ABSTRACT
It is axiomatic that every irrigated field should be provided with an adequate drainage system. The drainage system serves the following purposes.
The first and most important function of the drainage system is to remove excess ground water and salts from the soil profile in the root zone of the crops. It is a wellknown phenomenon that when an area is irrigated excessively over an extended period of time the ground water level rises. When it reaches a height which is within the capillary lift of the soil, the soil moisture is brought to the surface where it evaporates. Salts which were originally present in the irrigation water, or which were dissolved in the soil, are thus concentrated on the land surface by the so-called "teakettle effect". This causes soil salinity (and sometimes alkalinity) which are harmful to plant growth. Historians and archeologists have discovered vast areas of sterile, saline wasteland, which were in past times productive under irrigation. These are found in Mesopotamia, North Africa, the Near East, and the Far East, where great empires once existed. It is a historical fact that very few of the ancient civilizations based upon irrigation have survived the ravages of soil salinity which they themselves created. An exception to this is the Egyptian civilization which survived because of the leaching effect of the annual overflow of nonsaline Nile water. In modern times, the rate of salinization and land destruction has been greatly accelerated, especially in areas irrigated with plentiful, low cost water which contains dissolved salts. In the Bushehr district of southeast Iran, for example, a very large area of date palms was irrigated by surface methods using water diverted from a contiguous river which, in its upper reaches, crosses several salt domes. Within a period of 25 years the ground water table rose from 12-or 15-m depth to within 1 m from the ground surface, and the entire plantation is now threatened with destruction. Unfortunately, no provision for drainage had been made in the original plan and it will now be necessary to uproot rows of mature trees to make room for the drainage canals. Similar examples may be found in many other countries. The function of a drainage system, in such cases, is twofold. It must lower the ground water levels to well below the maximum depth of the root zone. This permits greater root development, better aeration of the soil, and better plant nutrition. The drainage system must also provide an outlet for the accumulated salts which can be washed down from the upper soil layers and leached out through the drainage water. This leaching may take place during the season of rains, if the natural precipitation is adequate for this purpose, or it may be induced by a deliberately excessive application of irrigation water for leaching purposes.
