General evolution of the concept of the hydrological cycle, by R. Nace

Introduction

It has been said that history at best is only an approximation of truth, and much that has been called history is only an approximation of probability.The hydro- logical cycle is the central concept of modern hydrology.When and where the full- fledged concept of a cycle first arose is unknown.Severa1 passages in the 'Old Testa- ment of the Christian Bible seem to imply the concept of a cycle, and the idea may antedate the Christian Era by several millenia.In the Old Testament (See Amos, V.8) one passage seems to imply a cycle:

"Seek him ... that calleth forth the waters of the sea, and poureth them out upon the face of the earth;the Lord is his name." The book of Amos dates from the eighth

century B.C., but the idea had much older roots.An oft-quoted passage from Eccle- siastes (1:7), dates from the tenth century B.C.:

"All the rivers run into the sea;yet the sea is not full; unto the ?lace from whence the rivers come, thither they return again." The ideas of Moses (about 1400 B.C.),

summarized by Professor Tonini seem to be a hybrid combination of ideas and legends handed down from several prior millenia. These deal with the nature of the universe and with the Noachian De1uge.They contain no hint of a hydrological cycle.The much later Lamentations of Daniel contains a nearly compiete list of hydrometeorological phenomena, but no idea of a cycle is implied.

Possibly Theophrastos the Greek thought in terms of a cycle several hundred years B.C.Not until comparatively modern times, however, was the complete concept of the hydrological cycle stated in fully coherent terms.

Written observations of weather and climate were made at least as early as 460 B.C. (Hippocrates).Early observers could have measured precipitation easily.Nothing was needed but a catchment receptacle and a measuring scale, both of which were available.However,many ancient peoples apparently saw no need for records, although they did observe weather phenomena and develop ideas.Reportedly, weather proverbs were inscribed on clay tablets as early as 4000 B.C.

The history of science is a history of the birth and development of ideas.In general, scientific understanding comes from the slow ripening of knowledge.Sudden intuitive flashes of insight, springing fullblown from the minds of individuals, actually are rare.The apparently abrupt birth of an insight commonly is the cul- mination of a long period of development during which one thinker after another approaches or skirts an idea.Commonly we know the name only of the individual who finally crystallized or recorded an Idea. So it is, for the most part, with hydrology. The origins of hydrological thought are lost in the mists of antiquity.

not merely as an adjunct of civilization, but as a stimulus to the development of civilization itself.It was no accident that the earliest civilizations arose in arid and semiarid regions, seemingly almost simultaneously in several parts of the world.The practice of irrigation was an essential ingredient of those civilizations. Management of irrigation systems required regional authority, so irrigation was an political evolution.Ur and some other ancient cities may have originated as administrative centres for irrigation districts.However, large-scale engineering works and water-management practices could not have arisen full blown without a long antecedent period of experimentation and small-scale practice.Modification and management of water regimes to any degree, however small, implies some knowledge of water action, even though the knowledge is wholly empirical.Thus a useful body of knowledge about practical hydrology existed perhaps as early as 6000 R.C.

Water had a crucial role in history,

Mesopotamia and the NiZe VaZZey

Opinions among archeologists differ on the question whether flood irrigation was developed first in Mesopotamia or in the Nile Valley.The weight of evidence seems to favciur Mesopotamia.Sumerian civilization seems to have arisen a few centuries before 3000 B.C., based on irrigation.Long before that, however, by about 6000 B.C., irriga- tion was practised in some middle eastern areas (Hole, 1966, p. 606,607) .Cities

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surrounded by irrigated fields evolved around 4000 B.C.In the Nile Valley, basin- type irrigation began a little earlier than 3000 B.C.Some authors place the date earlier and dispute the priority of Sumeria.

Invention of the Sumerian cuneiform script during the fourth millenium B.C. marks the beginning of recorded history. The record is 1aconic.Sumerian clay tablets and the inscriptions of ancient Egypt tell US much about accumulated knowledge concern- ing medicine, mathematics, geology, mineral- ogy, astronomy, cultivated crops, the armed conquests of kings, and something about the political and social systems.They give few clues to hydrological insights.

standards was wild and unpredictable.The princfpal water sources of the system were geographically remote, and flooding depended on the time and nature of meteorological conditions that were beyond the ken of the farmers on the Mesopotamian plain.The Sumerians, therefore, tended to ascribe river behaviour to the unpredictable caprice of various deities.

The Nile is a much less erratic river. Although the height of its flood could not be predicted, the time of flooding could be foreseen with considerable confidence.The Egyptians were an intensely practical people and it is doubtful that they had much scient- ific curiosity about floods.They were inter- ested in flood heights, however, because from the flood heights they knew what lands would be reached by flood waters and hence would yield crops that could be taxed. Recording of annual flood heights, therefore, began earlier than 3000 B.C.AS Professor Tonini notes, Egyptian forecasting, measure- ment, use and regulation of Nile flood waters is still worthy of admiration as an example of applied hydrology.

