1. CHAPTER II 2.1. Planning Irrigation systems
BASICS IN IRRIGATION ENGINEERING
2.1. Planning Irrigation systems
2.2. soil-plant-water relation –over view
2.3. Crop water requirement
2.4. Base, delta and duty

2. 2.2: Soil-Plant-Water Relationships
Soil- Plant Water relationships relate the properties of soil that affect the movement, retention and use of water.
It can be divided & treated as:
Soil-water relation
Soil-plant relation
Plant-water relations
Movement
Retention
utilization

3. 2.2: Soil-Plant-Water Relationships cont…
Simple analogy:
Soil – Water Reservoir
Plant Roots – pump with many inlets
Rate of pumping = f(character of the pump)
Rate of extraction = f(character of soil)
The physical and chemical properties of soil are very important in irrigation.

4. Soil Suitability for agricultural practices
Soil is a very important agricultural complement with out which no agricultural is possible.
It is important to study the soil characteristics to say a particular soil type is suitable for agriculture or not.
The process whereby the suitability of land for different uses such as agriculture is assessed is known as land evaluation.
Land evaluation for agricultural purpose provides information for deciding ‘which crops to grow where’ and other related crops.
Hence, before a land is put certain land uses, its suitability for that particular land use should be evaluated.

5. Soil Suitability for agriculture cont..
Soil map provides us with detailed information on soils that are utilized for land capability classification.
This indicates the suitability or unsuitability of the soil for growing crops.
Land capability classification is an interpretive grouping of soils based on inherent soil characteristics, external land features and environmental factors that may restrict the use of the land for growing varieties of crops.

6. Soil Suitability for agriculture cont..
For land capability classification we need information on:
1) The susceptibility of the soil to various factors that cause soil damage & decrease in its productivity.(we get this from soil map)
2) Its potential for crop production (obtained by growing crops).
Lands are first tentatively placed in different land capability groups on the basis of slope of the land, erosion and depth of the soil.
The suitability of soil for agricultural practices may be affected by physical and chemical soil characteristics.

7. Physical characteristics
1. Effective soil depth:-
The depth of the soil which can be exploited by crops is very important in selecting soils for agricultural purpose.
Experience has shown that many irrigated crops produce excellent yields with a well drained effective root depth of 90 cm.

8. Physical characteristics cont…
2. Water Holding capacity:-
This refers to the depth of water that can be held in the soil and available for plants.
A good soil from agricultural point of view should have a very good water holding capacity.
Clay soils have large water holding capacity, because drainage water is low in these soils.
Ideally loam soils are the best in this regard.
Sandy soils application losses are high and
in clay soils drainage and aeration is difficult.

9. Physical characteristics cont…
3. Non – capillary porosity:-
High values of non-capillary porosity is desirable,
lower values of porosity and high values of bulk density hinders root development and expansion.
4. Topography: -
A leveled land is the most suitable for agriculture.
Because, the water for irrigation can easily be conveyed and less conservation and management practices are required.
Where as, in sloppy soils, the more is the land wasted in bunds and channels in surface irrigation and there fore that cost for land development per unit area will be high

10. Physical characteristics cont…
5. Texture:-
It is the weight percentage of the mineral matter that occurs in each of the specified size fractions of the soil.
It is the particle size of the soil
It is the relative proportions of sand silt and clay, (Particles sized groups smaller than gravel i.e. < 2 mm in diameter).
It is the number and size of its mechanical particles after all compounds holding them together have been destroyed.
Loamy soils are the best texture for agriculture.
Deviation either into sandy or clayey texture will reduce the value of the land for agriculture.

11. Physical characteristics cont…
6. Soil Structure:
It refers to the manner in which primary soil particles are arranged into, secondary particles or peds or aggregates.
It is the arrangement of soil particles
Soil structure determines the total porosity, the shape of individual pores and their size distribution,
Hence it affects: -
- Retention & transmission of fluids in the soil
- Germination, root growth,
- Tillage, Erosion etc.

