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OCR for page 32
The Hydrologic Sciences
Science draws its excitement from new and wondrous views of
nature. By punching holes in the heavens, the telescope has revealed
the immensity of the universe and thereby captured the imagination
and support of the public for astronomy. At the other extreme, the
microscope and the particle accelerator, through revelation of the
structure of matter, have served the same purpose for materials sci-
ence, molecular biology, and atomic physics.
The drama of geophysics is now vividly transmitted by views of
our living, changing planet from space platforms. This perspective
has inspired multidisciplinary efforts to describe and understand the
interactive functioning and continuing evolution of the earth's com-
ponent parts. In turn, these efforts have brought a fuller appreciation
of the central role that the global circulation of water plays in the
interaction of the earth's solid surface with its atmosphere and ocean,
particularly in regulating the physical climate systems and the biogeo-
chemical cycles.
The global distributions of rainfall, snowfall, evaporation, and ac-
cumulated surface and subsurface water affect the local extent and
global distribution of biomass and biological productivity. Changes
in land cover and biological productivity can, in turn, affect hydro-
logic processes on both local and global scales. Water exerts thermostatic
control over local air temperature wherever evaporation or snow cover
occur. Water movement couples the land with the oceans through
the solution, entrainment, and transport of minerals and sediments;
32
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THE HYDROLOGIC SCIENCES
33
both liquid water and ice are powerful agents of erosion and join
with plate tectonics in shaping the land surface.
This realization of the importance of water to the earth system at
geophysical space and time scales has profound implications for the
research and educational infrastructure of hydrologic science. We
cannot build the necessary scientific understanding of hydrology at a
global scale from the traditional research and educational programs
that have been designed to serve the Pragmatic needs of the enci-
. .
neer~ng community.
THE UNIQUENESS OF WATER ON THE EARTH
The surface of the earth has abundant liquid water, yet our neigh-
boring "terrestrial" planets Venus and Mars have little. Venus,
Earth, and Mars all have atmospheres with clouds and solar-forced
circulation. The primary constituents of the clouds sulfuric acid on
Venus, dust on Mars, and water on Earth are markedly different,
however. The major components of the earth's atmosphere (nitrogen
and oxygen) are controlled by biological processes, whereas the Venusian
and Martian atmospheres (both carbon dioxide) are governed by abiotic
processes.
Theories for the uniqueness of water on the earth fall into two
classes, genetic and evolutionary. The genetic theory holds that chemical
equilibrium of accreting gas and dust in the solar nebula led to the
formation of solid constituents richer in hydrated minerals at greater
distance from the proto-sun. Once these minerals were incorporated
into Venus, Earth, and Mars, their water and other volatiles were
released to varying degrees over time in the formation of planetary
atmospheres. The evolutionary theory, on the other hand, contends
that the planetismals began with similar inventories of volatiles and
that subsequent events, perhaps including meteoritic impacts, led to
their current composition.
In any event, the higher accretion temperature and tectonic activ-
ity of Venus led to heavy outgassing, followed by irreversible photo-
dissociation of any water into hydrogen, which escaped to space, and
oxygen, which reacted with surface elements. The carbon dioxide
remained to create a runaway greenhouse effect, resulting in Venus's
current hot (464°C), dry surface.
On Mars the outgassing has been limited by lower accretion tem-
peratures and by the absence of tectonic activity, but there is evidence
of surface erosion by some flowing liquid, presumably water. The
fate of this water is unknown. The (largely) carbon dioxide atmo-
sphere of Mars is thin, and the current surface temperature is a cold
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34
OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
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-53°C, leading to the presence of seasonal polar caps of frozen carbon
dioxide and to speculation that there may be extensive subsurface
frozen water.
It appears that the earth also once had a carbon dioxide atmo-
sphere that was sharply reduced by some unique process (most probably
biological), and that its water is the result of tectonically driven out-
gassing over geological time.
It is particularly important to the dynamics and energetics of the
earth system that all three phases of water (solid, liquid, and vapor)
coexist over the range of earthly temperatures and pressures. This is
unique to the earth among the terrestrial planets (see the phase diagram
of water given in Figure 2.1~.
THE EARTH'S HYDROLOGIC CYCLE
Powered by the sun, the phase changes of water on the earth in-
volve storage and release of latent heat that at once drive the atmospheric
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THE HYDROLOGIC SCIENCES
1 0,000
1 ,000
100
-
~ 10
Cal
-
L1J
G 1
U'
In
UJ 0.1
AL
0.01
0.001
0.0001
-200 -100 0
35
j ICE
Jupiter -
- · Uranus ICE
· Pluto
_
- ICE
Mars ~
/
WATER
· Earth /
`/. WATER VAPOR
triple pt
-
-
-
-
1 1 1 1
-
-
-
-
~ Venus -
Mercury (daylight side) |
100 200 300 400 500
TEMPERATURE, °C
FIGURE 2.1 Planetary positions on the phase diagram of water.
circulation and redistribute both water and heat globally. Condens-
ing in the atmosphere where it releases its latent heat, the liquid (or
solid) water falls as precipitation, runs to the sea, and through evaporation
regains its cargo of latent heat and returns through the atmosphere
to wash the land again. This process is called the hydrologic cycle. It
is the framework of hydrologic science (Figure 2.2) and occurs across
a wide spectrum of space and time scales. It affects the global circulation
of both atmosphere and ocean and hence is instrumental in shaping
weather and climate.