Neither in Egypt nor in Mesopotamia did agriculture depend on rainfall, so there was no apparent incentive for measuring rainfall, although this was well within the capabilities of the people.The Middle East, nevertheless, was one of the cradles of meteorological data gathering.In the first century A.D., Claudfus Ptolemaeus, the geographer, kept a meteoro- logical record at Alexandria, Egypt.Reports appeared at about the same time on regular observations of rainfall effectiveness in Palestine.The longest modern record of

The Euphrates-Tigris system, by ancient

meteorological data for the Middle East, however, is that of Jerusalem, which was not begun until 1860.

Asia In areas of rainfall agriculture the idea of measuring rainfall as a basis for taxation arose much later than that of measuring flood heights.Given enough soil moisture to permit ploughing and planting, the subsequent amount and distribution of rain would be an index of crop productivity and hence a guide to taxation.In India measurements for such pur- poses were begun as early as the latter part of the fourth century B.D.That has generally been considered as the birth-time of rain- gauging.

Now, however, Professor Chow informs us that in China observations of rain, sleet, snow and wind were recorded about 1200 B.C.Raingauges were used as early as 1000 B.C. and systematic raingauging was well established by about 200 B.C.Other writers assert that the earliest known upper air observations were made with kites which had been invented as a war instrument by the Chinese about 206 B.C.

Chow shows that a dynamic concept of the hydrological cycle arose in China at least by 900 B.C. and was developed in ensuing centuries, though most of the records for time before 200 B.C. have been destroyed.Although some thinkers and writers continued to deal with the water cycle and to make observations, science and techno- logy were denigrated for a thousand years thereafter in China. Stagnation was the rule there as in the western world,although for different reasons.

Chow mentions that Chinese observers referred to some wells as "eyes of the sea" recognizing that in at least some wells water levels fluctuate with the tides.It is safd also that some Chinese refer to rivers as threads of the sea.Coupled with their observations on precipitation and evaporation it is evident that the Chinese recognized the unity of all water and hence had the basis for understanding the water cycle.

selves with water early in the Christian Era, but either their contribution was small or it has not been summarized in Western literature.

Japanese observers also concerned them-

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During the nineteenth century Asian hydrology came strongly under the influence of Europeans, especially in connexion with irrigation schemes.Nevertheless,Asians made independent studies and contributions, notably in Thailand and Japan, and some extensive records in China, dating from the second century to modern times have received modern study and analysis. Significant among these are records for sedimentation and floods of the Hwang-ho and other major rivers.

out, the history of hydrology in Asia is fragmentary at best.Much insight could be obtained by further study, and this should be encouraged.

Obviously, as Professor Chow points

Greek hydrology

Hydrology, as we know it, is an organized system of concepts and knowledge that has been accumulated during thousands of years, but the principal enduring tenets of our scientific faith have been committed to writing largely during the past 300 years. It is a popular conceit among many scientists that scientific reseaqch is a relatively modern invention, stemming from the inno- vations of men like Galileo and Newton

mentioned and who, judged in the context of their own time and environment, were great thinkers.They were intensely curious about the world and the universe.Though many of their ideas now seem absurd, we have no ground for feeling superior.They built up the intel- lectual and scientific superstructure on which we stand.Basic research is merely an inquiry along the prevailing boundaries of human knowledge.ßut there have always been prevailing boundaries and man has always been curious.We should marvel at how far some men saw when they had no superstructure on which to stand.

the early Greeks to have reasonably good information about their ideas and thinking processes.Much that has been written about them is misleading and even contradictory. They have been accused of being philosophical debaters to whom it never occurred to make an experiment.But how much experimentation and research could be done without a thermo- meter, a barometer, a telescope, a microscope, a watch, a current meter or even a simple weir?Although the Greeks actually did make some elementary experiments,they are more notable as astute observers and logical thinkers.Though mistaken about many things, their mistakes were logical deductions from

We have sufficient records concerning

during the sixteenth and seventeenth centuries. observations - However, Phylon of Byzantium (283-247 B.C.) is credited with having first stated the general principle that repeated experiments and research should precede the enunciation of scientific theories.Archeologica1 evi- dence shows that men had a good empirical working knowledge of water behaviour thousands of years ago, for otherwise they could not have developed and maintained large irrigation systems.Al1 science begins with empiricism.The scientific method consists of observing facts and correlating these facts by means of theories.The validity of a theory is then tested against new or additional facts.Contrary to a popular notion, the outstanding mark of the great scientist is not machine-like thinking, but a soundly logical mind, keen powers of observation and brilliant imagination. Therefore, scientists are distinguished from others less by their methods than by their state of mind.