12. Physical characteristics cont…
7. Soil Consistence:
Is the resistance of the soil to deformation or rupture.
It is determined by the cohesive and adhesive properties of the entire soil mass.
Structure deals with size, shape and distinctness of natural soil aggregates, and consistence deals with strength and nature of the force between particles.
It is important for tillage or traffic consideration.

13. Physical characteristics cont…
Soil Consistence Terms:
- Consistence is described for three moisture levels: wet, moist & dry.
For instance, a given soils may be sticky when wet, firm when moist and hard when dry.
The terms to describe soil consistency include: -
1) Wet soil - non sticky, sticky, non plastic, plastic
2) Moist soil - loose, friable, firm
3) Dry soil - loose, soft, and hard.

14. Physical characteristics cont…
8. Soil Permeability and Hydraulic Conductivity
Permeability - is the ease with which liquids, gases and roots pass through the soil.
Hydraulic conductivity- is the permeability of the soil for water. I.e. the ease with which the soil pores permit water movement.
It controls the soil water movement.

15. Physical characteristics cont…
The major factors affecting hydraulic conductivity are
texture and structure of soils.
Sandy soils have higher saturated conductivity than finer textured soils.
Soils with stable granular structure conduct water rapidly than those with unstable structural units, since they will not break down when get wetted.
Fine textured soils during dry weather because of their cracks allow water rapidly then the cracks swell shut, and drastically reduce water movement

16. USDA Textural Triangle

17. Soil structure types and their effect on downward movement of water (Source: USDA, 1997)
Single grain Blocky Plate-like
Rapid Moderate Slow
Granular Prismatic Massive
Rapid Moderate Slow

18. Physical characteristics cont…
2. Salinity (soluble salt content) – hydraulic conductivity
When the quantity of salts in irrigated land is too high, the salts accumulate in the crop root zone.
These salts create difficulty to crops in extracting enough water from the salty solution. Osmotic effect
Thus, for the land to be of high value for irrigation, the soluble salt content should be low as much as possible.

19. Physical characteristics cont…
3. Amount of Exchangeable sodium:- hydraulic conductivity
When the amount of exchangeable sodium is high in the soil, the soil will have large amount of Na+ in the form of colloid. Soil dispersion
This results in tremendous reduction of the permeability of the soil.
This in turn makes it difficult to the cop to get sufficient water and causes crusting of seedbeds.
Such a soil is called Black alkali soil.
Hence, the amount of exchangeable sodium should be low in agricultural lands.

20. Physical characteristics cont…
4. Soil Reaction (PH) :
PH of a soil is a measure of its acidity or alkalinity.
It is a measure of the concentration of hydrogen ion in a soil.
Mathematically,
Excessively low or high pH values are not good for proper growth and adequate yield production as they bring about acidity or alkalinity in the soil.

21. Key roles of soils in any ecosystem
In general, in any ecosystem, (a farm, forest, regional water shed etc.) soils have five key roles
Medium for plant growth:
It supports the growth of higher plants by providing a medium for plant roots and supplying nutrient elements that are essential to the entire plant.
2. Regulator of water supplies:
Its properties are the principal factor controlling the fate of water in the hydrologic system.
Water loss, utilization, contamination, and purification are all affected by the soil.

22. Key roles of soils in any ecosystem cont..
3. Recycler of raw materials:
With in the soil, waste products and dead bodies of plants, animals and people are assimilated, and their basic elements are made available for reuse by the next generation of life.
4. Habitat for soil organisms:
It provides habitats for living organism, from small mammals and reptiles to tiny insects to microscopic cells.
5. Engineering medium: In human - built ecosystem, soil plays an important role as an engineering medium.
It is not only an important building material (earth fill, bricks) but provides the foundation for virtually every road, airport, and house we build.

