Inadequate drainage of surplus waters ruins soils, keeps ground-water tables injuriously high and causes standing, stagnant water, or local flooding. Uncontrolled discharge of drainage waters can result in the inundation of the territory of downstream countries. Large tracts of land are agriculturally unproductive because of excess water in or on the soil, due to inadequate land drainage. Water in excess of that needed by plants may inhibit growth of the economically important parts of the plant. High water content also lubricates the soil particles and frequently leads to unstable conditions unsuitable to machine and other crop operations. In the extreme, when land is drowned, waterlogged or marshy, it may be of no value to crop production. Excess water creates problems in agricultural production over vast areas.
There are no figures to indicate the scale of the problem on a world wide basis, but it has been estimated that, in the USA alone, about a quarter of agricultural land would benefit from land drainage. Inadequate drainage on irrigated lands presents a different facet of the same problem. Badly drained irrigated lands can become marshy or salty and have to be abandoned. There are throughout the world 20-25 million hectares of now saline and barren lands which were once fertile. The loss of irrigated lands to agriculture is continuing, and every year between 200,000 and 300,000 hectares of irrigated lands fall victim to salinization or revert to marshland.
Land drainage can have negative side effects, such as: the increasing of drought hazard in rain-fed agriculture during dry spells; the decomposition of organic matter in the soil; the disappearance of peats through oxidation upon exposure to air and sunshine; the formation of acids in potentially acid sulphate soils; and the impoverishment of indigenous flora and fauna. The negative effects of land drainage usually owe their source to adverse soil conditions in the area being drained; pollution of drainage water is usually caused by other activities in the area, such as over-fertilization, the application of chemicals, or the use of poor-quality irrigation water. If the water being discharged through the drain is polluted by elements leached from the soil, this can have a harmful effect on the environment. The polluting elements may be fertilizers, toxic chemicals, salts, or acids. These may severely affect the quality of the water, or even render it unfit for further use, and cause the ecosystem to be gravely disturbed. Land drainage as such in areas prone to inundation is mostly an improper measure, as it creates new pathways for the water and may thus strengthen the inundations and cause additional erosion.
Drainage has long been regarded by many decision-makers as a step towards national prosperity because of the increase in agricultural land and the control of flooding. The examples of the unforeseen results of drainage are found in almost every river system of Europe, and in many cases the ill-effects are still accumulating, since the cure to one problem is often the cause of several more.