All civilizations, ancient and modern, have produced men who had the qualities

The ideas of some early Greek thinkers are known to us only indirectly, from later written reports of what they said and did. During the Hellenic period (time from Thales in the sixth century B.C. to the first of Aristotle's disciples) the Greeks were not only heirs to knowledge from the East and Middle East; by their own efforts they were developing science into an independent intellectual discipline.They sought not only to amass information but to consolidate knowledge.They believed that the universe is orderly and therefore understandable and predictable.The Greeks contributed so much to science, including hydrology, that it is impossible to do justice to their ideas and accomplishments in one section of a short paper.Tonini's summary, though brief, is highly informative,

have been the first philosopher who tried to interpret natural phenomena scientifically, rather than on the basis of mythology or theology.Thales supposed that all substances

Thales of Miletos (640?-546 B.C.) may

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originally came from water and eventually would return to water.0n the record, at least, this was man's first attempt to reduce the bewilder- ing diversity of matter to a common denominator. However, so far as the hydrological cycle is concerned, Thales was far off the track.He believed that rivers are derived from the sea, but he had the derivation in reverse.Wind forces sea water into the earth; once inside, overburden pressure forces the water upward into mountains from which the water runs out to form rivers.Neverthe1ess Tonini sees in Thales' ideas some hint of a hydrological cyc1e.Anaximander and Anaximenes, though they had some interesting ideas about hydrometeoro- logical phenomena, added little to real know- ledge about hydrology,

Xenophanes of Colophon (570-470 B.C.) established a turning point in Greek philosophy, setting it on the path that led to Plato and Aristotle.He said that "The sea is the source of the waters, and the source of the winds. Without the great sea, not from the clouds could come the flowing rivers or the heaven's rain; but the great sea is the father of clouds, of rivers and of winds."Tonini has a good discussion of Xenophanes.

Agrigentum and others also had some interest- ing ideas about earth, air, fire and water but added nothing of consequence to the concepts of hydrology.

teacher of Pericles, had some interesting ideas about meteorology and hydrology.Anaxagoras believed that the sun raises water from the sea into the atmosphere, from which it falls as rain.Rainwater gathers in great underground reservoirs from which rivers flow.No new water is generated within the earth, but the reservoirs fil1 during the rainy season; some reservoirs are larger than others so some rivers are perennial while others are not.The river water, of course, returns to the sea.

The sketch does not mention overland run- off from precipitation, probably because that seemed obvious and quantitatively minor. Nor does it mention the subcycle of trans- piration by vegetation, but what else could be expected at a time when men knew relatively nothing about plant physiology?Making due allowance for lack of substantive data 2,500 years ago, Anaxagoras formed a concept of the hydrological cycle which was qualitatively essentially correct.The gaps and inaccuracies

Beraclitus of Ephesus, Empedocles of

Anaxagoras of Clazomene (500-428 B.C.),

in his concept are perfectly understandable, Much of Greece and other parts of the Mediter- ranean area are limestone country, subter- ranean caverns containing water were well known, and many of these yield water copiously. Anaxagoras was basically right within the geographic and geologic limitations of his observations,

None of Anaxagoras' writings have been preserved and his ideas are known only from the writings of Aristotle, who summed them up more than 100 years later, only to reject them with unconcealed contempt.

A few later Greeks contributed some variations to scientific thought, but not to hydrology, until the time of Herodotus of Alicarnassus.He made some important observa- tions in Egypt, which he described as a product of Nile sedimentation.His ideas about hydrology, however, were mostly naive and erroneous, as noted by Tonini.Various later philosophers speculated about water. Like many of their predecessors, their ideas about the behaviour of the Nile river figured prominently, but they added little or nothing to realistic concepts of hydrology.

Plato, mentor of Aristotle, instigated some rapid advances in Greek thinking.For example, he assumed that the universe was created by an organizing mind.Therefore, - the universe is understandable.Hydrologically, however, Plato was retrograde.He said that a series of interconnecting subterranean channels communicate with their source, a vast subterranean reservoir of water which he called Tartarus.Perpetua1 surging to and fro of the waters of Tartarus caused the flow of rivers and springs.Al1 waters of rivers and seas arise from and return to mythical Tartarus. Apparently ,however , he did recognize the meteoric origin of some spring waters.

Aristotle of Stagira (Stavros) (384- 322 B.C.) accepted that some springs are fed by meteoric water, but he believed that the main mass of water in rivers originates in great underground caverns by transforma- tion of air into water.His ideas about meteorology were logical on the basis of what he could observe without instrumental observation, but were largely erroneous.

