The Earth as Modified by Human Action

by George P. Marsh

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 furnishing 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 pushing the bottom up till the water all runs out over the brim, or, in other words, by turning the vessel inside out.

The quantity of water thus withdrawn from the Nile is enormous.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 quantity 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 illustration, 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 unsurpassed, 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 canals 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 Canal were nearly equal to those for the Suez Canal, falling little short of 100,000 cubic yards, without counting feeders and accessory lines amounting to a length of 3,000 miles. This canal, according to a recent article in the London Times, waters a tract of land 320 miles long by 50 broad. The Jumna Canal, 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 canals now in construction will water as much more. The Indian irrigation canals are generally navigable, some of them by boats of large tonnage, and the canals return a net revenue of from five to twenty per cent. on their cost.]

The cultivable area of Egypt, or the space 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 quantity 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 space 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 canals.] 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 canals, 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 canals 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 shores would certainly have been morasses rather than sands.