The Earth As Modified By Human Action Part 27

"The extent of soil artificially dried by drainage is constantly increasing, and the water received by the surface from precipitation flows off by new channels, and is in general carried off more rapidly than before. Must not this fact exercise an influence on the regime of springs whose basin of supply thus undergoes a more or less complete transformation "--Bernhard Cotta, Preface to Paramelle, Quellenkunde, p.

vii., viii.

The effects of agricultural drainage are perceptible at great depths. It has been observed in Cornwall that deep mines are more free from water in well-drained districts than in those where drainage is not generally practised.--Esquiros, Revue des Deux Mondes, 15 Nov., 1863, p. 430.

See also Asbjornsen, Torv og Torvdrift, p. 31.]

Geographical and Meteorological Effects of Aqueducts, Reservoirs, and Ca.n.a.ls.

Many of the great processes of internal improvement, such as aqueducts for the supply of great cities, railroad cuts and embankments, and the like, divert water from its natural channels and affect its distribution and ultimate discharge. The collecting of the waters of a considerable district into reservoirs, to be thence carried off by means of aqueducts, as, for example, in the forest of Belgrade, near Constantinople, deprives the grounds originally watered by the springs and rivulets of the necessary moisture, and reduces them to barrenness.

[Footnote: See a very interesting paper on the Water-Supply of Constantinople, by Mr. Homes, of the New York State Library, in the Albany Argus of June 6, 1872. The system of aqueducts for the supply of water to that city was commenced by Constantine, and the great aqueduct, frequently ascribed to Justinian, which is 840 feet long and 112 feet high, is believed to have been constructed during the reign of the former emperor.] Similar effects must have followed from the construction of the numerous aqueducts which supplied ancient Rome with such a profuse abundance of water. [Footnote: The unhealthiness of the Roman Campagna is ascribed by many mediaeval as well as later writers to the escape of water from the ancient aqueducts, which had fallen out of repair from neglect, or been broken down by enemies in the sieges of Rome.] On the other hand, the filtration of water through the banks or walls of an aqueduct carried upon a high level across low ground, often injures the adjacent soil, and is prejudicial to the health of the neighboring population; and it has been observed in Switzerland and elsewhere, that fevers have been produced by the stagnation of the water in excavations from which earth had been taken to form embankments for railways.

If we consider only the influence of physical improvements on civilized life, we shall perhaps ascribe to navigable ca.n.a.ls a higher importance, or at least a more diversified influence, than to aqueducts or to any other works of man designed to control the waters of the earth, and to affect their distribution. They bind distant regions together by social ties, through the agency of the commerce they promote; they facilitate the transportation of military stores and engines, and of other heavy material connected with the discharge of the functions of government; they encourage industry by giving marketable value to raw material and to objects of artificial elaboration which would otherwise be worthless on account of the cost of conveyance; they supply from their surplus waters means of irrigation and of mechanical power; and, in many other ways, they contribute much to advance the prosperity and civilization of nations. Nor are they wholly without geographical importance. They sometimes drain lands by conveying off water which would otherwise stagnate on the surface, and, on the other hand, like aqueducts, they render the neighboring soil cold and moist by the percolation of water through their embankments; [Footnote: Sismondi, speaking of the Tuscan ca.n.a.ls, observes: "But inundations are not the only damage caused by the waters to the plains of Tuscany. Raised, as the ca.n.a.ls are, above the soil, the water percolates through their banks, penetrates every obstruction, and, in spite of all the efforts of industry, sterilizes and turns to mora.s.ses fields which nature and the richness of the soil seemed to have designed for the most abundant harvests. In ground thus pervaded with moisture, or rendered COLD, as the Tuscans express it, by the filtration of the ca.n.a.l-water, the vines and the mulberries, after having for a few years yielded fruit of a saltish taste, rot and perish.

The wheat decays in the ground, or dies as soon as it sprouts. Winter crops are given up, and summer cultivation tried for a time; but the increasing humidity, and the saline matter communicated to the earth--which affects the taste of all its products, even to the gra.s.ses, which the cattle refuse to touch--at last compel the husbandman to abandon his fields and leave uncultivated a soil that no longer repays his labor."--Tableau de l'Agriculture Toscane, pp. 11, 12.] they dam up, check, and divert the course of natural currents, and deliver them at points opposite to, or distant from, their original outlets; they often require extensive reservoirs to feed them thus retaining through the year acc.u.mulations of water--which would otherwise run off, or evaporate in the dry season--and thereby enlarging the evaporable surface of the country; and we have already seen that they interchange the flora and the fauna of provinces widely separated by nature. All these modes of action certainly influence climate and the character of terrestrial surface, though our means of observation are not yet perfected enough to enable us to appreciate and measure their effects.

Antiquity of Irrigation.

