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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 (464C), 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 :: :: i: :: :~ ~~ : If: i: : r PA~~v~r~D~r~ w~r:~a~:~a:~n~~ ~~ ^~C~:~! L~LV~:~I ~~ ~:~: ~~.:~: ~ aft: ~ i:: :: ~ ~ :~ :: :~:: ~ :: :~ i: ~ ~ i:::: i:: : :~::~:::~:~: ~ :: :~~ : i:: :~ :: ~~ :: ::: :~ :~ i: :~:: i: ::: ~~ ~1~972~th~e~:ap~t:ner: :~Y ~~spacec~t~retu:rn:ed~:~th~t~:n s~t~c:~re-m~oteiv~ ~:~ ~ :: :: :~ : ::: Sensed Images ~~c:~ls stand ~~ val~ys~o~n~ ~03e~plar~rs. ~~: ~ ~ ~~: ~ ~ ~ ~ ~ i~ ~ ~~ ~ ~ i~ ~ ~ i. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~ _? ~ ~ ~ ~ ~~ ~ ~ ~ ~~ ~ ~ ~~ ~ ~ ~ ~ ~: ~ ~~: ~~ ~~ ~ ~~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~ ~~ i~ ~ ~ ~~ ~ ~~ ~ ~ :~ ~~ ~ i~ ~~ ~ ~~ ~ 1 quest y Ant Be ate ~7' s~:anc Or By ~1~98~t debit As m, sate: retur~ned~:~thousands~:~ of ~~6igh~-resol ution ~~p~ictui erg shod 0pry; -~flu~vial~g~these: :~l~andforms.~Mar~s ~ had ~~beer'~ttiolight stop beta ~ ~ ~ ~~ :: ~ ~~ ~ ~ ~ ~ ~ ~ ~ ~ ~: .: ~ ~ ~ ~: ~: ~ ~ ~ ~~ ~~ ~~ ~ ~~ ~ ~: ~ ~ ~: ~ ~~ ~ ~ ~: ~ ~~ ~~ ~ :~ ~ ~~ ~~ ~ ~~ ~~ :~ ~ ~~ ~~ ~~ ~ ~: ~: ~~ ~~ ~ A anew: w pose : im~itecl~:~o ati e components Sac ~~r~ernai~a~ec : fr~e~ o:~u:t:~:~i~ts:~history. The: fIw~ial~::~thanr~ls~a~nd~ voles Anti A: ~ ~ ~ ~ d i~stant~::past::~Mars~apparentl y ~ h~:~dlisp l ayed a~:d ynam i:c ~hydr~ic~i cy~cle ~ : ~ ~ :~ ~ :: ~ ~ : ::: :: :~ ~ :: :: :::: ~: ~:: :~: ~: ~: ~ ~ ~ ~: ~:~ :~ :: :: :~ :~:~ ~: ~ :: :~ :~: ~ ~ ~ ~ ~ ~ ~ ~ ~ ; ~ ~ ~ ~ ~::~ ~ ~n~ t 1e ~p~ an~eta~r~f:~sc~len~ces,~comp~ar~l~so~n~s~anc ~ an:a og~ies~ncti - - gen-:~ :~erate ~the~hypotheses~ that~ stimu late~t~b:ui~l^~ng~i~of~iew~ theorW:~We~now :i ~:~:~know~ that~ ~at:~:~:~l~east~ ~one~:~ oth:er ~i~DI:a:n:et~sWi~dei~t:h6~earth~:~h~a~:n~artive~: s y s t e m ~ ~ t h a t j ~ : g e n e r a t : e d ~ ~ r u n o f f ~ ~ ; : ~ ~ O n ~ M a r s ~ ~ t h e ~ ~ r e m n a ~ I t s ~ ~ ~ f i t h ~ i s ~ ~ ~ I c I:~system~ ~` ~l~s~p ay~:~stra:~:e:~:parac ox~es~re~ ative~:~:~:t:he~m~liar~eadh~:s~:m~.~:~:~ ~: ~:~ ~rtl~an ~tha~n~nels ~am ~u:p~ :~1~00 ~:k:m:~ wi~de~and ~2~00~0~k~m~ lioi~:~1he~j~ar:ise~ : ~ ~ ~: :: ~ ~ : ::: : r ~ i ~ ~ ~ ~. rrom~sc~r~e:~areas~:~ co:~ ~ apseo~ terra~:i~n~i:cating:~:~: ~t Ilu~i(:i~c~a~:~to~the~ ~surface from~:subsurface:~so:wrces.~:~:Seeing~:the~e~ects~c~large sca:le~exfiltration~ ~ : ~:: i: :: ~ ~: : ~ ~ ~ ~ :: ~: :: ~: :: ~ ~ ~ ~ ~ ~ ~ T: :: ~ ~ :~ ~ ~ i~ ~ ~ ~ : : ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ; ~ ~ ~ ~ ~ ~ ~; ~ ~ ~ ~ ~ ~ ~ ~ ~ on~Martian~topography ~h~a:s~given~geom rph:~ogi~s~new: ideas ab~he : ~o r i g i r'~ of ~ J a nd~sca pes ~o n~ ~e~e a rth~ ~ i~n~ ~regi o n s ~ w he~re~in g ~u n d ~ ~ ~ ~: ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~: ~ ~ ~? ~ ~ ~ ~ ~ ~: ~ ~ ~ :~ ~: ~ ~ ~ ~ ~ ~ ~: ~ ~ ~ ~ ~ ~ ~: ~ ~: :water~rat: 1~er~:~:an:~ surmce~runon~: ls ~ t~oom~nant~agen~t~:carv~ng~t~r:~ ~:val~leys.:: ~ :~ :: ~ ~ :~ ~: ~ ~ ~ ~ ~ :~ :: ::: ::: ~:: ::: :~: ~ ::~ ::~ C~-~ :: :~: ~: r~ : ::: : : ~: a ~ T : :~ ~ ~ ~ ~: ~ ~ ~ ~: :~ ~ ~ :~ ~ :~ ~ ~ ~ ~::~:~: :~ ~;~ ~;~ ~ ~ ::~: : ~ :o~m~parl son~ ot~ ::p~rocesses on ~t~ne~:two ~p la:nets :~s hou Id~ :~reveal ~m~h ~: a:boUt~::~ ~:~ ~:the~cha:racter ~ of:~ru~h~off Droduct~ion~ark:f~its~:~intera:ction~:~wi+~Ma~rv Sr~:lr( - r'`a~ : ~4 1 ~ ~ca ~ ~_ ~ : ~ ~ : ~ ~ ~: : ~ :: ~: ~ : ~ :: ~ ~ :: :~ :~:: -53C, 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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36 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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38 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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40 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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42 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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52 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. Kerr, R. A. 1988b. Comets were a clerical error. Science 241:532. 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. 28(1):3-22. Meinzer, O. E., ed. 1942. Hydrology. Physics of the EarthIX. McGraw-Hill, New York. (Republished by Dover, Mineola, N.Y.) National Aeronautics and Space Administration Advisory Council, Earth System Sci- ences Committee. 1986. Earth System Science~verview. NASA, Washington, D.C., 48 pp. National Research Council. 1982. Scientific Basis of Water-Resource Management. Na- tional Academy Press, Washington, D.C., 127 pp. National Research Council. 1986. Global Change in the Geosphere-Biosphere. National Academy Press, Washington, D.C., 91 pp. Philip, J. R. 1978. Water on earth. Pp. 35-59 in Water: Planets, Plants and People. A. K. McIntyre,~ed. Australian Academy of Science, Canberra. 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 criticalcomparison. Annul Rev. Earth Planet. Sci. 15:171-172. Rainwater, F. H., and W. F. White. 1958. The solusphere- its inferences and study. Geochim. Cosmochim. Acta 14:244-249.

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THE HYDROLOGIC SCIENCES 61 Schneider, S., and R. Londer. The Co-Evolution of Climate and Life. Sierra Club Books, San Francisco, 563 pp. United Nations Educational, Scientific, and Cultural Organization (UNESCO). 1971. Scientific Framework of the World Water Balance. Technical Papers in Hydrology 7. UNESCO, Paris. U.S. Geological Survey. 1968. Water of the World. USGS 0-288-962. U.S. Government Printing Office, Washington, D.C.