Lesson Note On Land Drainage Systems

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.

Lesson Note On Introduction to Land Drainage

Lesson 1 Introduction to Land Drainage

1.1 What is Drainage?

Irrigation and drainage constitutes a subset of water resources system and are crucial for human survival Land drainage, or the combination of irrigation and land drainage, is one of the most important input factors to maintain or improve agricultural productivity. To enlarge the present cultivated area, more land must be reclaimed than the land that is lost due to urban/industrial development, roads and land degradation. However, in some areas, land is a limiting resource, whereas in other areas, agriculture cannot expand at the cost of nature.

Drainage is a reverse process of irrigation. It is broadly defined as the removal (disposal) of excess water from a land (usually agricultural land). The terms ‘drainage’, ‘land drainage’, ‘agricultural drainage’ and ‘field drainage’ are used as synonyms in practice. Since drainage (land drainage) is necessary not only for the removal of excess surface water or groundwater but also for removing salts from the soil, a precise definition of drainage has been given by the constitution of the International Commission on Irrigation and Drainage (ICID, 1979). According to ICID (1979), land drainage is defined as follows:

“Land drainage is the removal of excess surface and subsurface water from the land to enhance crop growth, including the removal of soluble salts from the soil”.

The above definition of land drainage (or drainage) is well known and is used worldwide.

1.2 Objectives of Drainage

Plant roots require a favorable environment to extract water and nutrient solutions to meet the plant’s requirement. For most crops, soil moisture ranging from field capacity to 50% of the field capacity in the root zone is considered ideal. Only a few crops such as rice and jute need standing water on the field at certain stages of their growth. Chemically, a neutral and non-saline soil is ideal for proper growth and yield of most food crops. Excess water and/or high salt concentration in the root zone or at the land surface do not allow the plant roots to function normally. As a result, the plant growth and yield are adversely affected. In the extreme cases of waterlogging and salinity, the seeds may not germinate and the plants may wilt permanently. The result is a loss of agricultural production. Land drainage, as a tool to manage excess surface water and groundwater levels, plays an important role in maintaining and improving crop yields:

· Drainage prevents a decrease in the productivity of arable land due to rising water tables and the accumulation of salts in the root zone.

· Drainage is the only way to reclaim the land which is not cultivated due to waterlogging and salinity problems.

Agricultural land drainage in essence is both a preventive and a curative measure for the prevention of physical and chemical degradation of soils and for the reclamation of already degraded lands. Thus, drainage of agricultural lands is an effective technique to maintain a sustainable agricultural system as well as to avoid environmental damage.

1.3  Drainage Problems in India

Waterlogging and salt accumulation are major constraints to sustainable agricultural production in most countries of the world, especially in developing countries (including India). In India, drainage problem is acute in the states of Punjab and Haryana, while it also prevails in the command areas of other states. Broadly speaking, waterlogging is a situation of an agricultural land when the root zone gets saturated. Such a condition restricts normal air circulation, reduces the oxygen level and increases carbon dioxide level in the root zone. On the other hand, salt affected soils are those in which the concentration of salts in the root zone adversely affects the normal root activity. Both the waterlogging and salt affected soils produce detrimental effects on crop growth and yield as well as cause environmental degradation. Waterlogging and salinity of agricultural lands are caused due to natural causes or artificial causes (i.e., human interventions). Important natural causes are high rainfall during the rainy season, unfavourable topography, backwater entry from rivers, seawater intrusion, high evaporation during long dry periods, and the salts present in the soil. On the contrary, important human factors are unscientific management of land and irrigation water, use of poor-quality water for irrigation, adoption of unscientific and non-sustainable cropping pattern, and obstruction of natural outlets because of urbanization and construction of highways and railways.

1.3.1 Definition, Classification and Impact of Waterlogging

(1) What is Waterlogging?