We know little of the history of the extinct civilizations which preceded the culture of the cla.s.sic ages, and no nation has, in modern times, spontaneously emerged from barbarian and created for itself the arts of social life. [Footnote: I ought perhaps to except the Mexicans and the Peruvians, whose arts and inst.i.tutions are not yet shown to be historically connected with those of any more ancient people. The lamentable destruction of so many memorials of these tribes, by the ignorance and bigotry of the so-called Christian barbarians who conquered them, has left us much in the dark as to many points of their civilization; but they seem to have reached that stage where continued progress in knowledge and in power over nature is secure, and a few more centuries of independence might have brought them to originate for themselves most of the great inventions which the last four centuries have bestowed upon man.] The improvements of the savage races whose history we can distinctly trace are borrowed and imitative, and our theories as to the origin and natural development of industrial art are conjectural. Of course, the relative antiquity of particular branches of human industry depends much upon the natural character of soil, climate, and spontaneous vegetable and animal life in different countries; and while the geographical influence of man would, under given circ.u.mstances, be exerted in one direction, it would, under different conditions, act in an opposite or a diverging line. I have given some reasons for thinking that in the climates to which our attention has been chiefly directed, man's first interference with the natural arrangement and disposal of the waters was in the way of drainage of surface. But if we are to judge from existing remains alone, we should probably conclude that irrigation is older than drainage; for, in the regions regarded by general tradition as the cradle of the human race, we find traces of ca.n.a.ls evidently constructed for the former purpose at a period long preceding the ages of which we have any written memorials.

There are, in ancient Armenia, extensive districts which were already abandoned to desolation at the earliest historical epoch, but which, in a yet remoter antiquity, had been irrigated by a complicated and highly artificial system of ca.n.a.ls, the lines of which can still be followed; and there are, in all the highlands where the sources of the Euphrates rise, in Persia, in Egypt, in India, and in China, works of this sort which must have been in existence before man had begun to record his own annals.

In warm countries, such as most of those just mentioned, the effects I have described as usually resulting from the clearing of the forests would very soon follow. In such climates, the rains are inclined to be periodical; they are also violent, and for these reasons the soil would be parched in summer and liable to wash in winter. In these countries, therefore, the necessity for irrigation must soon have been felt, and its introduction into mountainous regions like Armenia must have been immediately followed by a system of terracing, or at least scarping the hillsides. Pasture and meadow, indeed, may be irrigated even when the surface is both steep and irregular, as may be observed abundantly on the Swiss as well as on the Piedmontese slope of the Alps; but in dry climates, ploughland and gardens on hilly grounds require terracing, both for supporting the soil and for administering water by irrigation, and it should be remembered that terracing, of itself, even without special arrangements for controlling the distribution of water, prevents or at least checks the flow of rain-water, and gives it time to sink into the ground instead of running off over the surface.

The summers in Egypt, in Syria, and in Asia Minor and even Rumelia, are almost rainless. In such climates, the neccssity of irrigation is obvious, and the loss of the ancient means of furnis.h.i.+ng it helps to explain the diminished fertility of most of the countries in question.

[Footnote: In Egypt, evaporation and absorption by the earth are so rapid, that all annual crops require irrigation during the whole period of their growth. As fast as the water retires by the subsidence of the annual inundation, the seed is sown upon the still moist, uncovered soil, and irrigation begins at once. Upon the Nile, you hear the creaking of the water-wheels, and sometimes the movement of steam-pumps, through the whole night, while the poorer cultivators unceasingly ply the simple shadoof, or bucket-and-sweep, laboriously raising the water from trough to trough by as many as six or seven stages when the river is low. The bucket is of flexible leather, with a stiff rim, and is emptied into the trough, not by inverting it like a wooden bucket, but by putting the hand beneath and pus.h.i.+ng the bottom up till the water all runs out over the brim, or, in other words, by turning the vessel inside out.

The quant.i.ty of water thus withdrawn from the Nile is enormous. Most of this is evaporated directly from the surface or the superficial strata, but some moisture percolates down and oozes through the banks into the river again, while a larger quant.i.ty sinks till it joins the slow current of infiltration by which the Nile water pervades the earth of the valley to the distance, at some points, of not less than fifty miles.] The surface of Palestine, for example, is composed, in a great measure, of rounded limestone hills, once, no doubt, covered with forests. These were partially removed before the Jewish conquest.

[Footnote: "Forests," "woods," and "groves," are frequently mentioned in the Old Testament as existing at particular places, and they are often referred to by way of ill.u.s.tration, as familiar objects. "Wood" is twice spoken of as a material in the New Testament, but otherwise--at least according to Cruden--not one of the above words occurs in that volume.

In like manner, while the box, the cedar, the fir, the oak, the pine, "beams," and "timber," are very frequently mentioned in the Old Testament, not one of these words is found in the New, EXCEPT the case of the "beam in the eye," in the parable in Matthew and Luke.

