Basic concepts of drainage and leaching

Useful for B. Tech Agril Engg Graduates

1. Basic Concepts of
Drainage and Leaching
Md. I. A. Ansari
Department of Agricultural Engineering
(e-mail: [email protected])

2. Watershed
A watershed is defined as an
area that drains to a common
point.
That common point may be a
lake, an outlet to a river, or any
other point.

3. Watershed

6. Soil

7. Composition of Soil System
•Solid: 50 % (Mineral:45%, Organic matter:
5%)
•Water+ air:50%
•Optimum growth of Plants: 25 % water
+25% air

10. Soil Water
•When water is added to dry soil either by
rain or irrigation, it is distributed around
the soil particles, where it is held by
adhesion and cohesive forces.
•Water displaces air in the pore spaces and
eventually fills the pores.
•When all the pores, large and small are
filled, soil is said to be saturated and it is
at its maximum retentive capacity.
•Soil moisture tension:0 atm.

11. Measuring soil moisture with a tensiometer - a soil moisture
gauge

13. Gravitational Water
•Gravitational water is free water moving
through soil by the force of gravity.
•It is largely found in the macropores of
soil.

14. Capillary Water
•Capillary water is water held in the
micropores of the soil, and is the water
that composes the soil solution.
•Capillary water is held in the soil because
the surface tension properties (cohesion
and adhesion) of the soil micropores are
stronger than the force of gravity.
•Capillary water is the main water that is
available to plants as it is trapped in the
soil solution.

15. Field Capacity
•Amount of water in soil after free drainage
has removed gravitational water (2 – 3
days)
•Soil is holding maximum amount of water
available to plants
•Optimal aeration (micropores filled with
water; macropores with air)
•Soli moisture tension: 1/10 to 1/3 atm.

16. Hygroscopic Water
•Hygroscopic water forms as a very thin film
surrounding soil particles and is generally
not available to the plant.
•This type of soil water is bound so tightly to
the soil by adhesion properties that very
little of it can be taken up by plant roots.
•Since hygroscopic water is found on the
soil particles and not in the pores.
•Permanent wilting point: 7-32 atm.

21. Saturated Soil

22. Field Capacity

23. Permanent Wilting Point

30. Plant roots must have a favourable
environment to be able to extract water
and soluble nutrients to meet the plant’s
requirement.
Excess water or salt concentration in the
root zones or at the land surface do not
permit the plant roots to function properly
resulting in poor growth and yield of the
plants.

32. Waterlogging
•Waterlogging is defined as the state of
land in which the subsoil water table is
located at or near the surface with the
result that the yield of crops commonly
grown on it is reduced well below the
normal for the land.
•The soil becomes waterlogged when the
amount of water infiltrating or seeping into
it is sufficient to fill all the pore space in
soil profile.

34. Water in Soil After Heavy Rain

35. Water-Logging

38. Causes of Water-Logging
•Over Irrigation: The excess water percolates
and remains stored within the root zone of the
crops. This excess of water is responsible for
water logging.

39. Seepage from Canals:
•In unlined canal systems, the water percolates
through the bank of the canal and gets
collected in the low lying areas along the
course of the canal and thus the water table
gets raised.

40. Inadequate Surface Drainage:
•When the rainfall is heavy and there is no
proper provision for surface drainage the water
gets collected and submerges vast area. When
the condition continues for a long period, the
water table is raised.

41. •Obstruction in Sub-Soil Drainage:
•If some impermeable stratum exists at a lower
depth below the ground surface, then the
movement of the subsoil water gets obstructed
and this cause water logging in the area.
•Nature of Soil:
•The soil having low permeability, like black
cotton soil, does not allow the water to percolate
through it. So, in case of over irrigation or flood,
the water retains in this type of land and cause
water logging.

