CIGR Handbook of Agricultural Engineering, Volume I Land and Water Engineering, Chapter 5 Irrigation and Drainage, Part 5.6 Land Drainage

Abstract

First paragraph: Land drainage deals with the control of waterlogging and soil salinization in agricultural lands. In flatlands, a first problem emerges if soil infiltration rates are low and rainfall or irrigationwater stands on the ground surface, generally in small depressions or at the edges of the irrigation basin. This problem can be solved by leveling and smoothing the land and providing it with a uniform slope for excess water to flow through furrows or shallow ditches toward the surface drainage outlet. Surface water is discharged into a collector drain through pipes to prevent the erosion of the open ditch bank. Waterlogging of the root zone due to the presence of perchedwater tables also occurs if the percolation rate is lower than the amount of water infiltrated, because of poor internal drainage of the soil. In flatlands, percolation can be improved by means of subsoiling to break hardpans and other types of less pervious layers of the topsoil. Thus, the soil structure, porosity, and hydraulic conductivity are enhanced and thus so is water percolation. In areas with insufficient natural drainage, subsurface drainage systems are required to maintain the groundwater table below the root zone, thus keeping it free from waterlogging and salinization. These systems consist of parallel, horizontal, generally perforated plastic pipes, although deep open field drains also are used as a first step of drainage development. Groundwater flows toward the lateral drains and through them to the outlet, where it is discharged into the collector drain by means of a rigid unperforated pipe. The drainage water, removed from the field through the surface and subsurface drainage systems, is conveyed through a network of main drains to the outlet, where it is discharged into a water body by gravity flow, by pumping, or through tidal gates. Land drainage has contributed to agricultural development in rainfed areas of the temperate regions, in irrigated lands of the arid and semiarid regions, and in the humid tropics. In the temperate humid regions, land drainage promotes good aeration of the root zone and provides moisture appropriate in the topsoil to ensure workability. Therefore, in these regions, drainage has been an efficient means to increase crop production and to decrease farming costs. In irrigated lands, drainage is indispensable to prevent the permanent hazard of waterlogging and salinization: Of 260 million ha of irrigated lands, approximately 60 million ha currently suffer from the effects of salinization [1] and FAO estimates that salt buildup has severely damaged about 30 million ha. In the humid tropics, particularly in lowland areas where, traditionally, only paddy is grown, land drainage is required to increase rice production and to diversify agriculture by growing dry-foot crops.