16. Water Logging and Salinity Part -2.pdf

Water Logging & Salinity P-2
CE-402: Irrigation Engineering
(CH-03)
Dr. Muhammad Ajmal
Lecturer Agri. Engg.
UET Peshawar

Land Drainage
In irrigated area two types of drainage can be provided:
(1) Surface drainage
(2) Sub-surface Drainage / Tile Drainage

1) Surface drainage
Surface drainage is the removal of excess water by using and
construction open ditches, field drains, land grading, and
relative structures.
Open drains which are used to remove water from
excess irrigated area and storm water are broad and
shallow called shallow surface drains.
It carry runoff to outlet drains which are large enough to carry
flood water, these drains called deep surface drains.
Land grading includes continuous land slope towards field drains.
Shallow surface drains are trapezoidal in cross section and
constructed to carry normal storm water.
Deep surface drains are constructed to carry storm water plus
excess irrigated water from shallow / tile drains.

Difference between Canal and Drain

2) Sub-Surface drainage / Tile Drainage
 Subsurface drains are required for soils with poor internal drainage
and high water table.
 Tile drains are pipe drains and made up of porous material circular in
cross section.
 Diameter may vary from 10 to 30 cm.
 These drains laid below ground level and connected with each other
by open joints.
(I) Envelope filter
 The trenches are back filled with sand and excavated material.
 The tile drains should not be placed below less permeable strata. When
it is situated below less permeable strata then they are surrounded by
graded gravels called ‘Envelope Filters’.
 It prevent inflow of soil into drain and increases effective tile diameter.

Fig . surface inlet draining the surface water into a tile drain.

Fig.Blindinlet or French drain.

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Fig. Cross-section of a tile drain in less pervious soils
(with a filter)
Fig. Cross-section of a tile drain inpervious soils
(without a filter).

(II) Outlets for tile drains
• Water from tile drain is discharged into some bigger
drains called surface drains.
• The water from tile drains may be discharged by
gravity or pumping.
(A) Gravity outlets
(B) Pump outlets

Fig. Gravityoutletfortiledrains.
Fig. Pump outlet for tile drains.

 Tile drains are aligned in
different ways depending
upon topography of the
area.
 Various types of layout of
tile drains
 Natural System
 Grid Iron System
 Herring Bone System
 Double Main System
 Intercepting Tile Drains
Layout of Tile Drainage

(1) NATURAL SYSTEM
 This system is adopted in
rolling topography where
drainage of isolated areas
is required.

(2) GRID IRON SYSTEM
 In this system laterals
are provided only on one
side of the main.
 It is adopted when land
is practically level.

(3) HERRING BONE SYSTEM
 In this system laterals
join the main from each
side alternatively.
 It is adopted when main
is laid in depression.

(4) DOUBLE MAIN SYSTEM
 It has two mains with
separate laterals for
each main.
 It is adopted when
bottom of depression is
wide.

(5) INTERCEPTING TILE DRAINS
 In this system there is
no laterals drains.
 A main is provided at
toe of slope.
 It is adopted when
main source of drainage
is from hilly land.

(5) INTERCEPTING TILE DRAINS

 Soil salinity refers to presence of salt in root zone of
soil. It adversely affects the growth of plants.
 General symptoms of salinity;
 Retarded growth.
 Smaller plants with few leaves.
 Dark green than normal leaves.
Soil Salinity

16. Water Logging and Salinity Part -2.pdf

The major cations concern in saline soils and waters are Na+, Ca2+, Mg2+, and K+,
and the primary anions are Cl–, SO4
-2 ,HCO3
- , CO3
2– , and NO3
-.

PROCESSES OF FORMATION OF SALT EFFECTED SOIL.
a) Salination
 Salination is the process of accumulation of soluble
salts by which saline soil are formed.
b) Desalination
 The process of leaching of soluble salts out of root
zone is called desalination.
c) Sodication
 Sodication is the process of accumulation
exchangeable sodium in the soil which results in the
formation of sodic soil.
d) Desodication
 The process of removal of exchangeable sodium from
the soil is known as desodication.

Soil Salinity
 If the concentration of harmful salts in the root zone
of a plant increases to such on extent that plant
growth is effected, this situation is called Salinity.
 Injurious salts that hinder plant growth are also
called Alkali salts.
 These salts are harmful when present in excess in the
root zone of the plants.
 For example NaCl, Na2CO3, Na2SO4 etc.
 Na2CO3 is the most harmful, also known as black salt.

 In soil, salts dissolve and move downward with
water.
 In waterlogged soil, water moves upwards due to
capillary. It bring up salts more and more in the root-
zone. Thus making soil solution excessively saline.
 The plant then faces hindrances in taking up moisture.
This results in permanent wilting of the plant.
 Areas where Ground Water table is near the ground
surface, dissolved salts are brought up, after evaporation
2-3 cm crust of salt is formed on the surface. This is called
Salt Efflorescence.
Soil Salinity (Contd.)

 While coming up, some salts also get deposited in the first
3-4 cm of soil layer below the ground surface.
 This salt has the corroding effect on the roots. Plant
growth is affected and ultimately the plant dies.
 This soil which cannot support salt growth and has
excessive soluble alkali salts is known as Saline Salt.
 Salinity is measured as the total amount of dissolved
salts in parts per thousand (sometimes called PSU or
Practical Salinity Units by scientists).
 Ten parts per thousand is equal to one percent.

