Land Drainage and Waterlogging



A. Drainage :-

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”.

 
                                                      

1. 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.

B. 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’.


1. 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:

  • 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).

  • 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.

  • 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.

2. 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 The extent of crop damage depends upon the magnitude, duration and frequency of the waterlogged condition and the degree of soil salinity.


                        

Salt Build-up in Soils :-

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. Furthermore, important anthropogenic causes for salinity development are the use of poor quality water for irrigation and the excess application of irrigation water.

                                         


The main causes of soil salinity and sodicity (alkalinity) are: 
  •  irrigation mismanagement,
  •  poor land leveling,
  • leaving land fallow during dry periods especially in regions of shallow water table,
  •  improper use of heavy machinery resulting in soil compaction,
  •  leaching without adequate drainage,


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.

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