in the stratigraphic record push
even further and ask what can be said about the drainage area and sedi-
ment supply the channel carried. These questions are central to the field of
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THE WATER CYCLE: AN AGENT OF CHANGE 77
paleohydrology and, in general, to the understanding of the relationship
between flow characteristics and channel morphology.
With sufficient topographic and grain size information, a calculation of
the flow that just fills a channel (bankfull flow) can be made with reason-
able accuracy. This is widely practiced on terrestrial channels and on Mars.
But what clues are there about how long the bankfull flow lasts, how often
it occurs, how often much larger flows occur, and whether there could be
sustained low flow? Empirical studies of direct measurements of terrestrial
channels provide some data. How can such findings be extrapolated to
other planets and, importantly, to other ungauged channels on Earth? The
prediction of flows in ungauged basins has generally relied on some mixture
of empirical characterization of regional runoff characteristics and quanti-
tative measures of basin properties (e.g., channel network structure). These
methods typically require data on precipitation, whereas on other planets,
the question being asked is, from channel morphology (and perhaps basin
characteristics) can scientists estimate the amount of precipitation needed
to create that morphology?
These questions point to another gap in the knowledge of terrestrial
hydrogeomorphic processes. Is there a climatic signature in river morphol-
ogy? For example, other factors being equal, will channels primarily fed by
snowmelt differ significantly from those experiencing only rare monsoonal
runoff events? This question has received little attention, yet lies at the heart
of understanding of how river morphology records hydrologic processes.
Progress on these questions about terrestrial hydrology and morphology
will greatly inform exohydrology studies.
CONCLUDING REMARKS
The aspects of the water cycle highlighted in this chapter present nu-
merous scientific challenges, from understanding the near-surface flux terms
foundational to Earth's metabolism and the global-scale natural drivers of
hydroclimatic variability, to extending lessons learned on Earth throughout
the universe. These are among the most important of the wide range of wa-
ter cycle topics. Recent advances in observing, measuring, analysis methods,
and modeling make addressing many of these challenges attainable. Pair-
ing these new advances with scientific challenges is a critical component of
accounting for and predicting the human footprint on Earth's water cycle.
For example, the coupling of modeling and observational advances could
allow for continuous, regionally detailed monitoring and prediction of
water-cycle dynamics, with forecasting times and accuracy extending ever
further into the future, potentially on interannual and longer time scales.
Modeling advances could also allow for simulation of alternative scenarios
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78 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
for water's future in a climate system that includes socioeconomic as well
as natural controls on its movement.
Finding solutions to the questions presented here requires research
along the traditional lines of hydrologic sciences to, for example, promote
scaling theories. Yet this effort also requires interdisciplinary research in
relatively new disciplines such as hydroclimatology and paleohydrology.
Entirely new disciplines, for example, exohydrology, will also generate
new thinking and activity to further understanding. Field studies, whenever
feasible and appropriate, are important.
As the committee noted in the beginning of this chapter, water does
not respond passively to physical processes governing Earth: it is a dynamic
agent whose influence is central to processes that produced the world as
society knows it and that will affect its evolution into the future. Water is
locally exhaustible, which is why it is critical to understand its dynamics.
Moreover, human intervention in the water cycle alters water's role on
the planet. All of the phases and states of the water cycle are linked, and
impacts of human activities on one component of the hydrologic cycle
are consequently circulated to other components. This chapter focuses on
the water cycle, but the next two chapters will revisit the water cycle as
the component that integrates water throughout all biological and Earth
systems. Chapter 3 extends this discussion beyond the processes that were
addressed above into a host of ecohydrological topics.
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