IRRIGATION. The artificial application of water to land (Lat. in, and rigare, to water or wet).
Normally vegetation grows on soil saturated by rain. Where rain is seasonal, deficient or practically nonexistent, as it is, for instance, in Egypt, the desiccation of the soil prevents vegetable growth. In such areas irrigation can give even better results than nature herself produces, because water is given just when it is wanted, instead of at the fitful times of rainfall. Irrigation water must be "sweet" or fresh. All ordinary fresh water con tains a small percentage of salt. Vegetation, however, will tolerate a much larger percentage, more especially if the drainage is good. The use of irrigation as an aid to the growth of vegetation is indeed an ancient art, as is shown by the biblical reference : "And a river went out of Eden to water the garden" (Gen. ii. o). The probabilities seem to point to Egypt as its place of origin. The irregularity of watering, caused by the small though potent degree of variation in height of each annual Nile flood, would prevent the Egyptian from regularly garnering such crops as were naturally producible in specific areas, and he must quickly have realized that he could take steps to correct the variation by artificially supplying water directly, by his own labour, to the fields he found it most convenient to depend upon, and thus have begun, at least in the Nile valley, the art of irrigation. The
paintings and sculptures of ancient Egypt do show that the peasantry baled up water at least 4,000 years ago. From Egypt the art may have spread to the ancient empires whose territories are now covered, at least in part, by the modern State of 'Iraq. Certainly the conditions on the Euphrates and the Tigris are com parable to those on the Nile.
Carthage, Phoenicia, Greece and Italy may have followed the same example. India and China are also countries which have progressed in this early art. The westward sweep of the Moors is no doubt responsible for its development in Spain. Wherever the Mohammedan penetrated, and circumstances proved favour able, he introduced irrigation.