42. •Incorrect method of Cultivation:
•If the agriculture land is not levelled properly and
there is no arrangement for the surplus water to flow
out, then it will create pools of stagnant water leading
to water logging.
•Seepage from Reservoir:
•If the reservoir basin consists of permeable zones,
cracks and fissures which were not detected during
the construction of dam, these may cause seepage of
water. This sub-soil water will move forward toward
the low lying area and cause water logging.

43. •Poor Irrigation Management:
•Excessive Rainfall:
•If the rainfall is excessive and the water gets no time
to get drained off completely, then a pool of stagnant
water is formed which might lead to water logging.

44. •Topography of the land:
•If the agricultural land is flat, and consists of
depression or undulations, then this leads to water
logging.
•Occasional Flood:
•If an area gets affected by flood every year and there
is no proper drainage system, the water table gets
affected and this cause water logging.

45. Rise of ground water level
Tidal water
Excessive irrigation
 Seepage from river, canal, higher irrigated
areas, hill sides
Poor drainage system
Other causes

46. As per Central Ground Water Board:
Water logged area: water table within 2m
below ground level
Critical water logged area: water table 2-3m
below ground surface
Safe area: water table below 3 m from land
surface

47. Effects of Water-Logging
Salination of Soil: Due to water logging the
dissolved salts like sodium carbonate, sodium
chloride and sodium sulphate come to the
surface of soil.
•When the water evaporates from the surface,
the salts are deposited there. This process is
known as salinization of soil.
•Excessive concentration of salts make the land
alkaline and does not allow the plants to thrive
and thus the yield of crop is reduced.

48. Lack of Aeration: The crops require some
nutrients for their growth which are supplied by
some bacteria or micro-organisms by breaking the
complex nitrogenous compound into simple
compound which are consumed by the plants for
their growth.
•But the bacteria requires oxygen for their life and
activity. When the aeration in the soil is stopped
by water logging, these bacteria cannot survive
without oxygen and the fertility of the land is lost
which results in reduction of yield.

49. Decrease in Soil Temperature:
•Due to the water logging the soil temperature is
lowered. At low temperature of the soil, the
activity of the bacteria becomes very slow and
consequently the plants do not get the requisite
amount of food in time. Thus the growth of the
plants is hampered and the yield also is reduced.
Growth of weeds and aquatic plants:
•Due to water logging, the agricultural land is
converted to marshy lands and the weeds and
aquatic plants grow in plenty. These plants
consume the soil nutrient and thus the crops are
reduced.

50. •Diseases of Crops:
•Due to low temperature and poor aeration, the
crops get some diseases which may destroy the
crops or reduce the yields.
•Difficulty in Cultivation:
•In water logged area it is very difficult to carry
out the operation of cultivation such as tilling,
ploughing. etc.
•Restriction of Root Growth:
•When the water table rises near the root zone the
soil gets saturated. The growth of the roots is
confined only to the top layer of the soil. So, the
crop cannot be matured properly and the yield is
reduced.

51. Other Problems
Water logging causes shift in cropping
pattern, ultimately leading to inefficient use
of land.
Land becomes partially unavailable for part
of the year or fully unavailable throughout
the year, leading to overall poor production
performance of agricultural sector.
The productivity of sensitive crops like
pigeon peas and maize reduces by 30 % to
50% of the production potential when they
are under waterlogged conditions.

52. Continued stagnation of water often leads
to accumulation of toxic bio-chemical
substances .
Iron and Managanese are available in
excess causing toxicity to the plant

54. Development of Salts
•When the water table is about two metres
from the soil surface, ground water can be
brought to the surface by capillary rise.
•Water is then evaporated from the soil
surface, leaving dissolved salts behind in
the root zone.

59. •WHAT IS SALINITY?
•Salinity is the amount of salt in the soil or water.
•The dominant salt in most saline soil is
common salt—sodium chloride (NaCl).
•Varying amounts of calcium, magnesium and
potassium chlorides and sodium sulfates can
also occur.
•If this water contains less than 3 grams of salt
per litre, the soil is said to be non saline . If the
salt concentration of the saturation extract
contains more than 12 g/l, the soil is said to be
highly saline.