Generally, the term ‘waterlogging’ refers to the condition of a land (soil) in which the water table comes within or very near the root zone due to which crop yields decrease below the normal yield or the land cannot be used for cultivation. The soil becomes waterlogged when the water fills up all the pore space present in the soil profile, and it remains waterlogged when drainage facility is inadequate or absent. This type of waterlogging is quite common in irrigated agricultural lands and is known as ‘subsurface waterlogging’ or simply ‘waterlogging’. According to FAO (FAO, 1973), waterlogged areas are those where soils are temporarily saturated or where the water table is too shallow such that capillary rise of groundwater encroaches upon the root zone and may even reach the soil surface. Moreover, waterlogging also occurs when water is stagnant on the land surface for considerable time due to absence of a proper outlet and insignificant infiltration. This type of waterlogging is known as ‘surface waterlogging’.

(2) Classification of Waterlogging

The working group on problem identification in Irrigated Areas, constituted by the Ministry of Water Resources, Government of India (MOWR, 1991) adopted the following norms for the identification of waterlogged areas:

(i)   Waterlogged Area: Water table within 2 m from the land surface.

(ii)  Potential Area for Waterlogging: Water table between 2-3 m from the land surface.

(iii)   Safe Area: Water table below 3 m from the land surface.

The above categorization does not consider the time of the year or type cropping season in relation to the water table depths and runoff accumulation over the crop land. Crops vary greatly in their rooting depth and susceptibility to waterlogging. The dry season crops are more susceptibility to waterlogging than the wet season crops. Therefore, it will be useful if the categorization of waterlogged areas is linked with the crop season or time of the year. The common approaches to express the water table depth from the soil surface are: (a) pre-monsoon (April/May) depth to water table, (b) post-monsoon (October/November) depth to water table, (c) seasonal (monsoon/winter/summer) or annual average depth to water table, and (d) sum of the number of days when water table is shallower than a specified depth. Out of these four approaches, the first two are the simplest approaches to express the water table depth from the soil surface.

A deep water table at pre-monsoon reduces the chances of soil salinization and ensures successful crop production during monsoon (kharif) season. A deep water table in the post-monsoon period helps maintaining timeliness of field operations for the winter (rabi) season crops. Keeping these facts in view, the following norms are suggested for the classification of different categories of waterlogged areas in India and other South Asian countries (Bhattacharya and Michael, 2003):

(i) Waterlogged Area: Water table is within 2 m from soil surface during pre-monsoon (April/May) or water table is within 1 m from soil surface during post-monsoon (October/November).

(ii) Critical Area for Waterlogging: When the water table is between 2 and 3 m from the soil surface during pre-monsoon and/or between 1 and 2 m during post-monsoon, it is considered as critical. In a critical area, waterlogging condition may develop within a short period of time if suitable measures are not adopted. Such measures are location specific and may comprise providing a drainage system, land development and scientific management of irrigation water.

(iii) Potential Area for Waterlogging: In monsoon Asia, irrigated areas with water table between 3 and 5 m during pre-monsoon may be considered as potential areas for waterlogging.

(3) Impacts of Waterlogging

The physical effects of waterlogging are: (i) lack of aeration in the root zone, (ii) difficulty in soil workability, and (iii) deterioration of soil structure. If the waterlogging prolongs for considerable time, it produces its chemical effect which is known as soil salinization. Both waterlogging and soil salinity adversely affect the growth and yield of crops (Figs. 1.1, 1.2 and 1.3). The extent of crop damage depends upon the magnitude, duration and frequency of the waterlogged condition and the degree of soil salinity.

Fig. 1.2. General relationship between crop yield and constant water table depth during growing season in

the Netherlands. (Source: Schwab et al., 2005)

Fig. 1.3. Influence of water table depth on nitrogen supplied by the soil. (Source: Schwab et al., 2005)

1.3.2 Salt Build-up in Soils

Soluble salts in the parent rocks which have weathered to form soil, seawater intrusion and high evaporation are the major natural causes for the salinisation of agricultural lands. Under a monsoon climate much of the accumulated salts are washed or leached out during the rainy season. However, high evaporation during the remaining dry and hot months in the year draws up the saline groundwater at shallow depths towards the land surface. The salts are left behind after the water evaporates (Fig. 1.4). Furthermore, important anthropogenic causes for salinity development are the use of poor quality water for irrigation and the excess application of irrigation water.