~ i_
Water's efficiency as a solvent makes low-
temperature geochemistry an intimate part of the hydrologic cycle.
All water-soluble elements follow this cycle at least partially, or completely
if they are in a chemical compound that is volatile as well as soluble.
The hydrologic cycle is thus the integrating process for the fluxes
of water, energy, and the chemical elements.
THE IMPORTANCE OF WATER ON THE EARTH
As far as is known, the earth alone supports life, and this life is
active geophysically and geochemically as well as biologically. As
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OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
/~,,~,
//
\ \
/~ ~ ~
/\: 1 ATMOSPHERIC SCIENCE
~_-
~ ^__^ ~
Ad\
FIGURE 2.2 Hydrologic science is a geoscience.
\
OCEAN SCIENCE ~ | |
examples, consider the role of biota in the cycling of oxygen, nitro-
gen, sulfur, and carbon on the earth:
· The oxygen in our atmosphere originally was released by plants
after they began to evolve 2 billion to 3 billion years ago, and it is
maintained by them through the photosynthetic decomposition of
water molecules.
· Certain bacteria (as well as lightning and combustion) act to
convert free atmospheric nitrogen into a chemical form that can be
used by plants and animals.
· In the process of decomposing organic material in the sediments
of swamps, marshes, and eutrophic lakes, bacteria use sulfates washed
from the atmosphere by preciptation, reducing them to volatile sul-
fides that return to the atmosphere for reoxidation.
· Photosynthesis removes carbon dioxide from the atmosphere.
Through respiration and decomposition plants and microorganisms
pump carbon dioxide into the soil, where it joins with that washed
out of the atmosphere by precipitation. Some of this carbon dioxide
reacts with rock minerals and forms various carbonates and bicarbonates.
These are dissolved in ground water and carried to the oceans, where
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THE HYDROLOGIC SCIENCES
37
they join other carbon compounds entering directly from the atmo-
sphere. By this mechanism primitive biota reduced the carbon diox-
ide concentration of the earth's early atmosphere. Contemporary life
forms still remove carbon dioxide in this way, and by incorporating
it into plant material. However, humans are changing this natural
balance by accelerating the release of carbon dioxide through combustion,
and bv modifying photosynthesis through deforestation.
In the oceans carbonaceous minerals are incorporated into animal
shells and then are deposited in ocean sediments; ultimately they
become part of the earth's crust. These sedimentary rocks may even-
tually become involved in volcanism, whereby their carbon dioxide
(as well as water entrapped in the marine sediments) is once more
released to the atmosphere.
The earth is unique among the terrestrial planets both because it
has an active water cycle and because it supports life. The water
cycle is an essential part of the planet's life support mechanism and,
to the extent that the biota are responsible for the earth's moderate
surface temperatures, the biota permit the water cycle to exist. This
synergism couples the animate and inanimate components of the earth
into an evolving system. The central role of water in the evolution
and operation of the earth system provides a rationale for seeing
hydrologic science as a geoscience whose stature equals that of the
ocean, atmospheric, and solid earth sciences.
EARLY SCIENTIFIC INSIGHTS
Concern for water as both a necessity of life and a possible hazard
has been with humans throughout their existence. Drought and flood
have driven the search for an understanding of water since the first
civilizations formed along the banks of rivers. Pioneering hydraulic
engineers built primitive dams, channels, and levees to meet their
practical needs. They sought to understand the vagaries of water
only when these engineering measures were insufficient.
Not until the rise of Hellenic civilization, about 600 B.C., did man
attempt to understand nature just for the sake of understanding. Early
Greek philosophers such as Herodotus, Hippocrates, Plato, and Aristotle
theorized about the source of rivers and rain but failed to develop a
complete understanding of the hydrologic cycle.
Precipitation was first measured in the fourth century B.C. by Kautilya
of India, and streamflow was monitored by Hero of Alexandria in
the first century A.D., but little further advance in understanding
occurred until the Renaissance. Bernard Palissy (1510-1590), a French
potter and naturalist who used his own field observations to build
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OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
revolutionary theories, was the first to state categorically (and counter
to Plato and Aristotle) that rivers can have no other source than rainfall.
He gave the first correct explanation of the hydrologic cycle for tem-
perate regions.