60. Characterization and Severity Classes of Salt
Affected Soils
•Soil salinity /Alkalinity can technically be
expressed in terms of:
• pH,
•Electric Conductivity (EC),
• Exchangeable Sodium Percentage (ESP)
•Electrical conductivity (EC) determines the
amount of salts.

61. Saline Soils
•Contains salt mostly chlorides and
sulphates of sodium, calcium and
magnesium
•pH<8.5
•EC>4 dS/m
•Exchangeable Sodium Percentage
( ESP)<15

62. Alkali Soils
•Alkali soils contain salts dominated by
bicarbonates, carbonates and
silicates of sodium.
•pH>8.5
•EC<4 dS/m
•ESP>15

65. Classification Of Salt Affected Soils
Class EC ESP pH

Saline soil > 4 <15 <8.5

Alkali soil < 4 > 15 > 8.5

66. Soil Salinity Class
Conductivity of the
Saturation Extract
(dS/m)
Effect on Crop
Plants
Non saline
0 - 2
Salinity effects
negligible
Slightly saline
2 - 4
Yields of sensitive
crops may be
restricted
Moderately saline
4 - 8
Yields of many
crops are
restricted
Strongly saline
8 - 16
Only tolerant
crops yield
satisfactorily
Very strongly
saline > 16
Only a few very
tolerant crops
yield satisfactorily
Table SOIL SALINITY CLASSES AND CROP GROWTH

67. Salinity and Plant Growth
•Excess soil salinity causes poor, uneven
growth and poor yields, the extent depend on
the degree of salinity.
•The primary effect of excess salinity is that it
renders less water available to plants
although some is still present in the root zone.
This is because the osmotic pressure of the
soil solution increases as the salt
concentration increases.

71. Salt in agricultural soils

73. Table. Relative salt tolerance of common field and orchard crops
Tolerant Semi-tolerant Sensitive
Barley Wheat, oats Apple
(Dub) grass Sorghum
Cotton Maize Lemon
Spinach Rice Pear
Date palm Tomato Field beans
Sugar beet Cabbage Green beans

74. •About 33% of all irrigated lands world-wide
are affected by varying degrees of salinity.
•10 % water logged area in Jharkhand

75. 160 million hectares of cultivated land in India

77. •In irrigated lands, drainage is
indispensable to prevent the
permanent hazard of waterlogging
and salinization.

78. •Drainage is the removal of excess water
and dissolved salts from the surface and
subsurface in the root zone of crops in
order to enhance crop growth.
•Drainage aids in leaching of salts, reduces
the chance of salt accumulation and
removes excess irrigation water.

79. •An agricultural system of draining fields
commonly consists of:
1) A Field Drainage System
2) A Main Drainage System
•move water from field system to
outlet
3) An Outlet
•terminal point of discharge into open
water system

82. Benefits of Drainage
A well-planned drainage system provides a
number of benefits:
better soil aeration,
 more timely field operations,
less flooding in low areas,
 higher soil temperatures,
better soil structure,
Nutrients are better utilized by plants in well
drained soils.
better root development,
higher yields, and improved crop quality.

84. Leaching
•The process of removal of soluble salts by
the passage of water through soil is called
leaching.
•It is the process of dissolving and
transporting soluble salts by downward
movement of water through the soil.
•Leaching is applying irrigation water in
excess of the soil moisture depletion level
to remove salts in the root zone.

85. Management of
Waterlogged and Saline
Soil through Drainage and
Leaching

90. Management Of Water -Logged Soils
•Drainage: Drainage removes excess water 
from the land and the root zone that is 
harmful for plant  growth. 
•surface drainage
•sub-surface drainage
•drainage well methods.
•Biodriange

91. •Prevention of percolation from Canals:
• The irrigation canals should be lined with
impervious lining to prevent the percolation of
water through the bed and banks of the canals.
Thus the water logging may be prevented.
•Intercepting drains may be provided along the
course of the irrigation canals in place where
the percolation of water is detected.
•The percolation water is intercepted by the
drains and the water is carried to other natural
water course.