23. Key roles of soils in any ecosystem cont..
6. Soils In relation to irrigation:
The capacity of the soil to accept, transmit or retain relatively large amounts of water (Water holding capacity of the soil) in a relatively short time should be measured.
The surface infiltration rates and the case of water movement through unsaturated and saturated layers (hydraulic conductivity) need to be measured quantitatively.
The amount, kind and distribution of clay minerals (Soil chemical properties) are specially important to water movement, relation and availability of plants.
Studies of cracking and structural changes under different management practices (helps surface sealing or a need of pre irrigation)

24. Soil- water relations
It means that physical properties of soil in relation to water
The rate of entry of water in to the soil
its retention,
movement
availability to plant roots are all physical phenomena.
Hence it is important to know the physical properties of soil in relation to water.

25. Soil- water relations cont…
Quantity of water in a soil as determined by its moisture content does not give a true indication of the soil ‘wetness’.
A clay soil, which on handling feels dry, can be at the same moisture content as a sandy soil, which feels wet.
A plant will have less difficulty extracting water from a sandy soil than from a clay soil at the same moisture content.  
The Concept of Soil Water Potential is therefore used in Soil/Plant/Water Relations

26. Soil- water relations cont…
The flow of water in any hydraulic system, including the soil-plant-water system, takes place from a state of higher to one of lower potential energy.
Soil moisture/water: the relative water content in the soil.
Due to the complex nature of the soil system, soil water can be best explained by taking the soil systems as a simple reservoir which contains solids (soil particles), liquid (water), and gas (air).
There is need for a soil ‘wetness’ which reflects the ease or difficulty of extraction of water from the soil by the plant.

28. Thus, Soil moisture can be expressed in the form of: a) Dry -weight basis MC = sample wet wt. - sample dry wt. sample dry wt. MC (% ) = wt. of water (Mw) x 100 wt. of solid (Ms) b) Volumetric basis V = Vw = Vw x 100 Vt Vs + Va + Vw V = ρb x MC Where, ρb = bulk density = ρb = Ms Vt c) Equivalent Depth: This is expressed in depth e.g. in mm. This is normally used in irrigation engineering. d = ρb x MC x D where: d is the equivalent depth of water applied (mm); D is the root zone depth (mm).

29. Volumetric Water Content & Equivalent Depth
(cm3)
Equivalent Depth
(cm3)
(g)
(g)

30. Volumetric Water Content & Equivalent Depth Typical Values for Agricultural Soils
Soil Solids (Particles): 50%
0.50 in.
1 in.
Very Large Pores: % (Gravitational Water)
0.15 in.
Total Pore Space: %
Medium-sized Pores: 20% (Plant Available Water)
0.20 in.
Very Small Pores: % (Unavailable Water)
0.15 in.

31. Measurement of Soil moisture
Determination of moisture content of a given soil is important in irrigation for the following three main points.
For suitable scheduling of irrigation. i.e. to estimate the amount and when to apply.
To estimate evapotranspiration needs of the crop
For proper interpretation of soil-plant-water relationships

32. Method of soil moisture determination
By Feel and touch: This is by far the easiest method.
Assessment by feel is good for experienced people who have sort of calibrated their hands.

33. Gravimetric Method:
known volume of soil samples are taken from the field, weighed, and then dried in an oven for 24hours at an average temperature of 105 0C.
After dried, the samples will be taken out from the oven and weighed again.
The difference in weight before and after drying is the amount of moisture present in the soil.
M= Mwet – Mdry x 100
Mdry
Gravimetric method is an accurate method but time consuming.
Moreover, the method is not practical for farm use, as the oven cannot ordinarily be owned by farmers.
But it is a standard against which other methods of moisture determination are compared.

34. Neutron Probe:
 It consists of a probe lowered down a hole in the soil
A box (rate meter or rate scalar) is at the top.
Within the probe is a radioactive source e.g. beryllium (435 years life span).
Close to the source is a detector.
The source emits fast neutrons,
some of which are slowed down when they collide with water molecules (due to hydrogen molecules).