The first comprehensive hydrologic field study, in which Palissy's
explanation was proved, was conducted by Pierre Perrault (1608-1680),
a French lawyer and member of a distinguished family who became
a hydrologist only after his dismissal (for embezzlement) as Receiver-
General of Paris. Edmund Halley (1656-1742), an English Astronomer
Royal, conducted experiments on evaporation after having vapor
condensation interfere with his celestial observations on clear nights.
He calculated that ocean evaporation is sufficient to replenish the
inflowing rivers. With this proof the hydrologic cycle was firmly
established, marking the beginning of scientific hydrology. During
this early phase of investigation, hydrology received attention as a
natural science worthy of study in its own right without concern for
utility.
THE AGE OF APPLICATIONS
For the rest of the seventeenth and eighteenth centuries, Europe
was transformed by the Industrial Revolution and the urbanization
that accompanied it. Physicists and members of distinguished fami-
lies lost interest in hydrology, and its development was left to engineers
concerned with the urgent matters of water supply and sanitation.
The subfield of hydraulics received great impetus then from civil
engineers such as Henri Pitot, Antoine de Chezy, and G. B. Venturi,
who were concerned with water supply and water power. The primary
refinement of the hydrologic cycle during this period was provided
by Jean-Claude de la Metherie (1743-1817), who explained that rain-
fall has three possible fates: (1) direct movement to streams, (2)
evaporation or transpiration and moistening of the soil, and (3) deep
percolation to feed springs.
Water-related science developed spottily in response to the needs
of engineering practice. Water scientists and engineers focused their
attention on drainage basins commonly having a characteristic horizontal
scale of 10 to 100 km. Because the early foundations of hydrologic
science were built on experience with the middle latitudes, some in-
advertent and long-lived biases were established. At middle latitudes,
the atmospheric processes driving catchment hydrology are dominated
by cyclonic motions having horizontal scales (e.g., 1,000 km) orders
of magnitude larger than those of the individual catchments being
studied. This disparity of scales encouraged the convenient assumption
that the catchment is a passive participant in the hydrologic cycle,
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THE HYDROLOGIC SCIENCES
39
producing no feedback to the atmosphere from either its surface state
or its streamflow. In addition, the hydrologic processes peculiar to
the nondeveloping desert and to tropical and cold regions received
little or no attention. In this context, T. I. Mulvaney (1822-1892), an
Irish engineer, was apparently the first to deal with the unsteady
rainfall-runoff problem in a catchment. His landmark 1851 work
relating precipitation and the resulting maximum flood discharge opened
a field of study that preoccupied applied hydrologists for the next
125 years. Applying higher mathematics for the first time in hydrology,
Philipp Forchheimer (1852-1933) in Germany and C. S. Slichter (1864-
1946) in the United States founded an elegant theory that describes
ground water flow.
Until late in the nineteenth century hydrologic research in the United
States remained the province of enterprising professors, inventors,
prospectors, and wealthy amateurs. However, at that time the growing
data needs of water management projects on large rivers (e.g., 100 to
1,000 km) led to the establishment of new public agencies, both federal
and state, including the U.S. Weather Bureau and the U.S. Geological
Survey (USGS). Government was now the prime mover in water
research in the United States, although primarily in support of the
practical missions of its agencies. The USGS, for example, was founded
in 1879 to produce maps and data of a geological nature about the
"products of the national domain."
The early history of the USGS embodies the development of hydrologic
science in the United States, particularly in such areas as sediment
transport (led by G. K. Gilbert), ground water (C. S. Slichter, O. E.
Meinzer, and C. V. Theis), and water chemistry. In surface water,
private consulting engineers such as R. E. Horton, Allen Hazen, L. K.
Sherman, and Adolf Meyer remained at the forefront of research well
into the twentieth century. O. E. Meinzer, as head of the ground
water group of the USGS, brought together in the 1920s a group of
geologists and hydraulic engineers to develop quantitative methods
for the study of ground water. Out of this group came pioneering
work on unsteady ground water flows by such leaders as C. V. Theis
and C. E. Jacob.
This period of federal and state agency dominance of hydrologic
science in the United States ended with the completion, in the 1950s,
of the water projects delayed by World War II, and with the concur-
rent rise of both the environmental movement and the culture of
government-supported university research.
In the early twentieth century the first English language textbooks
on hydrology were published by Daniel Mead (in 1904) and Adolf
Meyer (in 1919~. These authors were engineers, and the hydrology
subjects introduced into U.S. universities using these books were taught
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OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
largely in departments of civil engineering, where the focus was naturally
on questions of surface runoff, water supply, and floods.
