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Drainage for Sustainable Agriculture

R.M. Singh & D.K. Singh

The natural drainage systems are severely affected by the development processes and thus increased in waterlogged and salt affected areas. In major and medium irrigation projects due to inadequate designs coupled with poor management practices has raised groundwater table and in turn sizeable command areas are being affected both by water logging and soil salinization, Water logging and salt problem have been experienced in irrigation projects all over the country. Both adversely affect the growth and the yield of the crops. An area with water table within 2 m from the land surface is called as water logged area. It is potential to water logging if water table is between 2-3 m from the land surface. It has been estimated that around 16.71 million hectare land is affected by salt and waterlogging in India. The drainage is the remedy to these problems for sustainable agriculture.

Drainage is defined as the natural or artificial removal of surplus grounded surface water and dissolved salt from the land in order to enhance agriculture Production. In the case of natural drainage the excess waters flows from the fields to lakes, swamps, streams and rivers. However, in an artificial system surplus ground or surface water is removed by means of sub surface or surface conduits. Improved drainage create a healthier environment for plant growth, It conserve soil and water; and provide drier field conditions for ease in farm operations for the crop production. Agricultural drainage is must to realize the full benefit of the irrigation.

Certain drainage criteria must be used to determine drainage need. Drainage is broadly divided into two types i.e., Surface and subsurface drainage. The excess surface or sub surface water of an agricultural field can be removed by applying the appropriate drainage method. Drainage system may consists of field drains system and main drainage system. Water of field is designed through field drainage and sent to main drainage for moving towards outlet. Field drainage system may be divided into surface drainage by gravity flow and subsurface drainage by gravity or pumped flow. The main drainage may be divided into deep collectors consisted of pipe or ditches for subsurface drainage and shallow collectors consisted of channels or ditches for surface drainage. The collector drains flows to disposal drains and to outlets of the drainage system to some stream or depressions.

Drainage coefficient is defined as the amount of water that runs off from a given area and is to be removed in 24 hours. While designing surface drainage system, a low value of the drainage coefficient will lead to partial improvement in drainage though the cost of design may be relatively low, whereas a high value would increase the cost substantially without any additional gain in the removal of surface congestion. Estimation of 24 hr rainfall depth that might occur with a probability level generally of 20% or a return period of 5 years should be considered for agricultural drainage.

Field drains for a surface drainage system have a different shape from field drains for subsurface drainage. Those for surface drainage have to allow farm equipment to cross them and should be easy to maintain with manual labour or ordinary mowers. Surface runoff reaches the field drains by flow through row furrows or by sheet flow. In the transition zone between drain and field, flow velocities should not induce erosion. Field drains are thus shallow and have flat side slopes. Simple field drains are V-shaped. Their dimensions are determined by the construction equipment, maintenance needs, and their cross ability by farm equipment. Side slopes should not be steeper. Nevertheless, long field drains under conditions of high rainfall intensities, especially where field runoff from both sides accumulates in the drain, may require a transport capacity greater than that of a simple V-shaped channel. Without increasing the drain depth too much, its capacity can be enlarged by constructing a flat bottom, thereby creating a shallow trapezoidal shape.

All field drains should be graded towards the collector drain with grades between 0.1 and 0.3%. Open collector drains collect water from field drains and transport it to the main drainage system. In contrast to the field drain, the cross-section of collector drains should be designed to meet the required discharge capacity. Besides the discharge capacity, the design should take into consideration that, in some cases, surface runoff from adjacent fields also flows directly into the field drains, which then require a gentler side slope. When designing the system, maintenance requirements must be considered.

Attention must also be given to the transition between the field drains and the collector drains, because differences in depth might cause erosion at those places. For low discharges, pipes are a suitable means of protecting the transition. For higher discharges, open drop structures are recommended. A free board of 25% of designed depth is kept. Permissible values for average velocity of flow to avoid scouring may be adopted.

Subsurface drainage improvement is designed to control the water table level through a series of drainage pipes that arc installed below the soil surface. The subsurface drainage network generally outlets to an open ditch or stream. Subsurface drainage requires some minor maintenance of the outlets and outlet ditches. For the same amount of treated area, subsurface drainage improvements generally are more expensive to construct than surface drainage improvements.

Subsurface drainage may be achieved by tubewell drainage, open drains or subsurface drains (pipe drains or mole drains). Tube well drainage and mole drainage are applied only in very specific conditions. Subsurface (groundwater) drainage for water table and soil salinity in agricultural land can be done by horizontal and vertical drainage systems. Horizontal drainage systems use open ditches (trenches) or buried pipe drains. Parallel, herringbone, targeted and double main system layout could be adopted for subsurface drainage system. The spacing of drains could be evaluated using Hooghoudt or Ernst or Child method. Drainage system requires several materials to be used such as tiles, pipes, and envelope materials for its better functioning. Envelope materials such as gravel envelop and filter including geotextile filters can be used as per requirement. Various coefficients developed may be used for proper selection of the materials to realize effective drainage for sustainable agriculture.

