Irrigation and Permanent Agriculture

IRRIGATION AND PERMANENT AGRICULTURE D. W. Thorne and D. A. Anderson* Most early civilizations were based on irrigation agriculture. Eventually many of these civilizations fell. It is interesting to analyze the causes of decline of these cultures to see if any of the factors involved are pertinent in today's effort to maintain a permanent agriculture in the irrigated sections of the United States. Failures of Civilizations Founded on Irrigation mesopotamia One of the earliest great civilizations was built in the valley of the Euphrates and Tigris rivers in Asia Minor. Today, Baghdad, in the heart of these once productive plains, is located in a barren and desolate waste. In the valley are the remains of two of the largest irrigation canals of ancient times.' ^Olin, 1 9 1 3; Kinney, 1912). One of them, the Nahrwan Canal, was 250 miles long; it had an average depth of 30 to 50 feet, and it averaged 400 feet in width. Engineers estimate that this canal carried a volume of water.equal to. 10 times that of the river Thames in England. There is no general agreement on the causes of irrigation abandonment m Mesopotamia. Huntington (1924) favors a theory of increasing aridity. According to his studies, rainfall is considerably less in this area in modern times than in the days of Babylon. He believes present water resources could not possibly supply all the 2,000,000 acres under irrigation in ancient times. Huntington's theory of increasing aridity is opposed by Willcocks (1911) and Russell (1941). Willcocks concludes from a study of canal levels that the whole valley between the Tigris and Euphrates was never under irrigation at one time, and that the present water supply is sufficient to irrigate as much land as was ever irrigated at once. The maintenance of an extensive irrigation system requires a sympathetic and cooperative central government. It seems, therefore, that the destruction of government was more important than any other factor in breaking down the irrigation systems of Mesopotamia. The country was overrun by one warlike nation after another. In 1258 the invasion of the Mongols destroyed all the ancient irrigation systems and left Mesopotamia a desert. Two observations of Sir William Willcocks (1911 ) illustrate, however, some of the problems faced by these people. Around the site of ancient Babylon and south of it there are accumulations of soluble salts in the •Research associate professor of agronomy and soils, Utah Agricultural Experiment Station, and assistant professor of agronomy, Brigham Young University, respectively . Authorized for publication Sept. 15, 1942. 6 soil reaching amounts as high as five per cent. In all areas the banks of canals are built high with silt, showing that much labor was required to keep the canals clear. It is also reported that land in this area gradually passed into the hands of wealthy land-holding nobles who lived in the larger cities. Slave labor was brought in to run the farms. No one maintained an interest in careful farming. Water was used improperly and finally the soil became waterlogged. Without proper drainage, soluble salts came to the surface and much of the area was abandoned. CARTHAGE Carthage and the surrounding agricultural territory in North Africa reveal much the same story (Smith, 1902). After the third Punic War and the destruction of Carthage, the Romans founded an extensive irrigated agriculture where now are found sand dunes of the Sahara desert. The extensive canal systems were operated largely by slave labor. As the Roman Empire crumbled, labor could not be regimented to maintain the canals and the African project was gradually abandoned. Similarly, in Palestine and ancient America (Piescott, 1843, 1847) political upheavals seem of far greater importance than soil deterioration in the breakdown of irrigation agriculture . EGYPT Extensive irrigation has been practiced in the Nile valley in Egypt for more than 4000 years. (Peel, 1904; Willcocks, 1897). The longevity of irrigation agriculture there is often attributed to the manuring value of silt deposited on the land each year by Nile floods. The layer of silt deposited each year, however, averages only 1/25 of an inch in thickness or about three tons of soil per acre. While this addition has certainly...