THE STRUGGLE FOR SCIENTIFIC RECOGNITION
Although the International Union of Geodesy and Geophysics (IUGG)
formed an International Commission on Glaciers in 1897, the first
formal recogrution of the scientific status of hydrology was the formation
of the Section of Scientific Hydrology within the IUGG at its Rome
assembly in 1922. Two years later, at the 1924 Madrid assembly, this
new section established a commission on statistics charged with bringing
uniformity into the publication of hydrologic information by the na-
tional services a sip the interests of the era.
In 1922 the U.S. National Research Council's Committee for the
IUGG was called the American Geophysical Union (AGU). Its delegate
to the Rome assembly returned with a recommendation that a new
section of scientific hydrology be added to the AGU. The fate of this
proposal illustrates the status of hydrologic science within the U.S.
scientific community during the first half of the twentieth century.
Despite repeated recommendations by ad hoc committees and biennial
pleas from the IUGG, the leadership of AGU maintained for eight
years that active scientific interest in the United States did not justify
a separate section of scientific hydrology within the AGU.
Finally, in reviewing plans for transforming the AGU from a committee
of the National Research Council into an independent society, approval
for the new section was given. On November 15, 1930, the Section of
Hydrology of the AGU came into existence with O. E. Meinzer as
chairman and R. E. Horton as vice-chairman. At the next annual
meeting of the AGU (April-May 1931), Horton presented a comprehensive
analysis of the field, scope, and status of the science of hydrology as
seen at that time (Horton, 1931~. His definition of hydrology as a
science was as follows:
AS a pure science, hydrology deals with the natural occurrence, distri-
bution, and circulation of water on, in, and over the surface of the
earth.... More specifically, the field of hydrology, treated as a pure
science, is to trace out and account for the phenomena of the hydro-
logic cycle. (p. 190)
In defining the scope of hydrologic science, Horton went on to say:
Both the scope and problems of hydrology are closely related to the
various branches of applied hydrology. This is natural since it is mainly
in the applications that new problems arise and the scope of the science
is extended.... Its scope is limited to considerably less than the entire
field of water science. (p. 191)
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ME HYDRO[OGlC SCONCES
47
In using the hydrologic cycle to define the processes encompassed
by hydrologic science, Horton recognized the diversity of scales by
stating:
may natural exposed surface may be considered as a ~H area on which
me hydrologic cycle operates. This includes, far example, an isolated
tree, even a single leaf or Wig of a growing plant, the roof of a building,
me drayage bash of a dver-system or any of its ~~utaries, an undrained
glacial depression, a swamp, a glacier, a polar ice-cap, a group of sand
dunes, a desert plays, a lake, an ocean, or me Earn as a whole. (p. 192)
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OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
Throughout Horton's definitive work it is apparent that
1. the central focus was still on the conservation of water mass at
the scale of the river basin, where evaporation was characterized as a
"water loss";
2. concern was exclusively with the physics of the hydrologic
cycle, omitting any mention of chemistry and biology (with the exception
of trees and vegetation); and
3. the qualifier "natural" was used to exclude concern with the
effects of humans.
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THE HYDROLOGIC SCIENCES
51
exists for circulation of ground water at depths of from 10 to 15 km,
probably as a result of tectonically induced pressure gradients. However,
it is not yet known how to measure and characterize the transport
properties of these fractured rock masses.
The volume of ground water in the upper kilometer of the conti-
nental crust is an order of magnitude larger than the combined volume
of water in all rivers and lakes and is equivalent to the total of all
ground water recharge for about the last 150 years. The total volume
of ground water is equal to almost one-fourth of all the nonoceanic
water on the earth.
Discharges of ground water at topographically lower elevations of
the earth's surface enable streams to flow even during prolonged
rainless periods and after winter snows have melted. Coastal ground
water reservoirs may be recharged locally by intrusion (I) of salt
water from the ocean.
A major problem that recurs throughout geophysics is the repre-
sentation of spatially aggregated nonlinear behavior in the presence
of large spatial variability. In other words, given the dynamics at
microscale, how can behavior at the macroscale be represented? This
scale-transfer problem arises in the hydrologic sciences in attempts
to describe the coupled fluxes of heat and moisture across large land
surface elements, to couple the microscopic molecular processes of
chemical reactions to the macroscopic averages of ground water transport
equations, and to establish appropriate parameters for use in describing
the behavior of ground water plumes at field scale.
The largest masses of fresh water exist as ice in Antarctica's and
Greenland's ice sheets. This ice, with an average residence time of
about 10,000 years, participates very slowly in the hydrologic cycle.
However, the reservoirs are so large that small-percentage changes in
ice volume can cause major changes in sea level on time scales of 100
to 10,000 years. Wastage (W) occurs primarily by melting or calving
around the periphery. The Greenland ice sheet, if it melted, would
yield enough water to maintain the flow of the Mississippi River for
more than 4,700 years, and this ice sheet represents only 10 percent
of the total volume of icecaps and glaciers. The greatest single item
in the water budget of the earth, aside from the oceans, is the Antarctic
ice sheet. It contains about 64 percent of all nonoceanic waters. Melting
of just the small West Antarctic ice sheet would raise global sea level
by about 7 m! The stability of those portions of the Antarctic ice
sheet that are grounded below sea level, such as the West Antarctic
ice sheet, is a major unsolved problem. A sudden slide of this sheet
would cause sudden and calamitous sea level rise.
