105328852-6-Water-Logging by tahirjhatial


Drainage Engineering

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									Two major Impacts of Irrigation
1. Water logging; and

2. Salinity.
 When water table rises to such heights that the soil pores in the
  root zone become saturated, thus displacing the air, the land is
  said to be water logged.
 With restriction of normal circulation of air, decline in level of
  oxygen and increase in the level of carbon dioxide occurs.
 Over irrigation can raise the water table high enough to suffocate
  plant roots with water logging.
 The process of water logging starts even when water table is quite
  below the surface.
 Water rises on the surface due to capillary action.
    Capillary action depend upon the type of soil, being small
    height for coarse and sandy soil and large for fine grained soil.
 The water table which is considered harmful will depend on the
  type of crop, type of soil and the quantity of water.
 With respect to the type of crops, depth of water table may vary
  over a wide range from zero for rice to about 1.5 m for other

                    Norms of water logging
                 Nomenclature           Depth of water table
     1 Water logged                           < 2m
     2 Potential area for water logging        2-3 m
     3 Safe                                    >3m
Environmental Impacts of Water Logging
Environmental Impacts of water logging are caused by un-
thoughtful planning of irrigation system and result into:
   Loss in crop yield
   Water logging and salinity causes destruction of vegetation
    and crops.
   Causing dampness and therefore diseases like Malaria, etc
   Destruction of roads due to reduced bearing capacity of
    water logged soil
   Rise of water in buildings due to capillary action
   Appearance of salts on the walls
   Coming down of plasters in the buildings.
 Certain weeds grow very fast in the water logged soil and normal
  crops cannot compete with them thus suppressing the useful crop
  cultivated in the area.
 Due to reduced bearing capacity the agricultural machinery
  cannot operate in the fields and agricultural operations cannot be
  carried out.
      Remedial Measures of Water logging
There are two possible ways to get rid of the water logging:
1. Surface drainage
Surface drainage is the removal of excess water from the land surface to create
more favourable conditions for plant growth.
The water may be from excess precipitation; water applied in irrigation; losses
from conveyance channels and storage systems and/or water that has seeped
from ground water in upper reaches.
These can be broadly described as
i.    On-farm field drainage system
ii.   Intermediate drains (collector or carrier drains)
iii. Main drains ( or sub main drains)
iv.   Seepage drains
2. Sub-surface drainage
It is removal/control of ground water and removal/control of salts using
water as vehicle.
The source of water may be percolation from precipitation or irrigation
leakage from canals, drains or surface water bodies at higher elevation.
Any drain or well designed to control or lower the ground water is
considered sub-surface drainage.
They may be broadly classified in two categories as :
(a) Horizontal Drainage, and    (b) Vertical Drainage
(a) Horizontal drainage: It is accomplished by buried pipes or pipe less
(mole) drains and also by deep open ditches.
(b) Vertical Drainage: It consists of direct extraction of ground water to
lower the water table by a system of shallow tube wells spread in the
 When water table is close to the surface, and ground water
 rises to or is very close to the surface due to capillary action, it
 evaporates, leaving behind dissolved salts in the soil pores
 and ultimately on the surface.
 Evaporation in arid areas draws water up through the soil,
 bringing salts with it. Irrigation causes repeated evaporation,
 bringing more salts up.
 The whitish salts appearing on the surface usually is Sodium
 chloride (NaCl).
 Where water logging has occurred, at that place salinity is
 also produced.
 In Pakistan about 6.3 million ha are affected by different levels and
  types of salinity, out of which nearly half are under irrigated
 Soil salinity problems are particularly serious in the Sind province
  where some 70–80% of the soil is classified as moderately or severely
  saline (Smedema, 2000).
      Province-wise distribution of cultivated area and salt-affected area
                               (WAPDA, 2003)
                              Punjab Sindh      KPK      Baluchistan   Pakistan
   Cutivated area (Mha)       12.27   5.65      2.11        1.84        21.87
   Salt-affected area (Mha)   1.234   3.04      0.11        0.12        4.50
Spatial distribution of composite groundwater quality in the Indus
Basin (Qureshi et al., 2004).

                      Soil Salinization

 Prevention                               Cleanup

Reduce irrigation                         Flushing soil
                                          (expensive and
                                          wastes water)

                                          Not growing crops
                                          for 2-5 years

Switch to salt-
tolerant crops
(such as barley,                          Installing under-
cotton, sugar beet)                       ground drainage
                                          systems (expensive)

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