Aristotle asserted that in matters of science his predecessors had handed on nothing that "the most ordinary man could not have thought of".True science was to

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begin with him.He was a firm advocate of observation and taught that knowledge must advance.His prestige became so great that science almost ended with him.Aristotle made more mistakes and uttered more absurd- ities than any other philosopher who ever wrote down his ideas.His encyclopaedic mind ranged the whole gamut of human knowledge and speculation, so that it was impossible for him not to be wrong about many things.He claimed to be an omniscient speaker of the latest word of wisdom in all matters.His feebly gifted successors followed him almost blindly for over 1500 years.This was not Aristotle's fault.His successors were simply incapable of testing his ideas.ûwing to ecclesiasticism and authoritarianism it became heresy to question either Aristotle or the Bible.

generated some interesting ideas, as out- lined by Tonini.Among them was Theophrastos, who, as already noted, is sometimes credited with having the earliest essentially correct idea of the hydrological cycle.

Some of Aristotle's followers, however,

time, is poorly known because most of his writings have been lost or destroyed.However, it is generally believed, as already stated, that Theophrastos was the' first man who, at least on the record, had a basically sound understanding of the rudiments of the complete hydrological cycle.

The Romans did share the age-old curiosity about the source of the Nile River, and Nero sent a small expedition to seek the source.

Despite their great hydraulic works, no evidence has been found that Roman engineers as a group had any clear idea of a hydrolo- gical cycle.Roman science was too intensely practical to be pr0gressive.A rapid intellectual decline began in the fourth century, and the European Dark Ages that followed were scienti- fically dark indeed.

For the awakening that followed, much credit must be given to the Arabs, who pre- served Hellenic philosophy and science by translation.They also made original scientific contributions, especially in mathematics, astronomy and medicine.

Roman hydrology The Scientific Awakening

The Romans emerged from barbarism and achieved civilization under the influence of the Etruscans, who were masters of the arts of swamp drainage and irrigation.This gave the Romans their start towards becoming masters of hydraulic engineering construction of many kinds.However, the Romans added little to science, generally accepting the ideas of the Greeks, as noted by Tonini.They added little except trivial or erroneous details.However, the great Roman poet Lucretius organized the scientific knowledge inherited from the Greeks, and added his own judgements about which ideas were acceptable.

One Roman, however, a military engineer and architect, did have a reasonably clear concept of the general nature of the complete hydrological cycle.In about 15 B.C. Marcus Vitruvius Pollio, wrote that water which evaporates from the sea and other water bodies forms clouds, that rain from these penetrates the ground and emerges as springs.The idea probably was not original with him because he was acquainted with the writings of Theo- phrastos.Theophrastos (3711-288?), a Greek who wrote about 300 years before Vi-truvius'

I refer here to the awakening rather than to the birth of science because, as noted earlier, science was born millenia ago, and it merely slumbered during the so-called Dark and Middle Ages.At least three things made the awakening not only possible but inevitable.0ne was the incraasingly obvious barrenness of scholasticism and authoritar- ianism.The second was the infiltration of religious authority by ancient Greek philo- sophy and the eventual challenge to the intellectual supremacy of the Church.Men began to question authority and to question the universe.Thirdly, men began to make tools and instruments for making observations and measurements, rather than for use as toys or instruments of magic.Almost any advance in science requires making new measurements or observations or making old ones more accurately in the course of experiments.

The awakening seems to have occurred earliest in Italy, spreading quickly from there northward.Leonardo da Vinci (1452- 1519), Girolamo Cardano (1501-15761, and Galileo Galilei (1564-1642) may be mentioned, but we have no svace here to add to what has

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already been written about them.To the extent that they dealt with water they were primarily hydraulicians rather than hydrolo- gists, as here defined.It may be noted, however, that it is not clear from the Da Vinci passages quoted by Tonini, that Leonardo apparently had a double concept of the hydrological cycle.In the one concept he considered that sea water rises within the earth to the heights of mountains, analogous to the rise of blood from lower regions of the human body to the head, whence it may flow out through cuts.The other aspect of his concept was essentially in accord with the modern concept.Eiéher Leonardo believed that both processes operate, or he changed his mind as time passed.

Invention of the barometer, generally attributed to Evangelista Torricelli (1608- 1647) in 1643, and of the sealed thermometer by Ferdinand, Grand Duke of Tuscany in about 1641, made possible some major advances in hydrometeorology and hence in hydrology because they permitted new quantitative observations.Use of the barometer and other instruments was quickly taken up by Mariotte in France, who is generally considered to be the father of scientific experimental methods in France.Independently of the Englishman, Robert Boyle, he originated what we call Boyle's 1aw.Boyle apparently introduced the word hydraulics in scientific literature.

of Mariotte, was Domenico Guglielmini (1655- 1710), who tended to belittle Mariotte's work.His own method of work was largely by field observation rather than by laboratory experiment.According to Rouse and Ince (1963, p. 69), Guglielmini's primary contribution was a broad analysis of the role played by torrents and rivers "in what has since been named the hydrologic cycle".

