Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/378

 362 IRRIGATION IRRIGATION is the systematic application of water to land in order to promote present or prospective vegetation. Water, thus used for the general purpose of growing or increasing tlie crops on which animals and man have to subsist, is employed in special ways and at special times according to the particular end in view, the individual plant to be grown, and the very divergent conditions of soil and climate which have to be studied in different countries. Sometimes the art of irrigation is practised for the simplest of all reasons, to make up for the absence or irregular seasonal distribution of rain or for a local deficiency of rainfall; sometimes a particular crop is irrigated, because the plant is of an aquatic or semi-aquatic nature ; sometimes lands are irrigated for the sake of the encouragement to early growth afforded by the warmth of the water, or for the sake of the dissolved plant-food which it furnishes ; and sometimes the object is that the land may be enriched and its level raised by means of the deposit from the water used. In considering the vast importance of water to plant growth, it must be remembered that seeds must absorb a very large quantity of water before germination can begin; that the growth of the young plant, while still dependent upon the seed, involves the employment of a constant supply of water in order that the transference of nutrients from the stores in the seed to the newly developed parts may proceed without interruption ; that soils which do not contain more than 5 to 9 per cent, of moisture will yield none of it to the plant, and that when such low per centages of moisture are approached there is a constant struggle often fatal to the plant between the soil and the plant for water ; that during the period of the plant s active growth, the absorption of all mineral matter and all nitrogen compounds from the soil takes place through the medium of an exceedingly weak aqueous solution of these substances, which solution is indeed absorbed in such quanti ties that a single plant of barley needs the passage through it during the five months in which it occupies the ground of more than an imperial gallon of water. It should be also remembered that all vegetable produce when in a growing state contains an immense proportion of w r ater, often 70 to SO per cent., and sometimes as much as 92 to 96 per cent., the latter figures representing the percentage of water in turnips and watercress respectively. From all this it will be readily understood that artificial supplies of water are needed for vegetation in many dry countries. An illustration of this need presents itself in the district w r hich comprises parts of the south of Spain, Portugal, and Italy, including Sicily and Greece. Along this zone, which includes the Mediterranean coasts north of the rainless region of Africa, with its currents of hot dry air from the Sahara, the annual rainfall may be as high as 30 inches, but the amount during the summer quarter is but 4 per cent, of the whole. All the district suffers from droughts, which are often most severe. Again, in many parts of central and eastern Europe there are table-lands, as in Moravia, Poland, and parts of Russia, where the yearly rainfall is insufficient from 10 to 15 inches only. There are about twice as many rainy days in western as in eastern Europe. In very many of these rainless or arid countries and districts there are remains (mostly in ruins) of important ancient irrigation works; Spain, Sicily, and Syria furnish abundant examples of aqueducts and canals for agricultural irrigation. In Egypt, and in some parts of Persia, India, and China, artificial watering is employed for the reasons given above; while in Peru and many other parts of America the same scarcity or irregular distribution of rain occurs. Special reference will be made further on to the very important irrigation works of India. The next point to which reference has been made is the peculiar aquatic or semi-aquatic nature of some of the plants which are grown by means of artificial watering. Rice is the chief example of a plant of this kind ; a rice swamp is proverbial, and wherever rice is grown in China, in India, in Japan, in Egypt, or in Italy, the land is under water till the crop is just ready for harvesting. The third reason for irrigating mentioned above is the determining cause of nearly all the artificial watering of land in temperate climates. It is not performed because the soil is dry and hot, for it is carried out mainly in the wettest and coldest months of the year. It is not performed because the crop to be raised is of an essentially aquatic nature, for ordinary grasses and meadow herbage only are watered. But it is performed that growth may be stimu lated and fed, through certain agencies which the water brings to bear upon the vegetation in question. The water- meadows of England afford examples of this kind of irri gation. These are, in some instances, of immemorial origin, and may, like those of the Avon in Wiltshire and the Churn in Gloucestershire, be traceable back to Roman times. In the early part of the present century the system received further developments, but at present there is some tendency to depreciate the value of this kind of irrigation. A fourth reason for irrigation is found where the solid matter suspended in the water is valuable and valued for its richness as manure, and for the actual increase which its deposition on the land makes to the height or level of the country. In England this kind of irrigation is practised mainly in the estuary of the Humber. But wherever a decided deposition of fertilizing silt, clay, or mud from w r ater allowed to rest on the land takes place, there &quot; warping,&quot; the name given to this kind of irrigation, may be said to be practised. The waters of the overflowing Nile in Egypt act, partly at least, in this manner, for their dissolved constituents (about 10 grains per gallon) are perfectly insignificant when compared with those which are suspended. In addition to these various kinds of irrigation with ordinary water, there are several systems in which town sewage is employed. These involve the introduction of many new and complex conditions, and may be more conveniently considered under the heading SEWAGE. It is the irrigation determined by the third of the fore going reasons water-meadow irrigation that calls for more particular notice here. The subject may be con veniently treated in the following order: quantity of water ; quality of water ; influence of mining refuse on water-meadows; grasses suitable for water-meadows; changes in irrigated herbage ; methods of irrigation, in cluding (1) bedwork irrigation, (2) catchwork irrigation, (3) upward irrigation, and (4) warping ; management and advantages of water-meadows ; theory of irrigation of water-meadows. The article will close with some account of irrigation in India, and in Italy, France, and Belgium, and of the history of irrigation. Before the systematic conversion of a tract into water- meadows can be safely determined on, care must be taken to have good drainage, natural or artificial, a sufficient supply of water, and water of good quality. It might indeed have been thought that thorough drainage would be unnecessary, but it must be noted that porous subsoils or efficient drains do not act merely by carrying away stagnant water which would otherwise cool the earth, incrust the surface, and retard plant growth. They cause the soil to perform the office of a filter. Tims the earth and the roots of grasses absorb the useful matters not only from the water that passes over it, but from that which passes through it. These fertilizing materials are- found stored up in the soil ready for the use of the roots