This interesting fact, were other evidence wanting, would go far to prove that a great change had taken place in this respect between the periods when the Old Testament and the New were respectively composed; for the scriptural writers, and the speakers introduced into their narratives, are remarkable for their frequent allusions to the natural objects and the social and industrial habits which characterized their ages and their country.] When the soil began to suffer from drought, reservoirs to retain the waters of winter were hewn in the rock near the tops of the hills, and the declivities were terraced. So long as the cisterns were in good order, and the terraces kept up, the fertility of Palestine was unsurpa.s.sed, but when misgovernment and foreign and intestine war occasioned the neglect or destruction of these works--traces of which still meet the traveller's eye at every step,--when the reservoirs were broken and the terrace walls had fallen down, there was no longer water for irrigation in summer, the rains of winter soon washed away most of the thin layer of earth upon the rocks, and Palestine was reduced almost to the condition of a desert.

The course of events has been the same in Idumaea. The observing traveller discovers everywhere about Petra, particularly if he enters the city by the route of Wadi Ksheibeh, very extensive traces of ancient cultivation, and upon the neighboring ridges are the ruins of numerous cisterns evidently constructed to furnish a supply of water for irrigation. [Footnote: One of these on Mount Hor, two stories deep, is in such good preservation, although probably not repaired for many centuries, that I found ten feet of water in it in June, 1851.] In primitive ages, the precipitation of winter in these hilly countries was, in great part, retained for a time in the superficial soil, first by the vegetable mould of the forests, and then by the artificial arrangements I have described. The water imbibed by the earth was partly taken up by direct evaporation, partly absorbed by vegetation, and partly carried down by infiltration to subjacent strata which gave it out in springs at lower levels, and thus a fertility of soil and a condition of the atmosphere were maintained sufficient to admit of the dense population that once inhabited those now arid wastes. At present, the rain-water runs immediately off from the surface and is carried down to the sea, or is drunk up by the sands of the wadis, and the hillsides which once teemed with plenty are bare of vegetation, and seared by the scorching winds of the desert.

In fact, climatic conditions render irrigation a necessity in all the oriental countries which have any importance in ancient or in modern history, and there can be no doubt that this diffusion of water over large surfaces has a certain reaction on climate. Some idea of the extent of artificially watered soil in India may be formed from the fact that in fourteen districts of the Presidency of Madras, not less than 43,000 reservoirs, constructed by the ancient native rulers for the purpose of irrigation, are now in use, and that there are in those districts at least 10,000 more which are in ruins and useless. These reservoirs are generally formed by damming the outlets of natural valleys; and the dams average half a mile in length, though some of them are thirty miles long and form ponds covering from 37,000 to 50,000 acres. The areas of these reservoirs alone considerably increase the water-surface, and each one of them irrigates an extent of cultivated ground much larger than itself. Hence there is a great augmentation of humid surface from those constructions. [Footnote: The present government of India obtains the same result more economically and advantageously by constructing in many provinces of that vast empire ca.n.a.ls of great length and capacity, which not only furnish a greater supply of water than the old reservoirs, but so distribute it as to irrigate a larger area than could be watered by any system of artificial basins. The excavacations for the Ganges Ca.n.a.l were nearly equal to those for the Suez Ca.n.a.l, falling little short of 100,000 cubic yards, without counting feeders and accessory lines amounting to a length of 3,000 miles. This ca.n.a.l, according to a recent article in the London Times, waters a tract of land 320 miles long by 50 broad. The Jumna Ca.n.a.l, 130 miles long, with 608 miles of distributing branches, waters a territory 120 miles long with a breadth of 15 miles.

Other statements estimate the amount of land actually under irrigation in British India at 6,000,000 acres, and add that ca.n.a.ls now in construction will water as much more. The Indian irrigation ca.n.a.ls are generally navigable, some of them by boats of large tonnage, and the ca.n.a.ls return a net revenue of from five to twenty per cent. on their cost.]

The cultivable area of Egypt, or the s.p.a.ce between desert and desert where cultivation would be possible, is now estimated at ten thousand square statute miles. [Footnote: The area which the waters of the Nile, left to themselves, would now cover is greater than it would have been in ancient times, because the bed of the river has been elevated, and consequently the lateral spread of the inundation increased. See Smith's Dictionary of Geography, article "Aegyptus". But the industry of the Egyptians in the days of the Pharaohs and the Ptolemies carried the Nile-water to large provinces, which have now been long abandoned and have relapsed into the condition of desert. "Anciently," observes the writer of the article "Egypt" in Smith's Dictionary of the Bible, "2,735 square miles more [about 3,700 square statute miles] may have been cultivated. In the best days of Egypt, probably all the land was cultivated that could be made available for agricultural purposes, and hence we may estimate the ancient arable area of that country at not less than 11,000 square statute miles, or fully double its present extent."

According to an article in the Bollettino della Societa Geografica Italiana, vol. v., pt. iii., p. 210, the cultivated soil of Egypt in 1869 amounted to 4,500,000 acres, and the remaining soil capable of cultivation was estimated at 2,000,000 acres.] Much of the surface, though not out of the reach of irrigation, lies too high to be economically watered, and irrigation and cultivation are therefore at present confined to an area of seven thousand square miles, nearly the whole of which is regularly and constantly watered when not covered by the inundation, except in the short interval between the harvest and the rise of the waters. For nearly half of the year, then, irrigation adds seven thousand square miles to the humid surface of the Nile valley, or, in other words, more than decuples the area from which an appreciable quant.i.ty of moisture would otherwise be evaporated; for after the Nile has retired within its banks, its waters by no means cover one-tenth of the s.p.a.ce just mentioned.