Salt problem is a major cause of decreasing agricultural production in many of the irrigated areas. Irrigation with water of low salinity but with dominant anion, and migration of sodic salts in arid climate promote salinity. The main causes of soil salinity and sodicity (alkalinity) are: (i) irrigation mismanagement; (ii) poor land leveling; (iii) leaving land fallow during dry periods especially in regions of shallow water table; (iv) improper use of heavy machinery resulting in soil compaction; (v) leaching without adequate drainage, and (vi) adoption of improper cropping patterns and crop rotations. In irrigated agriculture, scientific management of water and land is the key to avoid waterlogging and salt problems.

Fig. 1.4. Surface salt due to evaporation from shallow and saline groundwater (Najafgarh Block of Delhi).

(Source: Bhattacharya and Michael, 2003)

Salinity is a major problem in many non-irrigated areas also where cropping is based on limited rainfall. In rainfed agriculture, surface drainage is required to prevent waterlogging and flooding of low lands which lead to soil salinity hazards. Salinity in dryland areas has been a threat to land and water resources in many parts of the world. In rainfed agricultural lands of coastal areas, seawater intrusion is the main cause of salinization during dry periods. In semi-arid areas of the world, the scarcity and the variability of rainfall and high potential evapotranspiration affect the water and salt balance in the soil. Low humidity, high temperature, and high wind velocity induce upward movement of soil solution resulting in a high concentration of salts at the land surface and within the root zone. In arid regions, various types of Sodium, Magnesium and Calcium salts are concentrated mainly in Chloride and Sulphate forms. In less arid regions, Sodium salts in the Carbonate and Bicarbonate forms enhance the formation of sodic soils due to the adsorption of Sodium in the soil exchange complex.

Table 1.1 presents approximate information on the waterlogged and salt affected areas in some of the states of India. In this table, waterlogged areas include within and outside the irrigated regions as well as coastal saline lands.

Table 1.1. Geographical, waterlogged and salt affected areas of some states in India (Bhattacharya and Michael, 2003)

Sl. No.	State	Geographical Area (Mha)	Waterlogged Area (Mha)	Salt Affected Area (Mha)
1	Andhra Pradesh	27.44	0.339	0.813
2	Bihar	17.40	0.363	0.400
3	Gujarat	19.60	0.484	0.455
4	Haryana	4.22	0.275	0.455
5	Karnataka	19.20	0.036	0.404
6	Kerala	3.89	0.012	0.026
7	Madhya Pradesh	44.20	0.057	0.242
8	Maharastra	30.75	0.111	0.535
9	Orissa	15.54	0.196	0.400
10	Punjab	5.04	0.199	0.520
11	Rajasthan	28.79	0.348	1.122
12	Tamil Nadu	12.96	0.128	0.340
13	Uttar Pradesh & Uttaranchal	29.40	1.980	1.295

1.3.3 Drainage Problems in Rainfed Areas

The progress of the net sown area and its break-up into unirrigated and irrigated areas in India is shown in Fig. 1.5 (FAI, 1998). Although the unirrigated area has decreased with increasing irrigation development, about 80 Mha of the cropped land is still unirrigated (rainfed). As the pace of irrigation development has slowed down in recent years, much of the cultivated area may remain unirrigated in the future. Thus, it is irrigation rather than drainage which should be of concern for rainfed areas. However, due to the diversity of climate and soil, even rainfed areas experience excess water during monsoon season and excess salts during dry season (non-monsoon season). For example, land inundation during the monsoon season and high soil salinity during the dry season prevent cultivation in the coastal areas of Medinipur District, West Bengal. Vast flat lands in south-western Haryana and south-western Punjab, despite a low annual rainfall, get waterlogged due to sudden rains and lack of drainage to clear out the runoff fast. Lands in the plains of Bihar and Uttar Pradesh (U.P.) are uncultivable during monsoon due to excess water. Thus, drainage is relevant even in the unirrigated areas to ensure crop production.