92. Canal Lining

93. Canal Lining

94. •Prevention of percolation from the
reservoirs:
•During the construction of dams, the
geological survey should be conducted on the
reservoir basin to detect the zone of permeable
formations through which water may
percolate. These zones should be treated
properly to prevent seepage.

95. •Control of Intensity of Irrigation:
•The intensity of irrigation may cause water logging so,
it should be controlled in a planned way so that there is
no possibility of water logging in a particular area.
•Economic Use of Water:
•If the water is used economically, then it may control
the waterlogging and the yield of the crop may be high.
So, Special training is required to be given to the
cultivators to realize the benefits of economical use of
water. It helps them to get more crops by eliminating
the possibility of water logging.

96. •Pumping of Ground water:
•A number of open well or tube wells are
constructed in the water logged area and the
ground water is pumped out until the goes down
to a safe level. The lifted ground water may be
utilized for irrigation or may be discharged to the
river or any water course.
•Construction of Sump Well:
•Sump Well may be constructed within the water
logged area and they help to collect the surface
water. The water from the sump well may be
pumped to the irrigable lands or may be
discharged to any river.

97. •Fixing of Crop Pattern: Soil survey should be
conducted to fix the crop pattern. The crops
having high rate of evapotranspiration should be
recommended for the area susceptible to water
logging.
•Providing Drainage System:
•Suitable drainage system should be provided in
the low lying area so that rain water does not
stand for long days. A network of sub-surface
drains are provided which are connected to the
surface drains. The surface drains discharge the
water to the river or any water course.

98. •Surface Drainage
•Subsurface Drainage

99. Surface Drainage
•Surface drainage involves the removal of 
excess water from the surface of the soil.
•Surface  drainage  is  accomplished  by 
smoothing out  small  depressions  (land 
smoothing) or grading an undulating land 
surface to a uniform slope, and directing 
water  to  a  natural  or  improved, 
constructed channel.

100. Surface Drainage
•Land forming is mechanically changing the land 
surface to drain surface water.  
•This is done by smoothing, grading, or leveling.  
•Land smoothing is the shaping of the land to a 
smooth  surface  in  order  to  eliminate  minor 
differences in elevation and this is accomplished 
by filling shallow depressions.
•Land grading is shaping the land for drainage 
done  by cutting,  filling  and  smoothening  to 
planned  continuous  surface  grade e.g.  using 
bulldozers or scrapers.

102. By shaping By grading

103. A surface  drainage  system is  most 
appropriate  on flat  land with  slow 
infiltration and low permeability soils. 
The principal types of surface drainage 
configurations are the random and parallel 
systems . 
The random system is adapted to slowly 
permeable soils with depressions too large 
to be eliminated by smoothing or shaping 
the land.
• 

106. Parallel Field Drainage System
•The parallel field drains collect the surface runoff 
and discharge it into the collector drain. 
•The spacing between the field drains depends 
on the size of fields, the tolerance of crops to 
ponding, and on the amount and costs of land 
forming. 
•.

107. The parallel system is suitable for flat, 
poorly drained soils with many shallow 
depressions. 
In a field that is cultivated up and down a 
slope, parallel ditches can be arranged to 
break the field into shorter lengths. 

112. •A  surface  drainage  system  sometimes 
includes diversions and interceptor drains. 
•Diversions, usually located at the bases of 
hills, are channels constructed across the 
slope  of  the  land  to intercept  surface 
runoff  and  prevent  it  from  overflowing 
bottomlands. 