The Nile receives not a single tributary in its course below Khartoum; there is not so much as one living spring in the whole land, [Footnote: The so-called spring at Heliopolis is only a thread of water infiltrated from the Nile or the ca.n.a.ls.] and, with the exception of a narrow strip of coast, where the annual precipitation is said to amount to six inches, the fall of rain in the territory of the Pharaohs is not two inches in the year. The subsoil of the whole valley is pervaded with moisture by infiltration from the Nile, and water can everywhere be found at the depth of a few feet. Were irrigation suspended, and Egypt abandoned, as in that case it must be, to the operations of nature, there is no doubt that trees, the roots of which penetrate deeply, would in time establish themselves on the deserted soil, fill the valley with verdure, and perhaps at last temper the climate, and even call down abundant rain from the heavens. [Footnote: The date and the doum palm, the sont and many other acacias, the caroub, the sycamore and other trees grow in Egypt without irrigation, and would doubtless spread through the entire valley in a few years.] But the immediate effect of discontinuing irrigation would be, first, an immense reduction of the evaporation from the valley in the dry season, and then a greatly augmented dryness and heat of the atmosphere. Even the almost constant north wind--the strength of which would be increased in consequence of these changes--would little reduce the temperature of the narrow cleft between the burning mountains which hem in the channel of the Nile, so that a single year would transform the most fertile of soils to the most barren of deserts, and render uninhabitable a territory that irrigation makes capable of sustaining as dense a population as has ever existed in any part of the world. [Footnote: Wilkinson states that the total population, which, two hundred years ago, was estimated at 4,000,000, amounted till lately to only about 1,800,000 souls, having been reduced since the year 1800 from 2,500,000 to less than 2,000,000.--Handbook for Travellers in Egypt. p. 10. The population at the end of the year 1869 is computed at 5,215,000.--Bollettino della Soc. Geog. Ital., vol. v., pt. iii., p. 215. This estimate doubtless includes countries bordering on the upper Nile not embraced in Wilkinson's statistics.] Whether man found the valley of the Nile a forest, or such a waste as I have just described, we do not historically know. In either case, he has not simply converted a wilderness into a garden, but has unquestionably produced extensive climatic change. [Footnote: Ritter supposes Egypt to have been a sandy desert when it was first occupied by man. "The first inhabitant of the sandy valley of the Nile was a desert-dweller, as his neighbors right and left, the Libyan, the nomade Arab, still are. But the civilized people of Egypt transformed, by ca.n.a.ls, the waste into the richest granary of the world; they liberated themselves from the shackles of the rock and sand desert, in the midst of which, by a wise distribution of the fluid through the solid geographical form, by irrigation in short, they created a region of culture most rich in historical monuments."--Einleitung zur allgemeinen vergleichenden Geographie, pp. 165, 166.

This view seems to me highly improbable; for great rivers, in warm climates, are never bordered by sandy plains. A small stream may be swallowed up by sands, but if the volume of water is too large to be carried off by evaporation or drank up by absorption, it saturates its banks with moisture, and unless resisted by art, converts them into marshes covered with aquatio vegetation. By ca.n.a.ls and embankments, man has done much to modify the natural distribution of the waters of the Nile; yet the annual inundation is not his work, and the river must have overflowed its banks and carried spontaneous vegetation with its waters, as well before as since Egypt was first occupied by the human family.

There is, indeed, some reason to suppose that man lived upon the banks of the Nile when its channel was much lower, and the spread of its inundations much narrower, than at present; but wherever its flood reached, there the forest would propagate itself, and its sh.o.r.es would certainly have been mora.s.ses rather than sands.

The opinions of Ritter on this subject are not only improbable, but they are contradictory to the little historical testimony we possess.

Herodotus informs us in Euterpe that except the province of Thebes, all Egypt, that is to say, the whole of the Delta and of middle Egypt extending to Hemopolis Magna in N. L. 27 degrees 45 minutes, was originally a mora.s.s. This mora.s.s was doubtless in great part covered with trees, and hence, in the most ancient hieroglyphical records, a tree is the sign for the cultivated land between the desert and the channel of the Nile. In all probability, the real change effected by human art in the superficial geography of Egypt is the conversion of pools and marshes into dry land, by a system of transverse dikes, which compelled the flood-water to deposit its sediment on the banks of the river instead of carrying it to the sea. The colmate of modern Italy were thus antic.i.p.ated in ancient Egypt.]

The fields of Egypt are more regularly watered than those of any other country bordering on the Mediterranean, except the rice-grounds in Italy, and perhaps the marcite or winter meadows of Lombardy; but irrigation is more or less employed throughout almost the entire basin of that sea, and is everywhere attended with effects which, if less in degree, are a.n.a.logous in character, to those resulting from it in Egypt.