The addition or subtraction of ice from smaller icecaps and moun-
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OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
TABLE 2.1 Typical Residence Times of Global Water
Compartment
Typical Residence Time
Deep ground water
Icecaps and glaciers
Oceans
Shallow ground water
Soil moisture
Seasonal snow
Rivers
Atmosphere
Plants
104 years
104 years
103 years
102 years
102 days
102 days
10 days
10 days
10 days
NOTE: These residence times are averages computed, for atmo-
sphere and ocean, by dividing the total water mass in the entire
reservoir by the current rate of either inflow or outflow. For the
ground water, "shallow" refers to the topmost kilometer of the
continental crust; the associated water mass and flux are poorly
known and thus so is the residence time. The residence time for
deep ground water is a crude estimate for depths of 1 to 10 km.
SOURCES: Dooge (1984); Philip (1978); UNESCO (1971).
fain glaciers, reacting on time scales of 10 to 100 years, may apprecia-
bly affect the flow of certain rivers. The melting of ice in these glaciers
appears to have caused one-third to one-half of the sea level rise
observed in the past century. Snow cover on land and sea ice on the
ocean vary rapidly and seasonally and exert a major influence on the
earth's radiation budget and on the circulation of both the atmosphere
and the ocean. Particularly needed are new observational techniques
to study and monitor the rates of snow accumulation and snow and
ice melt over remote areas. Because of the sensitivity of snow and
ice reservoirs to climate change, it is important to monitor closely the
extent of snow cover, the mass balances of mountain glaciers and ice
sheets, and the West Antarctic ice sheet with its fringing ice shelves.
The magnitudes of these major water storages and fluxes are shown
in Figure 2.6, and typical residence times are summarized in Table
2.1. How humbling it is to realize that despite man's seeming importance,
his existence is made possible by the less than 1 percent of the earth's
water that is directly available as fresh water.
Flux of Sediments
Water moves around on the earth's surface, and the large-scale
primary driver is gravity. In its relentless downhill course to the sea,
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THE HYDROLOGIC SCIENCES
53
water sculpts the landscape through the processes of erosion, trans-
port, and deposition. In so doing, it plays the key role in a geological-
scale tectonic-climatic feedback system. Tectonic and volcanic pro-
cesses lift the crust, creating the gradients that drive erosion, which
in turn gradually reduces the gradients (Figure 2.7~. If the elevation
changes are large enough they may affect climate, and the erosion
may be self-limiting by precipitation reduction as well.
In the process of shaping the landscape, runoff forms a tree-like
network of channels into which the flow becomes concentrated. A1-
though empirical "laws" describing the two-dimensional geometry
of these networks have existed for almost half a century, there is
little quantitative understanding of the dynamics of channel for-
mation or of the causal relationship between the three-dimensional
network structure and the precipitation driving the erosion. Such
understanding would reveal fundamental scaling relationships of surface
water hydrology over a broad range of spatial scales (i.e., 1 to 106
km2) and would have immediate applicability to flash flood forecast-
ing in ungaged watersheds and to parameterization of hydrologic
processes in regional and global models. It would also help answer
many fundamental geological questions about landscape formation.
In spite of decades of study, continuing deficiencies in our under-
standing of fluid turbulence seriously impair our ability to specify
the relative rate of transport of various sizes of grains and aggregates
-
~| CLIMATE |
EROSIONAL REMOVAL OF
MASS FROM SYSTEM
UPLIFT,
VOLCANISM
EXTRACTION OF
MAGMAS FROM MANTLE
TECTONIC INFLOW
OF MASS: CRUSTAL
SHORTENING
FIGURE 2.7 The tectonic-climatic feedback loop. SOURCE: Courtesy of B. L. Isacks
and the Cornell Andes Project.
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54
OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
TABLE 2.2 Continental Yield of Water, Water-Borne Sediments, and
Dissolved Solids
Annual Annual Dissolved Annual
Water Sediment Solids Dissolved
Area Yield Yield Concentration Load
Continent (106 km2) (103 km3) (loll kg) (ppm) (loll kg)
Africa 30.26 4.1 0.48 121 0.50
Asia 43.25 13.2 14.53 142 1.87
Australia 7.70 2.3 0.21 59 0.14
Europe 10.36 3.0 0.30 182 0.55
N. America 23.31 6.7 1.78 142 0.95
S. America 17.82 11.2 1.09 69 0.77
SOURCES: Dooge et al. (1973); Dooge (1984).
in streams and the duration of their storage at various locations within
the channel system. This is important not only for its contribution to
understanding erosion and deposition but also because many pollut-
ants are moved through the system by their being adsorbed on sedi-
ment particles.