Before Guglielmini's time, Bernardo Ramazzini (1633-1714) had published in 1691 a partial explanation of the hydrodynamics of artesian wells in the city of Modena.It remained, however, for Antonio Vallisnieri (1661-1730) to publish in 1715 a basically correct explanation of the artesian-well situation at Modena, including a correct explanation of the source of the water.

Another notable Italian, a contemporary

fimepgence of the Cyclical Concept

As for the hydrological cycle, the first approach to its rational analysis since before the Dark Ages occurred in France, which became the Mecca of Science during the seventeenth century.The earlier work of Palissy was astute but he made no experiments and he did not write in scholarly Latin, hence he had little impact on science.His principal fame was as a potter, geologist and martyr to the persecution of the Huguenots.Perhaps the most moving account ever written about Palissy as an individual was that of the great French literatus, Anatole France (1880), which seems to be unknown to most hydrologists.

Professor Tixeront has given us a concise and very informative outline of the development of seventeenth century French thinking leading up to the advent of Pierre Perrau1t.For Perrault he has presented a fair and astute analysis of some, but not all, of Pierre's accomplishments as recorded in his little book on the origin of springs. But Pierre does not emerge as an individual of flesh and blood with human traits of strength and weaknesses.Little is known directly about Pierre, but much can be inferred from writings about his family, especially a recent analysis by Soriana (1968) of the family history and character- is tics.

not be confused with the first appearance of a name for it.Both Perrault and Mariotte recognized the disposition of precipitation by evaporation and streamflow, for which they had nameS.Both recognized soil-moisture and ground-water recharge (no names), but Perrault had some erroneous notions about ground-water recharge in general.Pierre also had the concepts of bank storage in river valleys and return flow (no names). It is evident that he also recognized over- land flow and subsurface runoff.Many authors have described the hydrological cycle, but Pierre is one of the few in post-Biblical times who described its initiation and full development, beginning with the time of Creation (Perrault, 1674, p. 221-226).

dealt with evaporation in the disposition of Precipitation, they did not discuss the relative importance of marine and terrestrial

The first development of a concept should

Curiously, though Perrault and Mariotte

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evaporation for the replenishment of atmos- pheric moisture.It is implicit in Perrault's thinking, however, that he recognized derivation of at least some atmospheric moisture from the sea.He said that his find- ings in the upper basin of the Seine River were not just a local phenomenon but illus- trated a universal principle.Inasmuch as he said that about one-sixth of precipitation runs off to the sea, he must have recognized a return mechanism, otherwise continental air masses would soon become permanently depleted of vapour.

scientifically better trained than Perrault, is sometimes said to have been a "better" scientist than Perrault.Invidious comparisons are unnecessary.To the extent that Mariotte dealt with liquids, he would be called a hydraulician in modern parlance.Perrault dealt principally with concepts and made no attempt at quantitative precision.He said, in fact, that his data were imprecise and expressed a hope that they would be improved upon.

atmospheric arc of the hydrological cycle, so to speak.Professor Dooge has dealt at some length with Halley's life and work.It is interesting, as an aside, to know how Halley became interested in evaporation and atmos- pheric moisture.In about 1677, while making astronomical observations on St. Helena at an altitude of about 730 m, Halley noted that his telescopic lenses quickly became fogged by condensation, and paper became so moist that he could not write on it.These nuisances may have been the small seeds that later germinated in Halley's mind as he developed his concept of the hydrological cycle, including the atmospheric arc.

with water already in the atmosphere and from there described the disposition of preci- pitation.Halley sought and found the source of atmospheric water vapour and attempted to show a balance in the complete cycle of water move- ment.ûwing to the crudity of measurements and estimates, the main contribution of the three men was the concept of the cycle rather than proof by accurate measurement.The proof was delayed a hundred years until John Dalton took up the problem.

of the atom dates back to classical Greece, where it was generated and expounded by

Edmé Mariotte, a more versatile man and

It remained for Edmond Halley to close the

In summary, Perrault and Mariotte started

Many historians have noted that the idea

Leukippos and Demokritos.Although the atom, as now conceived, is something quite different from the Grecian concept, the atmmic concept, was never lost to sight.Efforts to interpret matter in terms of particles were made many times, notably by Isaac Newton and Daniel Bernouilli, before Dalton's time.Dalton, however, took the idea of the atom very seriously (he used the term "ultimate particle", rather than "atom") .His law of multiple proportions was a deduction from the atomic theory, not the converse, as has been supposed by some historians.Dalton showed that elements consist of distinctive atoms which combine according to simple laws, and large-scale combinations of atoms reflect these simple laws .Conversely, the massive behaviour of elements in combination gives information about the constituent atoms.In that connexion it is pertinent to mention the somewhat earlier work of the great French chemist, Antoine Laurent Lavoisier (1743-1794).Lavoisier, it will be recalled, demonstrated conclusively that air is a mixture, not a compound, of gases. He named two of the gases, oxygen and azote (nitrogen).He also proved that water is a chemical compound, which he synthesized by burning hydrogen.He was not the first person to do so, but he was the first to explain what actually happened.Thus, chemical study of water set the science of chemistry on its modern path.Dalton, in effect, took up where Lavoisier left off.

