2.1 Sources of Excess Water
Direct rainfall constitutes the major and most common source of excess water in an area. However, another major source of excess water in many cold and moderate climates is snowmelt water during spring seasons. Other sources of excess water are irrigation, seepage, runoff and flood water, which are mostly of local importance.
The occurrence of excess rainfall applies especially to humid climates. However, it may also occur in semi-arid climates following the common type of intense, heavy storm or in general during the rainy season. The drainage load from rainfall not only depends on the amount of rainfall but also on the storage capacity of the soil and on the rate of evapotranspiration. Part of the rainfall may be stored beneficially in the soil profile or be readily evaporated so that only the remaining excess water needs to be removed from the land.
2.2 Design Considerations for Land Drainage
In the ICID definition of drainage given in Lesson 1, the phrase ‘the removal of excess water’ indicates that land drainage (or drainage) is an action by man (i.e., artificial action) who must know how much excess water should be removed. Therefore, when designing a system for a given area, drainage engineers should use certain criteria to determine whether or not water is in excess. Water balance of the area to be drained is the most accurate tool to calculate the volume of water required to be drained (Bos and Boers, 1994).
Before carrying out the water balance of an area, a number of field investigations should be undertaken which would result in adequate hydrogeological, hydropedological and topographic maps (Bos and Boers, 1994), among other information. Also, all subsurface water inflows and outflows must be measured or estimated. The precipitation and relevant evapotranspiration data from the area under investigation should be analyzed. In addition, all relevant data on the hydraulic properties of the soil should be collected. The above processes in drainage surveys call for a sound theoretical knowledge of various subjects related to the field of Soil and Water Engineering. Detailed information on field investigations required for the design and implementation of drainage systems is given in Smedema and Rycroft (1983), Ritzema (1994), and Michael and Ojha (2006).
In some cases, a proper identification of the source of ‘excess water’ can avoid the construction of a costly drainage system. Some examples are as follows (Bos and Boers, 1994):
· If irrigation water causes waterlogging, the efficiency of water use in the water-supply system and at the field level should be studied in detail and improved.
· If the surface water inflow from surrounding hills is a major cause of excess water in an area, this water could be intercepted by a hillside drain which diverts the water around the agricultural area.
· If the problem of surplus water is caused by an inflow of saline groundwater, this groundwater inflow could be intercepted by a series of tubewells, which can dispose of effluent into a drain that bypasses the agricultural land.
· If an area is partially inundated due to the insufficient discharge capacity of a natural stream, a renovation of the stream may solve the drainage problem.
However, if the origin of excess water lies in the agricultural area itself (e.g., excess rainfall or extra irrigation water to meet the leaching requirement for salinity control), then the installation of drainage facilities within the agricultural area should be considered. Usually, drainage facilities consist of: (i) a drainage outlet, (ii) a main drainage canal, (iii) some collector drains, and (iv) field drains (also called ‘lateral drains’) as illustrated in Fig. 2.1.
Fig. 2.1. Schematic diagram of a drainage system. (Source: Bos and Boers, 1994)
The main drainage canal is often a canalized stream which runs through the lowest parts of the agricultural area. It discharges its water into a river, lake, or sea by means of a pumping station or tidal gate located at a suitable outlet point (Fig. 2.1). Main drainage canals collect water from two or more collector drains. Although collector drains preferably also run through local low spots, their spacing is often influenced by the optimum size and shape of the area to be drained by a field drainage system. However, the layout of collector drains is still somewhat flexible because the length of field drains can be varied and sub-collector drains can be designed. Furthermore, the length and spacing of field or lateral drains are kept as uniform as applicable. Note that both the collector drains and the field drains can be either open drains or pipe drains, which are decided based on a number of factors such as topography, soil type, farm size, and the method of field drainage.
2.3 Types of Drainage Systems
Three most commonly used techniques for removing (draining) excess water are: (a) surface drainage, (b) subsurface drainage, and (c) vertical drainage (also known as ‘tubewell drainage’). Besides these conventional drainage techniques, there is an emerging non-conventional drainage technique known as biodrainage which is described in Lesson 12. An introduction to the conventional drainage techniques is presented below, and their details are provided in later lessons.