Table 2.2 contains the best current estimates of sediment fluxes
from the earth's continents.
Flux of Dissolved Solids
Water is the universal solvent, and as it moves through the hydro-
logic cycle it dissolves and transports in solution solids as well as
gases. Rain falling onto soil surfaces contains various gases and sol-
ids in solution. As water infiltrates the soil and moves downward, it
picks up carbon dioxide from the soil, exchanges solutes with soil and
rock particles, and becomes less acidic. The percolating waters convey
their solute load through the ground water and into streams. This
water has a dissolved-mineral signature that is dependent on the
subsurface materials' properties, the flow path, and biological processes
that recycle minerals. Knowledge of these solutes and their chemical
kinetics can be used in tracer studies of subsurface water flow paths
and to understand the rates of continental degradation and soil for-
mation. Estimates of the total dissolved-solids runoff from the conti-
nents are given in Table 2.2.
Involvement of Biota
Water supports a variety of living organisms, and some have ma-
jor interactions with the hydrologic cycle. The thin soil cover, for
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THE HYDROLOGIC SCIENCES
55
instance, is a result of physicochemical weathering by water and sup-
ports a vegetation cover that constitutes about 99 percent of the earth's
terrestrial biomass. Through transpiration, the vegetation is respon-
sible for most of the water returned directly to the atmosphere from
the land, and the associated latent heat transfer is a major regulator
of land surface temperature. Through photosynthesis the vegetation
extracts carbon dioxide (a so-called greenhouse gas) from the atmosphere.
The removal of vegetation modifies runoff and, in certain climates,
may reduce local precipitation due to earth-atmosphere coupling.
The physical relationships among climate, soil, and vegetation
that determine the dominance and stability of specific vegetation
types at particular geographic locations are largely unknown, but
understanding such relationships is necessary to anticipating the
effects of climate change.
The soil also supports a variety of microorganisms that act on
complex organic materials and reduce them to simpler organic compounds
and ultimately to mineral form. Bacteria and fungi are particularly
important in the carbon cycle and in regulating the availability of
phosphorus, nitrogen, and sulfur. Their action results in the produc-
tion of carbon dioxide and the development of humus, the organic
detritus of decayed vegetation. Humus affects the infiltration and
water-holding capacity of the soil and its resistance to erosion. Mi-
croorganisms also are responsible for transforming atmospheric nitrogen
to a form usable by and essential to plants. While these well-known
organisms are active near the soil surface, recent investigations have
identified thousands of bacterial species up to 250 m below the surface.
It is interesting to speculate on their natural role, if any, in the hydrologic
cycle and on their possible use in the degradation of anthropogenic
contaminants.
Oceanic microorganisms are responsible for roughly one-half of
the earth's photosynthetic activity and therefore play a major role in
atmospheric chemistry and the chemical quality of precipitation.
Wetlands are a primary source of atmospheric methane (another
greenhouse gas) and perform a host of other hydrologic and biogeo-
chemical functions. Serious scientific study of this complex blame is
in its infancy, however.
In the last 500 years the hand of the human animal has been in-
creasingly felt on the hydrologic cycle. Energy production, farming,
urbanization, and technology have altered the albedo of the earth,
the composition of its soil and water, the chemistry of its air, the
amount of its forest, and the structure and diversity of the global
ecosystem. These actions of humans now extend to the "ends of the
earth" high latitudes, deserts, and mountains, where they affect sensitive
environments and where hydrologic data and understanding are ab-
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56
OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
sent. We must learn to incorporate human activity as an active com-
ponent of the hydrologic cycle in all environments.
Summary
The evolution of hydrologic science has been in the direction of
ever-increasing scale, from small catchment to large river basin to
the earth system, and from storm event to seasonal cycle to climatic
trend. Inevitably, increased scale brings increased complexity and
increased interaction with allied sciences. New questions arise, such
as the following:
· How do we aggregate the dynamic behavior of hydrologic pro-
cesses at various space and time scales in the presence of great natu-
ral heterogeneity? This fundamental statistical-dynamical problem
of hydrology remains unsolved.
· How can we employ modern geochemical techniques to trace
water pathways, to understand the natural buffering of anthropo-
genic acids, and to reveal ancient hydroclimatology?
· What can the soil, sediment, vegetation, and stream network
geometry tell us about river basin history and about the expected
hydrologic response to future climate change?
· What can we learn about the equilibrium and stability of moisture
states and vegetation patterns? Is "chaotic" behavior a possibility?