Introduction

The natural geo-physiographical and agro-ecological situations of India are one of the major factors in causing surface water logging and development of salt affected areas in. The natural drainage systems are severely affected by the development processes and thus increased in waterlogged and salt affected areas. The other major factor is the development of man-made major and medium irrigation systems, where huge quantity of water is being transported into new geo- hydrological arid and semi-arid regions. The lack of working experiences in these regions caused inadequate designs coupled with poor management practices has raised groundwater table and in turn sizeable command areas are being affected both by water logging and soil salinization. National Commission on Agriculture, Govt. of India (NCA 1976) defined an area as waterlogged when the water table causes saturation of crop root zone soil, resulting to restriction to air circulation, decline in oxygen and increase in carbon dioxide levels.

The Working Group on Problem Identification in Irrigated area, constituted by the Ministry of Water Resources, Govt. of India (MOWR 1991) adopted the following norms for identification of waterlogged areas:

(i) Waterlogged area : Areas with water table within 2 m from the land surface'

  • (ii) Potential area for waterlogging : Areas with water table between 2-3 m from the land surface
  • (iii) Safe area : Areas with water table below 3 m from the land surface.

The physical effects of waterlogging are lack of aeration in the crop root zone, difficulty in soil workability and deterioration of soil structure. Its chemical effect is soil salinisation. Both adversely affect the growth and the yield of the crops. The extent of drop damage depends upon the magnitude, duration and frequency of the waterlogged condition and the degree of soil salinity. Salt problem is a major cause of decreasing agricultural production in many of the irrigation project areas. Salinity may be a major problem in many non-irrigated areas where cropping is based on limited rainfall. The various agencies evaluated the status of water-logging and soil salinization problems in these areas. However, the officially accepted one is the estimates of Working Group, (1991).

Table 1. Water logged and Salt affected areas in million hectares

Irrigated Command Area

Country as a whole

Source

Water

logged

Saline

Alkali

Total

Water

logged

Saline

Alkali

Total

Working Group of MoWR

2.46

3.06

0.24

5.76

-

-

-

-

MoA, GoI

-

-

-

-

8.53

5.50

3.58

17.61

Water logging and salt problem have been experienced in irrigation projects all over the country. The examples are Chambal Command areas in Rajasthan and M.P., Indira Gandhi Canal Project in western Rajasthan, Kosi and Gandak Project Commands in Bihar, the Tungabhadra Project area in Karnataka, the Nagarjunasagar Project area in Andhra Pradesh and the Kakrapar Project area in Gujarat. Construction of drainage canals, field drains and avoiding wastage of canal supplies have been adopted as remedial measures. However, lack of maintenance, operational constraints of large irrigation projects, construction of highways, railway embankments and other obstructions, without providing for adequate drainage facility are still the major factors for water logging (Singh et al., 2011). In the Chambal Command area soils became water logged with a few years of introduction of irrigation. In many coastal areas excessive groundwater exploitation has caused seawater intrusion, worsening the salinity problem. Extent of waterlogged and salt affected areas for some states in India has been presented in Table 2.

There are extensive low lying areas in the rice growing coastal belts of eastern and south eastern regions of India where poor drainage seriously affects crop production in the monsoon season. The agricultural drainage is the remedy to these problems for sustainable agriculture. Reclamation of water logged/saline affected land by scientific and cost- effective methods should form a part of command area development programme. The drainage system should form an integral part of any irrigation project right from the planning stage. Some examples are the Sardar Sarovar Project in Gujarat the Narmada Canal Project in Rajasthan, Madhya Pradesh, the Indira Sagar Project in Madhya Pradesh, the Subarnarekha Barrage project in West Bengal, the Arjun Sahayak Pariyojana in UP, the Bodwad Parisar Sinchan Yojana in Maharashtra and many others (Singh et al. 2011). Sustainable agriculture could be achieved if components of sustainable development, environment, society and economy remain in balance (Ott 2003 and Adams, 2006). Different types if agricultural drainage systems to manage water logging and soil salinity have been discussed in this chapter.

Table 2. Geographical, waterlogged and salt affected areas of some states in India

State

Geographical area, million ha hectares

Water logged area, million ha hectares

Salt affected area, million ha hectares

Andhra Pradesh

27.44

0.339

0.813

Bihar

17.40

0.363

0.400

Gujarat

19.60

0.484

0.455

Haryana

4.22

0.275

0.455

Karnataka

19.20

0.036

0.404

Kerala

3.89

0.012

0.026

Madhya Pradesh

44.20

0.057

0.242

Maharashtra

30.75

0.111

0.534

Orissa

15:54

0.196

0.400

Punjab

5.04

0.199

0.520

Rajasthan

28.79

0.348

1.122

Tamil Nadu

12.96

0.128

0.340

Uttar Pradesh &

29.40

1.980

1.295

Uttaranchal

Total

258.43

4.528

7.006

(Source : Ghosh 1991 and Tyagi 1999)

Drainage for Sustainable Agriculture Drainage

Drainage is defined as the natural or artificial removal of surplus ground and surface water and dissolved salt from the land in order to enhance agriculture production. In the case of natural drainage the excess waters flows from the fields to lakes, swamps, streams and rivers. However, in an artificial system surplus ground or surface water is removed by means of sub surface or surface conduits (Source: FAO Glossary of Land and Water Terms). Improved drainage create a healthier environment for plant growth, It conserve soil and water; and provide drier field conditions for ease in farm operations for the crop production. Adequate drainage is required to improve soil health and soil water plant interaction for enhanced water productivity to ensure sustain ability (Singh et al., 2009). Agricultural drainage is must to realize the full benefit of irrigation.

 
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