113. A subsurface drainage system is  a 
man-made system that induces excess 
water and dissolved salts to flow through 
the soil to pipes or open drains, from 
where it can be evacuated.
To remove excess water from the root 
zone, subsurface drainage is used. 
This is done by digging open drains or 
installing pipes, at depths varying from 1 
to 3 m.
Subsurface Drainage

114. Subsurface Drainage
•Subsurface drainage systems are installed in 
flatlands to control the groundwater level in 
order to achieve water and salt balances 
favorable for crop growth. 
•Two  types  of  man  made  systems  of 
subsurface drainage i.e.
• (i) horizontal
•and (ii) vertical 
•Vertical  drainage  is  mainly  achieved  by 
pumping out groundwater through the tube 
wells.
• 

115. Sub-Surface Drainage Using Ditches

116. Sub-Surface Drains Using Buried Drains

118. Subsurface Drain-open drain

119. •Clay and concrete pipes (often referred to as 
tiles) are usually made with 10 cm internal 
diameter. 
•Two design variables of subsurface drainage 
system are drain spacing and drain depth, 
which control water table depth. 
• In general, the depth of drains may be limited 
between 1.5 and 2.0 m. 
•The length of lateral drains varies from 200 to 
600 m depending on available natural slope 
and layout of the area. 
• 

120. •Envelope  (filter)  materials are  provided 
around the pipe drains to facilitate water 
flow into the drain and to prevent the entry 
of soil particles into the drain. 
•The most common envelope materials are 
graded gravel.

126. The four patterns of tile drainage 
system are:
- Random
- Parallel
- Herringbone
- Double Main

129. Drainage Designs
The major considerations in drainage
design include:
 Drainage Coefficient
Drain Depth and Spacing
Drain Diameters and Gradient
 Drainage Filters

130. Capacity
- Velocity
- Hydraulic gradient
- Channel depth
- Cross section

131. Drainage Coefficient
Drainage coefficients are used in determining the
design capacity of a system. The drainage
coefficient is that rate of water removal to obtain
the desired protection of crops from excess surface
and subsurface water.
For subsurface drainage, the coefficient is
usually expressed as a depth of water to be
removed over a safe period of time, usually 24
hours.
For surface drainage, the coefficient may be
expressed as a flow rate per unit area.

132. Ditch Cross-section
•Open ditches for drainage are designed
with trapezoidal cross-sections.
•The size of the ditch will vary with the
velocity and quantity of water to be
removed.
•The Manning's formula is used for design.

133. •Channel side slopes are determined by
the soil texture and stability.
•Recommended side slopes are: 1: 1 for
clay, 1.5: 1 for silt loam, 2 : 1 for sandy
loam and 3 : 1 for loose sandy soils.

134. General Flow Equation
Q = vA
Flow rate
(m
3
/s)
Avg. velocity
of flow at a
cross-section
(m/s)
Area of the
cross-section
(m
2
)
Equation 1

137. Design Depth of Drain
•The deeper a drain is put, the larger the spacing and
the more economical the design becomes.
•Drain depth, however, is constrained by soil and
machinery limitations.
•
•Table : Typical Drain Depths(D)
•Soil Type Drain Depth (m)
•Sand 0.6
•Sandy loam 0.8 - 1.0
•Silt loam 0.8 - 1.8
•Clay loam 0.6 - 0.8
•Peat 1.2 - 1.5

138. Pipe Drain Systems
•Pipe drain materials:
–concrete and clay tile
–concrete pipe
–corrugated metal pipe
–bituminous-fibre pipe
–plastic pipe

140. Mole Drainage System
Mole drains are unlined circular or oval
underground earthen channels, formed
within highly cohesive or fibrous soils by a
mole plough.
The mole plough has a long blade-like
shank to which is attached a cylindrical
bullet-nosed plug, known as a mole.