2.3.1 Surface Drainage
Surface drainage can be defined as (ASAE, 1979): “Surface drainage is the removal of excess water from the soil surface in time to prevent damage to crops and to keep water from ponding on the soil surface, or, in surface drains that are crossed by farm equipment, without causing soil erosion”. Surface drainage is a suitable technique where excess water from rainfall or surface irrigation cannot infiltrate into the soil and move through the soil to a drain, or cannot move freely over the soil surface to a natural/artificial drainage channel. Surface drainage problems occur in flat or nearly flat areas, in the areas having uneven land surfaces with depressions or ridges preventing natural runoff, and in the areas where there is no outlet. A detailed discussion of surface drainage technique is provided in Lesson 3.
2.3.2 Subsurface Drainage
Subsurface drainage is defined as ‘the removal of excess soil water in time to prevent damage to crops because of a high water table’. Subsurface drainage problems occur in the areas having shallow water table (e.g., canal commands), which occurs due to substantial groundwater recharge and sluggish subsurface outflow. Subsurface field drains can be either open ditches or pipe drains, but nowadays they are mostly pipe drains. Pipe drains are installed underground at depths normally ranging from 1 to 3 m (Bos and Boers, 1994). Excess groundwater enters the perforated field drains and flows by gravity to an open or closed collector drain. A detailed discussion of subsurface drainage technique is provided in Lesson 4.
2.3.3 Vertical Drainage
Vertical drainage or tubewell drainage can be defined as the ‘control of an existing or potential high water table or artesian groundwater condition’. It is accomplished using shallow or deep tubewells; sometimes open wells are also used. Most tubewell drainage systems consist of a group of wells spaced with a sufficient overlap of their cones of depression so as to control the water table at all points in an area. When draining newly-reclaimed clay soils or peat soils, the drainage engineer has to estimate land subsidence due to drainage of these soils, because this will affect the drainage design (Bos and Boers, 1994). The problem of land subsidence can also occur in the areas drained by tubewells. A detailed discussion of vertical drainage technique is provided in Lesson 12.
Irrespective of the technique used to drain a given area, it is evident that the drainage technique must fulfill the local need to remove excess water. These days, the ‘need to remove the excess water’ is strongly influenced by a concern for the environment. The design and operation of all drainage systems must ensure sustainable agriculture in the drained area and must minimize the pollution of rivers and lakes from irrigation return flow or drainage effluent (Bos and Boers, 1994). The quality of drainage effluent is generally inferior because it often contains significant amounts of sediments, agricultural chemicals (fertilizers and pesticides) and other contaminants. Therefore, proper disposal of drainage effluents is a serious concern in most canal commands of the world, especially in developing countries.
References
ASAE (Surface Drainage Committee) (1979). Design and Construction of Surface Drainage Systems on Farms in Humid Areas. Engineering Practice EP 302.2, American Society of Agricultural Engineers (ASAE), Michigan.
Bos, M.G. and Boers, Th.M. (1994). Land Drainage: Why and How? In: H.P. Ritzema (Editor-in-Chief), Drainage Principles and Applications, International Institute for Land Reclamation and Improvement (ILRI), ILRI Publication 16, Wageningen, The Netherlands, pp. 23-31.
Michael, A.M. and Ojha, T.P. (2006). Principles of Agricultural Engineering. Volume II, M/s Jain Brothers, New Delhi, India.
Ritzema, H.P. (Editor-in-Chief) (1994). Drainage Principles and Applications. International Institute for Land Reclamation and Improvement (ILRI), ILRI Publication 16, Wageningen, The Netherlands.
Smedema, L.K. and Rycroft, D.W. (1983). Land Drainage. Batsford Academic and Education Ltd., London.
Suggested Readings
Murty, V.V.N. and Jha, M.K. (2011). Land and Water Management Engineering. Sixth Edition, Kalyani Publishers, Ludhiana, India.
Michael, A.M. and Ojha, T.P. (2006). Principles of Agricultural Engineering. Volume II, M/s Jain Brothers, New Delhi, India.
Schwab, G.O., Fangmeier, D.D., Elliot, W.J. and Frevert, R.K. (2005). Soil and Water Conservation Engineering. Fourth Edition, John Wiley and Sons (Asia) Pte. Ltd., Singapore.