water retaining its identity when it vapor- izes, being diffused in the air, not trans- formed into air or combined with it in any new compounds.He made another pronouncement which, though it now seems elementary, had profound implications at that time: namely, that evaporation from land must be less than precipitation, otherwise no rivers could flow. This,in effect, was the obituary of the notions that cold air in caverns is transformed into water that feeds springs, and that exhalations from the interior of the earth are the source of springs.

Dalton went further, recognizing that evaporation and precipitation are opposite processes, caused by the opposite effects of heat and cold.Then he reached the qualitat- ively correct conclusion that mountainous areas receive more precipitation than lowlands because of what are now called orographic

Dalton pronounced stoutly in favour of

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influences.Da1ton's measurements, experiments and deductions were sufficiently consistent to set hydrologists on the correct path towards the measurement and estimation of water balanceS.Dalton was, in fact, the first person of record to attempt to strike a water balance for a large area, namely, a good share of the British Isles.Moreover, he estimated each component of the water cycle separately.The result was a discrepancy or imbalance.It is worth noting that, even today, if we estimate each item separately, without adjustment, we still have imbalances, whether for the British Isles or any other area.

calculated the water power available by subtracting evaporation from precipitation. This implies recognition of the basic water balance equation, P = R +E.Kane mentioned earlier work on this topic dating back at least fifty years.

Dooge's account of the work and importance of Thomas Mulvaney deals more with methodology than with basic concepts.?lulvaney's main contribution is perhaps best summarized in Professor Dooge's final paragraph.Mulvaney went beyond the idea of a water balance based on measurement of all its elements in an effort to predict runoff on the basis of measured rainfall ratio and watershed characteristics. His work led to the current form of the genetic theory of runoff and thence to unit hydro- graph methods.Mulvaney recognized the effect of human activity on the hydrological cycle, laying the foundation for a new departure "that can be linked to some of the concepts of hydrologic processes that are the subject of active research" today.

scientists to early developments in the evolution of the concept of the hydrological cycle is difficult to evaluate owing at least in part to lack of availability of Russian contributions in western languages. Chemists, physicists and oceanographers have been more alert, and Russian contri- butions in their fields are well known. G.P. Kalinin's paper deals chiefly with the twentieth century and much of it concerns methodology rather than the evolution of ideas.

Russian interest in water balances during the eighteenth and nineteenth centuries,however, is well recognized, including pioneering work in the Caspian

In 1845 the Irishman Sir Robert Kane (1845)

The contributions of Russian and Soviet

Basín.Russian scientists, moreover, were among the first to recognize and attempt to evaluate the importance of vapour fluxes from maritime sources as contributors to inland continental precipitation.

Recognizion by Veselovskiy and Voyeikov of the significance of horizontal vapour fluxes in the water cycle in the middle and latter part of the nineteenth century had already been anticiptetd in principle by Halley and Dalton.Da1ton's formulation of what has since been called Dalton's law or "law of partial pressuresl'laid the founda- tion for the subsequent scientific work on evaporation and vapour fluxes.0n the other hand, Russian scientific contributions to the study of large-scale heat and water balances, beginning early in the twentieth century, are widely known and appreciated.

Professor Kalinin's paper devotes considerable space to changes of sea level and their hydrological significance. Large changes through geologic time, owing to the waxing and waning of glaciers and icecaps, are undisputed.Cmal1er changes, of the order of one or two millimetres per year during recent decades are open to dispute, both as to their actuality and their cause.Geodetic data are too imprecise for the world as a whole to support positive conclusions.The same is true of information about variations in the amounts of water in ocean basins and in land areas.Much has been written about changes in mean sea level, but the latest analyses of evidence seem to indicate that recent small changes are caused by warping of ocean basins and changes in their capacities.

It seems unlikely that a definitive history will be written until more historians consult original sources and devote considerable effort to the study of individuals and the circumstances of their accomp1ishments.Darmstaedter (1927) aptly said,

"We owe the great men of science not only a careful reading of their works, but also a study of their lives and characters so that they may appear to us as real characters.We can then better understand how they came to undertake and accomplish their tasks and appreciate in what way their achievements reacted in determining the course of their careers".

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Italian contributors, as well as those from Russian and other parts of Europe are not really well known.As is well illustrated by Professor Chow's paper, only fragments of Asiatic contribution are known.