Fig. 1.5. Progressive development of net sown, irrigated and rainfed areas in India during 1950-2000

(the last values are extrapolated).(Source: FAI, 1998)

1.3.4 Technical Limitations and Current Status of Land Drainage

Making major changes in the physical, morphological, and chemical properties of the land and water resources are infeasible. Equally infeasible is to change the climate of a region. However, the occurrence of waterlogging and salinity problems can be substantially reduced when proper attention is given to the factors listed above. The man-made causes, which are mainly concerned with the development and use of land and water resources, are theoretically easier to prevent and even to rectify. The rectification is, however, expensive, and the prevention has proved to be elusive up to now. Therefore, we are seriously concerned about the adverse impacts of waterlogging and salinity on agricultural production. Also, agriculture sector needs a serious attention because of the fact that while land and the water resources are limited in quantity and degradable, human population is gradually increasing in most Asian and African countries. This necessitates more agricultural productivity per unit of land and water, which will be possible only if further deterioration of land and water resources is avoided or minimized, degraded lands are reclaimed and these two vital resources are utilized judiciously.

Among the various activities in the agricultural production system, drainage is perhaps the most neglected in India as well as in many other developing countries. The misuse of irrigation water is slowly but inevitably leads to drainage problems. Of great relevance in the context is the history of land and groundwater degradation due to their unscientific use in different parts of the world. In 1876, the Reh Commission had cautioned against undermining the importance of agricultural drainage in the irrigated areas of India. According to (Bower and Hufschmidt, 1984), irrigation and drainage, as practiced in the developing countries, is functionally inefficient, technology primitive, economically unremunerative and environmentally degrading. Also, in the past, there have been an unspecified number of recommendations of a large number of seminars and symposia, highlighting the necessity of land drainage in enhancing and sustaining agricultural production. Most recently, there are the crisp observations of the Standing Committee of Agriculture (Lok Sabha Secretariat, 1996) of the 11^th Lok Sabha of India, on the undesirable neglect of the agricultural drainage in the irrigated areas of India. Thus, modernization of irrigation and drainage is urgently needed in India as well as in many other developing countries across the world.

References

Bhattacharya, A.K. and Michael, A.M. (2003). Land Drainage: Principles, Methods and Applications. Konark Publishers Pvt. Ltd., New Delhi, India.

Bower, B.T. and Husfschmidt, M.M. (1984). A conceptual framework fro analysis of water resources management in Asia. Natural Resources Forum, 8(1): 343-356.

FAI (1998). Fertilizer Statistics. The Fertilizer Association of India (FAI), New Delhi, India.

FAO (1973). Drainage of Salty Soils. FAO Irrigation and Drainage Paper 16, Food and Agriculture Organization of the United Nations, Rome.

ICID (1979). Amendments to the Constitution, Agenda of the International Council Meeting at Rabat. International Commission on Irrigation and Drainage (ICID), Morocco, ICID, New Delhi, India, pp. A-156-163.

Lok Sabha Secretariat (1996). Fourth Report of the Standing Committee on Agriculture of the 11^th Lok Sabha. New Delhi, India.

MOWR. (1991). Report of the Working Group on Problem Identification in Irrigated Areas with Suggested Remedial Measures. Ministry of Water Resources (MOWR), Government of India, New Delhi.

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.

Suggested Readings

Bhattacharya, A.K. and Michael, A.M. (2003). Land Drainage: Principles, Methods and Applications. Konark Publishers Pvt. Ltd., New Delhi, 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.