.
 Salinity causes destruction of vegetation and crops.
 Salinity has corrosive action on plant roots.
 If the salts are alkaline, then soil pH increases.
 If the soil pH increases to 8.5, it effects the growth of
plant.
 If increases to 11.0 then plant becomes infertile.
 If the salts are acidic, then its lower the pH.
 For acidic salts with pH low than 4, plants
cannot absorb nutrients and die

Categorisation of salt affected soils
1. Saline
2. Sodic/ Alkali soil
3. Saline-Sodic soil
1.Saline soil
a) Physico-Chemical Characteristics
i) Electrical Conductivity (EC) of the saturation soil extract is
more than 4 dSm-1
(>4)
ii) pH of the soil is less than 8.5 (< 8.5)
iii) Exchangeable Sodium Percentage (ESP) is less than 15 (<15)
b) Physical Characteristics
i) Soil Structure- Usually good
ii) Infiltration rate- High
iii) Soil Aeration- Good c) Colour- Usually white

2. Sodic Soil (Black-alkali soil)
i) EC of the saturation soil extract is less than 4 dSm-1
(<4)
ii) pH of the soil is more than 8.5 (> 8.5)
iii) ESP is higher than 15 (>15)
i) Soil Structure - very poor
(soil is in highly dispersed condition)
ii) Infiltration rate - very poor
iii) Soil Aeration - very poor
c) Colour- Usually black
(Organic Matter dissolves at high pH appearing black colour)

3. Saline-Sodic Soil
i) EC of the saturation extract is higher than 4 dSm-1
(>4)
ii) pH of the soil is lower than 8.5 (< 8.5)
iii) ESP is higher than 15 (>15)
i) Soil Structure - good
ii) Infiltration rate - good
iii) Soil Aeration - good
c) Colour- Usually white

1. Rise In water table
 A rise in water table with a high evaporation results in build
up of dissolved salts.
 High evaporation and transpiration rates with poor drainage
of soil is the major cause of salinity of soil.
2. Climatic Conditions
 In the region of heavy rainfall, leaching of soil takes place
due to heavy infiltration.
Causes of Soil Salinity

 In arid and semi-arid climates rainfall is small, hence
the leaching is limited and the soil is likely to become
saline and alkaline
3. Quality of irrigation water used
 If the quantity of dissolved salts in irrigation water
exceeds a certain limit, the excess salts accumulate in the
root zone and create salt problem.
4. Nature of soil and under irrigation
 Heavy soils like clay have poor permeability, leaching is
extremely slow. Hence, they are more prone to salinity and
alkalinity compared to sandy soils.
Causes of Soil Salinity (Contd.)

Causes of Soil Salinity (Contd.)
 Brackish water can be used for sandy soils (light soils)
if enough water can be provided to satisfy irrigation and
leaching requirements.
 Under-irrigation by sprinkler may results in salinity due to
non availability of leaching water.

Effects of salt effected soil in plant growth
1. Osmotic effect
 Increase in salinity reduce amount of water available
to crops, it result increase in osmotic pressure so
plants required additional energy to absorb water
from saline soil.
 Only halophytes* are adapted to such soil.
*Halophytes are salt-resistant or salt-tolerant plants
that thrive and complete their life cycles in soils or
waters containing high salt concentrations. Despite high
salt content in the tissues of halophytes, they can be
grown and harvested as food or animal fodder.

Effects on plants growth
2. Specific ion effect
 Salinity cause specific ion effect(uptake of specific ion at
the expense of other ions).
 This result in accumulation of toxic amount of sodium,
chloride and boron ions in plants.
3. Salt tolerance of crops
 Salinity effects salt tolerance of crops (ability of crop to
survive and produce economic yield on saline soil).

improves and becomes culturable.
 The reclamation of salt affected land is costly and should
be resorted to only if necessary.
 V
arious measures adopted for reclamation depends upon
the reason causing the salinity or alkalinity. Some of
these measures are discussed:
1.Anti water logging measures
 Adequate artificial drainage and pumping etc to lower
the ground water table.
 As water logging conditions reduce, the salt affected land
Reclamation of Salt Affected Land

2. Use of proper quality of water for irrigation
 If the salinity or alkalinity of land has been caused due
the continued use of poor quality water, its replacement with
proper quality of water improves the land condition.
3. Choice of Crops
 Proper choice of salt tolerant crops, crop rotation etc.
increases fertility and reclamation of salt affected land.
 V
arious salt tolerant cropsare: barley, sugar beet,
tobacco, turnips, mustard, cotton, sugar-cane and
some varieties of grasses.

 Research scientists have also evolved salt tolerant varieties
of rice, wheat, bajra etc. The acidic nature of cell saps in the
roots. These crops also neutralize the alkalinity of the soil.
3. Leaching of soil salts
 In order to regain the salt affected land, the salt has to
be leached out by supplying additional quantity of water.
 Leaching water can be added along with irrigation water
where there is no limit on supply.
 For reclamation of land covered with encrusted salt
layers, alternate flooding and surface drainage can remove
appreciable quantities of soluble salts

4. Chemical Methods
 Chemical such as gypsum, sulphur, sulphuric acid, hydrochloric
acid may be added to salt effected soil . These chemical lower
the soil pH, reacts with soluble carbonates and replace
exchangeable sodium.
 Use of gypsum is popular due to low cost and cheap
availability
 Chemical reaction that take place is
2NaY + CaSO4 = CaY2 + Na2SO4
where Y represents the exchangeable constituent.
Owing to the replacement of Na ion by Ca ion, soil regains
its normal character making it permeable, fibrous, aerated and
fit for agriculture.

5. Addition of organic matter
 Addition of organic matter help to increase soil permeability
through promotion of soil aggregation and release of carbon
dioxide during respiration that dissolve lime.
6. Special tillage practices
 Seedbed preparation ,deep ploughing, are necessary for saline
soil.

16. Water Logging and Salinity Part -2.pdf

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16. Water Logging and Salinity Part -2.pdf