In earth sciences it is a generally accepted aphorism that the present is the key to the past.1 suggest that the past is also a key to the future.When we know better where man has been, we may be able to guide him better to where he should go.

can be summarized very briefly.Prehistoric and colonial developments need not be considered because they contributed little to the concept of large-scale water balances. Within the past hundred years North American hydrologists capitalized avidly on the ante- cedent brilliant laboratory studies in Europe, Beginning shortly after the middle of the nineteenth century, North America became a major world scene of hydrological studies with special emphasis on field data and studies.This permitted the testing, elabora- tion and modification of theories and exten- sive developments of instruments and methods for studies at all scales, both small and large.

moved to the forefront in ground-water surveying and research and seem to be main- taining leadership.

Relatively little has been written on the history of hydrology in North America and this deficiency should be remedied.As stated in my basic paper, it may be too early for objective evaluation of accomplishments during the past thirty years.Nevertheless, such an evaluation should be begun.

Development of hydrology in North America

At the turn of the century North Americans

Conclusion

The history of hydraulics up to 1957 has been covered reasonably well by Rouse and Ince (1963).But hydrology, as here defined, is a set of concepts about the hydrological cycle and the interplay of its processes.Hydraulics is an essential branch of physics in the study of hydrology, but it does not constitute hydrology anymore than does mathematics or chemistry, which are also essential.

of so-called hydrology, but they are heavily Many books contain sketches of the history

weighted with hydrometry, hydraulics and mathematicS.No definitive history of hydrology has been written, and to prepare one would be a major task.A recent book by Biswas (1970) is useful but it is more a chronicle than a history and it, also, is heavily weighted with hydraulics, much of which is a repetition of parts of Rouse and Ince.

Re ferences

In addition to the references quoted, use has been made of contributions by Messrs. Chow, Dooge, Eckoldt, Kalinin, Nace and Tonini.Their papers form a separate publi- cation by Unesco in the series "Technical papers in hydrology", entitled "Contributions to the general evolution of the concept of the hydrological cycle".

Biswas, A.K. 1970. History of hydrology. New York, American Elsevier Publishing Co., 336 p.

Darmstaedter, Ludwig. 1927. The life of Edmé Mariotte. Jour. Chemical Education. v. 4, no. 3, March, p. 320-322.

France, Anatole. 1880. Notice historique et Bibliographique. In: Oeuvres complktes de Bernard Palissy, publiées d'après les textes originaux. Paris, Charavay Frères, p. v-xxvii.

Hole, Frank. 1966. Investigating the origin of Mesopotamian civilization. Science, v. 153, no. 3736, p. 606-610.

Kane, §ir Robert, 1845. The industrial resources of Ireland,chap. III. (Not seen.Cited by Austin Bourke, letter to U.S.Nat. C o m . for the I D , 23 April 1973).

Perrault, Pierre. 1674. De t'origine des fontaineS.Paris, Pierre le Petit, 229 p. (Reprinted at Paris by Jean & Laurent d'Houry, same number of pages, in 1678).

Rouse, Hunter; Simon Ince. 1963. History of hydraulics, New York, Dover Publications, 269 p. (First published in 1957 by the Iowa Institute of Hydraulic Research).

culture savante et traditions populaires. Paris, Editions Gallimard, 525 p.

SoriSoo, Marc. 1968. Les contes de Perrault,

48

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General evolution of the concept of the hydrological cycle, by R. Nace

Abstruct

Hydrology is an organized system of concepts and knowledge about water and its behaviour in the hydrological cycle.It is often confused with hydrau1ics.a branch of physics which, however essential it may be in hydrological studies,is not hydrology.The origins of the concept of the hydrological cycle are ancient but obscure.Some biblical passages have been inter- preted to imply a cyclical concept,but these are debatab1e.h ancient Sumeria and Egypt,water was a management problem,and archeological records contain no hint of a cyclical concept. Some Greek thinkers may have had a conceptually sound but rudimentary idea of a cycle,but the complete concept of the hydrological cycle,stated in fully coherent terms,is a relatively modern deve1opment.A dynamic concept of the cycle may have arisen in China by 900 B.C.,but this had no impact on western thought.China underwent an equivalent of the European Dark Ages and science was at a virtual standstill,worldwide, for at least a thousand years.

century,when men began to make instruments,measurements and deductions,rather than accepting ecclesiastical or Aristotelian authority.Leonardo da Vinci was a principal catalytic innovator at that time.Attitudes spread quickly northward and by the seventeenth century France was in the vanguard of scientific achievement.Scientific hydrology was born there but British thinkers also were becoming increasinglv active and took a strong position in the eighteenth century.By the nineteenth century most of the advanced countries of the world were active in many aspects of the water cycle and were beginning to think in terms of continental and global phenomena.The twentieth century has witnessed a nearly universal awakening to the importance of the hydrological cycle and the evolution of much international collaboration.Hydrology may be on the verge of its Golden Age.