These and many other fundamental problems of hydrology must
be addressed to provide the ingredients for solving the sharpening
conflicts of humans and nature. Many, if not most, will require coor-
dinated multidisciplinary field studies conducted at the appropriate
scales. Others, such as the measurement of unknown oceanic pre-
cipitation and evaporation, will require sensors, often satellite-borne,
that are still undeveloped. Progress in many areas of hydrologic
science is currently limited by a lack of (high-quality) data.
HYDROLOGIC SCIENCE AS A
DISTINCT GEOSCIENCE
In 1931 Horton identified a hydrologic science of limited scope
that was motivated by engineering practice to understand the quantity
and movement of water at the small catchment scale. Over the next
30 years, first Meinzer (1942) and later Langbein's committee (Ad
Hoc Panel on Hydrology, 1962) expanded the scope to embrace the
full life history of water on the earth, including its chemical properties
and its relation to living things. That definition needs modification
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THE HYDROLOGIC SCIENCES
57
now to reflect an evolving understanding of the science and to specify
clear administrative boundaries for the science.
How have our perceptions of hydrology changed? Hydrologic
science can now be seen as a geoscience interactive on. a wide range
of space and time scales with the ocean, atmospheric, and solid
earth sciences as well as with plant and animal sciences. The new
perceptions concern the interaction of the components and the range
of scales.
Our perceptions of the necessary administrative boundaries also
have changed. The ubiquity of water on the earth and its indispens-
ability to life do not make hydrologic science out of all geoscience
and biology. Forging a separate identity for hydrologic science re-
quires specifying and claiming its central elements, and locating
its administrative boundaries as a flexible compromise between
precedent and scientific completeness. The scope of hydrologic sci-
ence does not involve developing the physics, chemistry, and biology
of water within the ocean and atmosphere reservoirs, for these processes
are firmly in the recognized domains of the sibling geosciences. Such
clear precedents do not apply to characterizing the lake and ice reser-
voirs, however. Limnologists and glaciologists are divided over their
administrative homes. Indeed, in the United States the central scientific
society for limnologists (particularly those with biological interests)
is the American Society of Limnology and Oceanography (ASLO),
and within the National Science Foundation (NSF) their primary research
home is in the Division of Ocean Sciences. Many physical and some
chemical limnologists consider themselves hydrologists, however, arguing
that lakes are merely wide places in rivers. Glaciologists deal with a
wide variety of snow and ice problems. Melting glaciers and snow
cover, frozen ground, and lake and river ice have been a traditional
part of hydrology, while glacial dynamics, large ice sheets, sea ice,
and snow avalanches have not. In developing this report, the committee
included the study of snow, ice, and lakes within its definition of
hydrologic science.
To establish and retain the individuality of hydrologic science as a
distinct geoscience, its domain is defined as follows:
· Continental water processes the physical and chemical processes
characterizing or driven by the cycling of continental water (solid,
liquid, and vapor) at all scales (from the microprocesses of soil water
to the global processes of hydroclimatology) as well as those biologi-
cal processes that interact significantly with the water cycle.
(This restrictive treatment of biological processes is meant to in-
clude those that are an active part of the water cycle, such as vegetal
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58
OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
transpiration and many human activities, but to exclude those that
merely respond to water, such as the life cycle of aquatic organisms.)
· Global water balance the spatial and temporal characteristics of
the water balance (solid, liquid, and vapor) in all compartments of
the global system: atmosphere, oceans, and continents.
(This includes water masses, residence times, interracial fluxes, and
pathways between the compartments. It does not include those physical,
chemical, or biological processes internal to the atmosphere and ocean
compartments.)
These boundaries are illustrated in Figure 2.8, and the range of
process scales is shown in Figure 2.9.
The complex problems of global change illtu~unate the multidisciplinary
nature of hydrologic science and make clear the need for extensive
cross-discipline interaction in education as well as in research. The
problems we face pay no attention to organizational boundaries; thus
there are major areas of overlapping interest with other sciences and
frequent needs to blur the stated boundaries. This is both inevitable
and desirable. One example is the problem of the variability in space
and time of storm precipitation wherein the search for hydrologic
generalization demands incorporation of considerable atmospheric
dynamics and thermodynamics. Similar trespassing must occur in
'~4~Ns If.
Oceans
~ ~~\~
5G~ s~cea/se-
Region
FIGURE 2.8 Hydrologic science: a distinct geoscience.
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THE HYDROLOGIC SCIENCES
GLOBAL
1 0000 Km
1000 Km
1 00 Km
1 0 Km
1 Km
LOCAL
= _ GLOBAL GLOBAL _
WEATHER CO2
SYSTEMS VARIATIONS
DEVELOPMENT
OF MAJOR
c OIL RIVE .R BASINS
FORMATION
tIJ RUNOFF l l
—~— WE] rOESMCSALE MESOSCALE sol| DRAINAGE
(FLOODS) MOISTURE EROSION l
VARIATION SHALLOW
_ I GROUND
NUTRIENT CIRCULATION
THUNDEI ~ l _
1
TIME
~ , ~ STY; 1 ~ 11 o 1 ~ 4 1
SEC MIN DAY YEAR CENTURY
FIGURE 2.9 Illustrative range of process scales.