There are few things in European husbandry which surprise English or American observers so much as the extent to which irrigation is employed in agriculture, and that, too, on soils, and with a temperature, where their own experience would have led them to suppose it would be injurious to vegetation rather than beneficial to it. In Switzerland, for example, gra.s.s-grounds on the very borders of glaciers are freely irrigated, and on the Italian slope of the Alps water is applied to meadows at heights exceeding 6,000 feet. The summers in Northern Italy, though longer, are very often not warmer than in the Northern United States; and in ordinary years, the summer rains are as frequent and as abundant in the former country as in the latter. [Footnote: The mean annual precipitation in Lombardy is thirty-six inches, of which nearly two-thirds fall during the season of irrigation. The rain-fall is about the same in Piedmont, though the number of days in the year cla.s.sed as "rainy" is said to be but twenty-four in the former province while it is seventy in the latter.--Baird Smith, Italian Irrigation, vol. i., p.

196.

The necessity of irrigation in the great alluivial plain of Northern Italy is partly explained by the fact that the superficial stratum of fine earth and vegetable mould is very extensively underlaid by beds of pebbles and gravel brought down by mountain torrents at a remote epoch.

The water of the surface-soil drains rapdily down into these loose beds, and pa.s.ses off by subterranean channels to some unknown point of discharge; but this circ.u.mstance alone is not a sufficient solution. It is not possible that the habits of vegetables, grown in countries where irrigation has been immemorially employed, have been so changed that they require water under conditions of soil and climate where their congeners, which have not been thus indulgently treated, do not It is a remarkable fact that during the season of irrigation, when large tracts of surface are almost constantly saturated with water, there is an extraordinary dryness in the atmosphere of Lombardy, the hygrometer standing for days together a few degrees only above zero, while in winter the instrument indicates extreme humidity of the air, approaching to total saturation.--Baird Smith, Italian Irrigation, i., p. 189.

There are some atmospheric phenomena in Northern Italy, which an American finds it hard to reconcile with what he has observed in the United States. To an American eye, for instance, the sky of Piedmont, Lombardy, and the northern coast of the Mediterranean, is always whitish and curdled, and it never has the intensity and fathomless depth of the blue of his native heavens. And yet the heat of the sun's rays, as measured by sensation, and, at the same time, the evaporation, are greater than they would be with the thermometer at the same point in America. I have frequently felt in Italy, with the mercury below 60 degrees Fahrenheit, and with a mottled and almost opaque sky, a heat of solar irradiation which I can compare to nothing but the scorching sensation experienced in America at a temperature twenty degrees higher, during the intervals between showers, or before a rain, when the clear blue of the sky seems infinite in depth and transparency. Such circ.u.mstances may create a necessity for irrigation where it would otherwise be superfluous, if not absolutely injurious.

In speaking of the superior apparent clearness of the SKY in America, I confine myself to the concave vault of the heavens, and do not mean to a.s.sert that terrestrial objects are generally visible at greater distances in the United States than in Italy. Indeed, I am rather disposed to maintain the contrary; for though I know that the lower strata of the atmosphere in Europe never equal in transparency the air near the earth in New Mexico, Peru, and Chili, yet I think the accidents of the coast-line of the Riviera, as, for example, between Nice and La Spezia, and those of the incomparable Alpine panorama seen from Turin, are distinguishable at greater distances than they would be in the United States.] Yet in Piedmont and Lombardy irrigation is bestowed upon almost every crop, while in our Northern States it is never employed at all in farming husbandry, or indeed for any purpose except in kitchen-gardens, and possibly, in rare cases, in some other small branch of agricultural industry. [Footnote: In our comparatively rainless Western territory, irrigation is extensively and very beneficially employed. In the Salt Lake valley and in California, hundreds if not thousands of miles of irrigation ca.n.a.ls have been constructed, and there is little doubt that artificially watering the soil will soon be largely resorted to in the older States. See valuable observations on this subject in Hayden, Preliminary Report on Geological Survey of Wyoming, 1870, pp. 194, 195, 258-261.]

In general, it may be said that irrigation is employed only in the seasons when the evaporating power of the sun and the capacity of the air for absorbing humidity are greatest, or, in other words, that the soil is nowhere artificially watered except when it is so dry that little moisture would be evaporated from it, and, consequently, every acre of irrigated ground is so much added to the evaporable surface of the country. When the supply of water is unlimited, it is allowed, after serving its purpose on one field, to run into drains, ca.n.a.ls, or rivers.

But in most regions where irrigation is regularly employed, it is necessary to economize the water; after pa.s.sing over or through one parcel of ground, it is conducted to another; no more is usually withdrawn from the ca.n.a.ls at anyone point than is absorbed by the soil it irrigates, or evaporated from it, and, consequently, it is not restored to liquid circulation, except by infiltration or precipitation.