The scientific awakening in Europe began during the fifteenth

Evolution générale d u concept de cycle hydrologique, par R. Nace

Résumé

L'hydrologie est un ensemble organisé de théories et de connaissances concernant l'eau et son cycle.0n la confond souvent avec l'hydraulique,branche de la physique qui,pour aussi essentielle qu'elle puisse être dans les études hydrologiques, est néanmoins une science différente.La notion de cycle de l'eau a des origines anciennes mais obscures.11 semble qu'il y soit fait allusion dans certains passages de la Bible,mais ceux-ci sont sujets à contr0verse.A Sumer et dans 1'Egypte antique,l'eau posait avant tout un problème d'aménagement et rien dans l'archéologie n'indique que la notion de cycle existait chez ces peupleS.certains penseurs grecs s'en étaient peut-être fait une idée intellectuellement valable,encore que rudimentaire, mais cette notion,dans son acception intégrale et sa formulation systématique,est relativement moderne.Si une conception dynamique dy cycle de l'eau a fait son apparition en Chine au neuvième siècle avant J.-C.,elle n'a eu aucune influence sur la pensée occidentale.La Chine a traversé une période analogue au moyen âge européen et,dans le monde entier,la science n'a fait pour ainsi dire aucun progrès pendant au moins un millier d'années.

Le renouveau scientifique en Europe a débuté au quinzième siècle,lorsque l'homme s'est mis à fabriquer des instruments, à effectuer des mesures et à faire des déductions au lieu de s'en remettre au principe de l'autorité ecclésiastique ou aristoté1icienne.Léonard de Vinci fut un des grands novateurs et catalyseurs des courants de 1'époque.La renaissance gagna rapidement le nord et, au dixseptième siècle, la France se trouva à l'avant-garde du progrès scientifique.C'est 13 que prit naissance la science de l'hydrologie,mais les savants britanniques de leur côté jouèrent un rôle de plus en plus actif,qui s'affirma nettement au dixhuitième siècle.Au dix- neuvième siècle,la plupart des pays développés du monde

49

R. Nace

étudiaient de nombreux aspects dy cycle de l'eau et commen- çaient 2 les envisager dans une perspective continentale et mondiale.Au vingtième siècle,l'on assiste à une prise de conscience quasi universelle de l'importance de ce cycle et au développement d'une vaste collaboration internationale. L'hydrologie est peut-être à la veille de connaître son âge d'or

Evolución general del concepto de ciclo hidrológico, por R. Nace

Resumeri

Pe31o~e

La hidrologfa es un sistema organizado de conceptos y de conocimientos relativos al agua y a su comportamiento en el ciclo hidro1ogico.A menudo se la confunde con la hidraulica, una rama de la física que, por esencial que sea en los estudios hydrologicos ,no es hidrologia.Los orígenes del concepto del ciclo hidrológico son antiguos pero confusos, Algunos pasajes bíblicos se han interpretado en el sentido que existfa un concepto c€clico,pero ello es discutible.En el antiguo Egipto y en Sumer, el agua constitufa un problema de explotación de ese recurso, y de los vestigios arqueológicos no se desprende que existiera un concepto cíclico.

Es posible que aleunos pensadores griegos hayan tenido una idea conceptualmente correcta aunque rudimentaria de un ciclo, pero el concepto completo de ciclo hidrologico,enunciado en términos totalmente coherentes,es relativamente moderno.Cabe que en China,hacia el ano 900 a. de J.C. se haya enunciado un concepto dinamico del ciclo,pero no tuvo influencia alguna en el pensamiento occidental.La China tuvo un equivalente de los primeros siglos de la Edad Media europee,en que la ciencia estuvo virtualmente paralizada,en el mundo entero,durante mil anos por lo menos.

cuando el hombre comenzó a construir instrumentos,hacer mediciones y deducciones,en vez de acatar la autoridad ecle- sisstica o aristotélica. En esa época,Leonardo da Vinci fue uno de los principales catalizadores de la innovaci8n.Las nuevas actitudes se extendieron rapidamente hacia el norte,y en el siglo diecisiete,Francia se encontraba y en la vanguardia del propreso cientffico.La hidrologia cientifica nació en Francia, pero simultaneamente aumentaba la actividad de los pensadores ingleses qu en el siglo dieciocho adoptaron una firme postura en la materia.En el siglo diecinueve,la mayor parte de los paises adelantados del mundo ya desarrollaban actividades relacionadas con muchos aspectos del ciclo hidrolögico y empezaban a pensar en términos de fenómenos continentales y universales.En el siglo veinte se asistió a una toma de consciencia casi universal de la importancia del ciclo hidro- lÓgico,asf como al desarrollo de una gran colaboraci8n inter- nacional.La hidrologfa está posiblemente a punto de entrar en su Edad de Oro.

El despertar cientffico de Europa empezó en el siglo quince,

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