143. Mole drain formed

147. August, 2003 August, 2004 August, 2005
August, 2006 August, 2008
August, 2009
August, 2010 August, 2011 August, 2012

149. Waterlogging in mole drained plots
Mole drained plots after 4 hours of intense rains

150. Waterlogging in mole drained onion crop
Mole drained onion crop after 24 hours

152. Bio-drainage
•Bio-drainage refers to the drainage effect
of the high rate of withdrawal of ground
water by certain plants (e.g, eucalyptus,
poplar).
•This technology is applicable to physically
and chemically degraded lands.

153. •Biodrainage may be defined as “pumping of excess
soil water using bio-energy through deep-rooted
vegetation with high rate of transpiration.”
•The biodrainage system consists of fast growing tree
species, which absorb water from the capillary fringe
located above the ground water table.
•The absorbed water is translocated to different parts
of plants and finally more than 98% of the absorbed
water is transpired into the atmosphere mainly
through the stomata.
•This combined process of absorption, translocation
and transpiration of excess ground water into the
atmosphere by the deep rooted vegetation
conceptualizes bio-drainage.

156. •Plantation of tree having high
transpiration rate: Transpiration rate in
certain tree like hybrid poplar,Eucalyptus,
acacia, zyzyphus is very high.
•In transpiration process, the underground
water is consumed by trees, thus, lowering
the ground water table.

157. Reclamation of Saline Soil
•The term reclamation of saline soils refers
to the methods used to remove soluble
salts from the root zone.
•Flushing and leaching are the two major
operations done to reclaim saline soils.
•In flushing, salts are removed by surface
washing of the soil as compared to
pushing of salts below root zone.
•Good quality water is an essential input in
the reclamation programme.

158. •Leaching: This is by far the most effective
procedure for removing salts from the root zone of
soils.
•Leaching is the basic management tool for
controlling salinity.
•Water is applied in excess of the total amount used
by the crop and lost to evaporation.
•Leaching should preferably be done when the soil
moisture content is low and the groundwater table
is deep.
•.

159. •Leaching:
•In this process, the agricultural land is flooded
with water to a depth of about 20-30 cm. The
salt deposited on the surface are dissolved.
Some portion of salt is then drained off
through the subsoil drainage system and some
portion is removed by surface drainage
system. This operation is repeated several
times at specific intervals

160. Addition of Chemical Agent
•For improving he alkaline soil a chemical like
gypsum is generally added with irrigation water. The
gypsum neutralizes the alkaline effect of the soil and
yield of the crop is increased.

161. •In some parts of India for example,
leaching is best accomplished during the
summer months because this is the time
when the water table is deepest and the
soil is dry
•The amount of water needed is referred to
as the leaching requirement or the
leaching fraction.

162. Quantity of Water For Leaching
•It is important to have a reliable estimate
of the quantity of water required to
accomplish salt leaching.
•The initial salt content of the soil, desired
level of soil salinity after leaching, depth to
which reclamation is desired and soil
characteristics are major factors that
determine the amount of water needed for
reclamation.

163. •A useful rule of thumb is that a unit depth
of water will remove nearly 80 percent of
salts from a unit soil depth.
•Thus 30 cm water passing through the soil
will remove approximately 80 percent of
the salts present in the upper 30 cm of
soil.

164. •To remove salts from the soil, more
irrigation water is applied to the field than
the crops require.
•This extra water infiltrates into the soil and
percolates through the root zone.
•While the water is percolating, it dissolves
the salts in the soil and removes them
through the subsurface drains.

166. Leaching Requirement:
Amount of water needed to remove excess salts from saline
soils
LR = ECiw/ECdw
ECiwis =EC of irrigation water
ECdw = maximum acceptable salinity of the soil solution
If root zone needs 15 cm of water to be fully wetted, then
amount of water to be leached = 15*0.4= 6 cm
So supply 15 + 6 or 21 cm of water total to irrigate and
leach.
Example: if EC of irrigation water is 2.5 dS/m and crop
can tolerate an EC of 6 dS/m. What is LR?
LR = 2.5/6 = 0.4