We are safe, then, in saying that the humidity evaporated from any artificially watered soil is increased by a quant.i.ty bearing a large proportion to the whole amount distributed over it, for most even of that which is absorbed by the earth is immediately given out again either by vegetables or by evaporation; and the hygrometrical and thermometrical condition of the atmosphere in irrigated countries is modified proportionally to the extent of the practice.

It is not easy to ascertain precisely either the extent of surface thus watered, or the amount of water supplied, in any given country, because these quant.i.ties vary with the character of the season; but there are not many districts in Southern Europe where the management of the arrangements for irrigation is not one of the most important branches of agricultural labor. The eminent engineer Lombardini describes the system of irrigation in Lombardy as, "every day in summer, diffusing over 550,000 hectares [1,375,000 acres] of land 45,000,000 cubic metres [nearly 600,000,000 cubic yards] of water, which is equal to the entire volume of the Seine, at an ordinary flood, or a rise of three metres above the hydrometer at the bridge of La Tournelle at Paris." [Footnote: Memorie sui progetti per Pestensions dell' Irrigazione, etc., il Politecniso, for January, 1868, p. 6.]

Niel states the quant.i.ty of land irrigated in the former kingdom of Sardinia, including Savoy, in 1856, at 240,000 hectares, or not much Ices than 600,000 acres. This is about four-thirteenths of the cultivable soil of the kingdom. According to the same author, the irrigated lands in Franco did not exceed 100,000 hectares, or 247,000 acres, while those in Lombardy amounted to 450,000 hectares, more than 1,100,000 acres. [Footnote: Niel, L'Agriculture des Etats Sardes, p.

232. This estimate, it will be observed, is 275,000 acres less than that of Lombardini.]

In these three states alone, then, there were more than three thousand square miles of artificially watered land, and if we add the irrigated soils of the rest of Italy, [Footnote: In 1865 the total quant.i.ty of irrigated lands in the kingdom of Italy was estimated at 1,357,677 hectares, or 2,000,000 acres, of which one-half is supplied with water by artificial ca.n.a.ls. The Ca.n.a.l Cavour adds 250,000 acres to the above amount. The extent of artificially watered ground in Italy is consequently equal to the entire area of the States of Delaware and Rhode Island.--See the official report, Sulle Bonificazione, Risaie, ed Irrigazioni, 1865, p. 269.] of the Mediterranean islands, of the Spanish peninsula, of Turkey in Europe and in Asia Minor, of Syria, of Egypt and the remainder of Northern Africa, we shall see that irrigation increases the evaporable surface of the Mediterranean basin by a quant.i.ty bearing no inconsiderable proportion to the area naturally covered by water within it.

Arrangements are concluded, and new plans proposed, for an immense increase of the lands fertilized by irrigation in France and in Belgium, as well as in Spain and Italy, and there is every reason to believe that the artificially watered soil of the latter country will be doubled, that of France quadrupled, before the end of this century. There can be no doubt that by these operations man is exercising a powerful influence on the soil, on vegetable and animal life, and on climate, and hence that in this, as in many other fields of industry, he is truly a geographical agency. [Footnote: It belongs rather to agriculture than to geography to discuss the quality of the crops obtained by irrigation, or the permanent effects produced by it on the productiveness of the soil.

There is no doubt, however, that all crops which can be raised without watering are superior in flavor and in nutritive power to those grown by the aid of irrigation. Garden vegetables, particularly, profusely watered, are so insipid as to be hardly eatable. Wherever irrigation is practised, there is an almost irresistible tendency, especially among ignorant cultivators, to carry it to excess; and in Piedmont and Lombardy, if the supply of water is abundant, it is so liberally applied as sometimes not only to injure the quality of the product, but to drown the plants and diminish the actual weight of the crop. Gra.s.s-lands are perhaps an exception to this remark, as it seems almost impossible to apply too much water to them, provided it be kept in motion and not allowed to stagnate on the surface. Protestor Liebig, in his Modern Agriculture, says: "There is not to be found in chemistry a more wonderful phenomenon, one which more confounds all human wisdom, than is presented by the soil of a garden or field. By the simplest experiment, any one may satisfy himself that rain-water filtered through field or garden soil does not dissolve out a trace of potash, silicic acid, ammonia, or phosphoric acid. The soil does not give up to the water one particle of the food of plants which it contains. The most continuous rains cannot remove from the field, except mechanically, any of the essential const.i.tuents of ite fertility." "The soil not only retains firmly all the food of plants which is actually in it, but its power to preserve all that may be useful to them extends much farther. If rain or other water holding in solution ammonia, potash, and phosphoric and silicic acids, be brought in contact with soil, these substances disappear almost immediately from the solution; the soil withdraws them from the water. Only such substances are completely withdrawn by the soil as are indispensable articles of food for plants; all others remain wholly or in part in solution."