59
ONE MILLION ONEBILLION
YEARS YEARS
the areas of fluvial geomorphology, micrometeorology, and plant ecology
(to name but a few) because of the importance to the hydrologic cycle
of related processes such as erosion, energy flux, and transpiration.
The recent past has seen events that highlight the need for a sepa-
rate and strong science of hydrology:
· pressure for economic development in the more extreme cli-
mates of the world such as the tropics, deserts, and arctic and alpine
regions;
· realization of the capacity of humans to alter the hydrologic
cycle on all scales, including a global scale; and
· evidence that anthropogenic changes to the chemistry of the
earth's water are having harmful effects on the health of many humans.
Thus the science of hydrology has come to encompass a mix of
natural and altered physical, chemical, and biological systems as well
as to include important interactions with the engineering and social
sciences. There is little doubt that coping with these issues in a
timely fashion will require a much-improved scientific understand-
ing of the earth system and its component parts. Unified and coher-
ent treatment of hydrologic science is central to this larger effort.
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60
OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
SOURCES AND SUGGESTED READING
Ackermann, W. C. 1969. Hydrology becomes water science. Trans. AGU 50 (April):76-
79.
Ad Hoc Panel on Hydrology. 1962. Scientific Hydrology. U.S. Federal Council for
Science and Technology, Washington, D.C., 37 pp.
Biswas, Asit K. 1972. History of Hydrology. North Holland, Amsterdam, 336 pp.
Deevey, E. S., Jr. 1970. Mineral cycles. Sci. Am. 223(3):148-158.
Dooge, J. C. I. 1984. The waters of the Earth. Hydrol. Sci. J. 29(2):149-176.
Dooge, J. C. I. 1988. Hydrology in perspective. Hydrol. Sci. J. 33(1):61-85.
Dooge, J. C. I., A. B. Costin, and H. J. Finkel. 1973. Man's Influence on the Hydrologi-
cal Cycle. Irrigation and Drainage Paper. Special Issue 17. Food and Agriculture
Organization of the United Nations, Rome, 71 pp.
Eagleson, P. S. 1982. Hydrology and climate. Pp. 31-40 in Scientific Basis of Water-
Resource Management. National Research Council, National Academy Press,
Washington, D.C.
Forster, C., and L. Smith. 1990. Fluid flow in tectonic regimes. Fluids in Tectonically
Active Regimes of the Continental Crust. Mineralogical Association of Canada,
in press.
Horton, R. E. 1931. The field, scope, and status of the science of hydrology. Pp. 189-202
in Trans. AGU, Reports and Papers, Hydrology. National Research Council,
Washington, D.C.
Jones, P. B., G. D. Walker, R. W. Harden, and L. L. McDaniels. 1963. The Development
of the Science of Hydrology. Circular No. 63-03. Texas Water Commission, 35 pp.
Kasting, J. F., O. B. Toon, and J. B. Pollack. 1988. How climate evolved on the terres-
trial planets. Sci. Am. 261(February):90-97.
Kerr, R. A. 1988a. In search of elusive little comets. Science 240:1403-1404.
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Kerr, R. A. 1989. Double exposures reveal mini-comets? Science 243:13.
Langbein, W. B. 1981. A history of research in the USGS/WAD. Pp. 18-27 in Water
Resources Division Bulletin (Oct.-Dec.). U.S. Geological Survey.
Livingstone, D. A. 1964. Chemical composition of rivers and lakes. U.S. Geological
Survey Professional Paper 440 G.
Lovelock, J. E. 1979. Gala. Oxford University Press, New York, 157 pp.
Meter, M. F. 1983. Snow and ice in a changing hydrological world. Hydrol. Sci. J.
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Meinzer, O. E., ed. 1942. Hydrology. Physics of the Earth—IX. McGraw-Hill, New
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National Research Council. 1982. Scientific Basis of Water-Resource Management. Na-
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Philip, J. R. 1978. Water on earth. Pp. 35-59 in Water: Planets, Plants and People. A. K.
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Price, W. E., Jr., and L. A. Heindl. 1968. What is hydrology? Trans. AGU 49(21:529-533.
Prinn, R. G., and B. Fegley, Jr. 1987. The atmospheres of Venus, Earth, and Mars: a
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61
Schneider, S., and R. Londer. The Co-Evolution of Climate and Life. Sierra Club
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United Nations Educational, Scientific, and Cultural Organization (UNESCO). 1971.
Scientific Framework of the World Water Balance. Technical Papers in Hydrology
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U.S. Geological Survey. 1968. Water of the World. USGS 0-288-962. U.S. Government
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Representative terms from entire chapter:
hydrologic sciences