These opinions were confirmed, soon after their promulgation, by the experimental researches of other chemists, but are now questioned, and they are not strictly in accordance with the alleged experience of agriculturists in those parts of Italy where irrigation is most successfully applied. They believe that the const.i.tuents of vegetable growth are washed out of the soil by excessive and long-continued watering. They consider it also established as a fact of observation, that water which has flowed through or over rich ground is more valuable for irrigation than water from the same source, which has not been impregnated with fertilizing substances by pa.s.sing through soils containing them; and, on the other hand, that water, rich in the elements of vegetation, parts with them in serving to irrigate a poor soil, and is therefore less valuable as a fertilizer of lower grounds to which it may afterward be conducted. See Baird Smith, Italian Irrigation, i., p. 25; Scott Moncrieff, Irrigation in Southern Europe, pp. 34, 87, 89; Lombardini, Sulle Inondazioni etc., p. 73; Mangon, Les Irrigations, p. 48.

The practice of irrigation--except in mountainous countries where springs and rivulets are numerous--is attended with very serious economical, social, and political evils. The construction of ca.n.a.ls and their immensely ramified branches, and the grading and scarping of the ground to be watered, are always expensive operations, and they very often require an amount of capital which can be commanded only by the state, by moneyed corporations, or by very wealthy proprietors; the capacity of the ca.n.a.ls must be calculated with reference to the area intended to be irrigated, and when they and their branches are once constructed, it in very difficult to extend them, or to accommodate any of their original arrangements to changes in the condition of the soil, or in the modes or objects of cultivation; the flow of the water being limited by the abundance of the source or the capacity of the ca.n.a.ls, the individual proprietor cannot be allowed to withdraw water at will, according to his own private interest or convenience, but both the time and the quant.i.ty of supply must be regulated by a general system applicable, as far as may be, to the whole area irrigated by the same ca.n.a.l, and every cultivator must conform his industry to a plan which may be quite at variance with his special objects or with his views of good husbandry. The clas.h.i.+ng interests and the jealousies of proprietors depending on the same means of supply are a source of incessant contention and litigation, and the caprices or partialities of the officers who control, or of contractors who farm, the ca.n.a.ls, lead not unfrequently to ruinous injustice towards individual landholders. These circ.u.mstances discourage the division of the soil into small properties, and there is a constant tendency to the acc.u.mulation of large estates of irrigated land in the hands of great capitalists, and consequently to the dispossession of the small cultivators, who pa.s.s from the condition of owners of the land to that of hireling tillers.

Though farmers are no longer yeomen, but peasants. Having no interest in the soil which composes their country, they are virtually expatriated, and the middle cla.s.s, which ought to const.i.tute the real physical and moral strength of the land, ceases to exist as a rural estate, and is found only among the professional, the mercantile, and the industrial population of the cities.--See, on the difficulty of regulating irrigation by law, Negri, Idea su una Legge in materia di Acqua, 1864; and Agmard, Irrigations du Midi de L'Europe' where curious and important remarks on the laws and usages of the Spanish Moors and the Spaniards, in respect to irrigation, will be found. The Moors were so careful in maintaining the details of their system, that they kept in publio offices bronze models of their dams and sluices, as guides for repairs and rebuilding. Some of these models are still preserved.

--Ibidem, pp. 204, 205. For an account of recent irrigation works in Spain, see Spon, Dictionary of Engineering, article Irrigation.

As near as can be ascertained, the amount of water applied to irrigated lands is scarcely anywhere less than the total precipitation during the season of vegetable growth, and in general it much exceeds that quant.i.ty. In gra.s.s-grounds and in field-culture it ranges from 27 or 28 to 60 inches, while in smaller crops, tilled by hand-labor, it is sometimes carried as high as 300 inches. [Footnote: Niel, Agriculture des Etata Sardes, p. 237. Lombardini's computation just given allows eighty-one cubic metres per day to the hectare [two hundred and sixty cubic yards to the acre], which, supposing the season of irrigation to be one hundred days, in equal to a precipitation of thirty-two inches.

But in Lombardy, water in applied to some crops during a longer period than one hundred days; and in the marcite it flows over the ground even in winter. According to Boussingault (Economie Rurale, ii., p. 240), gra.s.s-grounds ought to receive, in Germany, twenty-one centimetres of water per week, and with less than half that quant.i.ty it is not advisable to incur the expense of supplying it. The ground is irrigated twenty-five or thirty times, and if the full quant.i.ty of twenty-one centimetres is applied, it receives more than two hundred inches of water, or six times the total amount of precipitation. Puvis, quoted by Boussingault, after much research comes to the conclusion that a proper quant.i.ty is twenty centimetres [eight inches] applied twenty-five or thirty times, which corresponds with the estimate just stated. Puvis adds--and, as our author thinks, with reason--that this amount might be doubled without disadvantage.--Ibidem, ii., p. 248, 249. In some parts of France this quant.i.ty is immensely exceeded, and it is very important to observe, with reference to the employment of irrigation in our Northern States, that water is most freely supplied in the COLDER provinces. Thus, in the Vosges, meadows are literally flooded for weeks together, and while in the department of Vancluse a meadow may receive, in five waterings of six and a half hours each, twenty-one inches of wnter, in the Vosges it might be deluged for twenty-four hundred hours in six applications, the enormous quant.i.ty of thirteen hundred feet of water flowing over it. See the important work of Herve Mangon, Sur l'emploi des eaux dans les Irrigations, chap. ix. Boussingault observes that rain-water is vastly more fertilizing than the water of irrigating ca.n.a.ls, and therefore the supply of the latter must be greater. This is explained partly by the different character of the substances held in solution or suspension by the waters of the earth and of the sky, partly by the higher temperature of the latter, and, possibly, partly also by the mode of application--the rain being finely divided in its fall or by striking plants on the ground, river-water flowing in a continuous sheet.

The temperature of the water is thought even more important than its composition. The sources which irrigate the marcite of Lombardy--meadows so fertile that less than an acre furnishes gra.s.s for a cow the whole year--are very warm. The ground watered by them never freezes, and a first crop, for soiling, is cut from it in January or February. The Ca.n.a.l Cavour--which takes its supply chiefly from the Po at Chiva.s.so, fourteen or fifteen miles below Turin--furnishes water of much higher fertilizing power than that derived from the Dora Baltea and the Sesia, both because it is warmer, and because it transports a more abundant and a richer sediment than the latter streams, which are fed by Alpine ice-fields and melting snows, and which flow, for long distances, in channels ground smooth and bare by ancient glaciers and not now contributing much vegetable mould or fine slime to their waters.]

The rice-grounds and the marcite of Lombardy are not included in these estimates of the amount of water applied. [Footnote: About one-seventh of the water which flows over the marcite is absorbed by the soil of those meadows or evaporated from their surface, and consequently six-sevenths of the supply remain for use on ground at lower levels.]

The meteorological effect of irrigation on a large scale, which would seem prima facie most probable, would be an increase of precipitation in the region watered. [Footnote: On the pluviometric effect of irrigation, see Lombardini, Sulle Inondazioni, etc., p. 72, 74; the same author, Essai Hydrologique sur le Nil, p. 32; Messedaglia, a.n.a.lisi dell' opera di Champion, pp. 96, 97, note; and Baird Smith, Italian Irrigation, i., pp. 189, 190.

In an article in Aus der Natur, vol. 57, p. 443, it is stated that the rain on the Isthmus of Suez has increased since the opening of the ca.n.a.l, and has enlarged the evaporable surface of the country; but this cannot be accepted as an established fact without further evidence.]

Hitherto scientific observation has recorded no such increase, but in a question of so purely local a character, we must ascribe very great importance to a consideration which I have noticed elsewhere, but which, has been frequently overlooked by meteorologists, namely, that vapors exhaled in one district may very probably be condensed and precipitated in another very distant from their source. If then it were proved that an extension of irrigated soil was not followed by an increase of rain-fall in the same territory, the probability that the precipitation was augmented SOMEWHERE would not be in the least diminished.

But though we cannot show that in the irrigated portions of Italy the summer rain is more abundant than it was before irrigation was practised--for we know nothing of the meteorological conditions of that country at so remote a period--the fact that there is a very considerable precipitation in the summer months in Lombardy is a strong argument in favor of such increase. In the otherwise similar climate of Rumelia and of much of Asia Minor, irrigation is indeed practiced, but in a relatively small proportion. In those provinces there is little or no summer rain. Is it not highly probable that the difference between Italy and Turkey in this respect is to be ascribed, in part at least, to extensive irrigation in the former country, and the want of it in the latter It is true that, in its accessible strata, the atmosphere of Lombardy is extremely dry during the period of irrigation, but it receives an immense quant.i.ty of moisture by the evaporation from the watered soil, and the rapidity with which the aqueous vapor is carried up to higher regions--where, if not driven elsewhere by the wind, it would be condensed by the cold into drops of rain or at least visible clouds--is the reason why it is so little perceptible in the air near the ground. [Footnote: Is not the mottled appearance of the upper atmosphere in Italy, which I have already noticed, perhaps due in part to the condensation of the aqueous vapor exhaled by watered ground ]

But the question of an influence on temperature rests on a different ground; for though the condensation of vapor may not take place within days of time and degrees of distance from the hour and the place where it was exhaled from the surface, a local refrigeration must necessarily accompany a local evaporation. Hence, though the summer temperature of Lombardy is high, we are warranted in affirming that it must have been still higher before the introduction of irrigation, and would again become so if that practice were discontinued. [Footnote: I do not know that observations have been made on the thermometric influence of irrigation, but I have often noticed that, on the irrigated plains of Piedmont ten miles south of Turin, the morning temperature in summer was several degrees below that marked at the Observatory in the city.]

The quant.i.ty of water artificially withdrawn from running streams for the purpose of irrigation is such as very sensibly to affect their volume, and it is, therefore, an important element in the geography of rivers. Brooks of no trifling current are often wholly diverted from their natural channels to supply the ca.n.a.ls, and their entire ma.s.s of water is completely absorbed or evaporated, so that only such proportion as is transmitted by infiltration reaches the river they originally fed.