35. Neutron Probe cont…
A cloud of slow neutrons (thermal neutrons) build up near the probe and are registered by the rate meter or rate scalar which measures the number of slowed down neutrons.
The method is quick but very expensive.
It is also dangerous since it is radioactive and must be used with care.

37. Soil Water Measurement Neutron Attenuation

38. Electrical Resistance Unit:
This consists of a porous body with two electrodes embedded into it.
The porous body when buried equilibrates with the soil water and the readings are obtained through the resistance meters attached to the electrodes.
Resistance units are measured and the instrument needs to be calibrated against matric suction or volumetric moisture content (V).

39. Electrical Resistance Unit:
Various porous bodies needed are gypsum, nylon or fiber glass.
The instrument is relatively cheap but it takes a long time to equilibrate or react e.g. 48hours.
The method is insensitive in wet soils <0.5 bars.
It measures from 0.5 to 15 bars and more.
Measure soil water potential (tension)
Tend to work better at higher tensions (lower water contents)

40. Electrical Resistance Blocks & Meters

42. Tensiometer:
Tensiometer operates on the principle that a partial vacuum is developed
in a closed chamber when water moves out through the porous ceramic tip to the surrounding.
A vacuum gauge or a water or mercury manometer can measure the tension.
The gauge is usually calibrated in centi-bars or milli-bars.
After the porous cup is put in the soil, the tensiometer is filled with water.

43. Tensiometer cont…
Water moves out from the porous tip to the surrounding soil (as suction is more in the soil).
A point is reached when the water in the tensiometer is at equilibrium with the soil water.
The reading of the gauge is then taken and correlated to moisture content using a calibration curve.
Practical operating range is about 0 to 0.75 bar of tension (this can be a limitation on medium-and fine-textured soils)

44. Tensiometer for Measuring Soil Water Potential
Water Reservoir
Variable Tube Length (12 in- 48 in) Based on Root Zone Depth
Porous Ceramic Tip
Vacuum Gauge (0-100 centibar)

46. Infiltration and soil Water movement
Infiltration is the entry of water into the soil.
It is a very important variable in irrigation design since it shows the rate at which water can move into the soil mass to replenish the root zone.
Infiltration rate of a soil is the rate at which water will enter the soil mass through the surface.
Infiltration rates into soils depend on soil texture and structure, density, organic matter content, hydraulic conductivity (permeability) and porosity.

47. Infiltration and soil Water movement cont.
As wetting time increases, the infiltration rate decreases and usually approaches a constant value, which in the case of heavy clays may be zero.
A general equation for the Infiltration rate (I) is the Modified Kostiakov equation:
I = (a tn )+ b mm/hr
Where:
a and n are constants; t is the elapsed wetting time and
b is the basic infiltration rate

48. Methods of Measuring Infiltration
For Flooded irrigation (border strip and basin), a double infiltrometer is normally used.
For furrow irrigation, the difference between inflows and outflows of water flowing through hydraulic flumes placed at different distances of test furrows represent the total infiltration
Ring infilterometer consists of two concentric cylinders, the inner about 0.3m diameter, the outer 0.6m.
Water is maintained at the same level in each cylinder, 25 mm above the soil surface, or more if the water level is likely to be higher during irrigation.

49. Methods of Measuring Infiltration cont..
The water infiltrating from the outer ring prevents lateral seepage by the water from the center cylinder.
By measuring the rate at which the water is added to the center cylinder, the infiltration rate can be found.

50. Double ring infiltrometer

51. Infiltration Rate vs. Time For Different Soil Textures

52. Cumulative Infiltration Depth vs. Time For Different Soil Textures

53. Soil Water Classification

54. Gravitational water:
It is the water in the large pores that moves downward freely under the influence of gravity
It drains out so fast that it is not available to the crops.
The time of draining out varies from one day in sandy soils to three days in clay soils.
Capillary Water:
It is the amount of water retained by the soil after gravitational water has drained out.
It is the water in the small pores which moves because of capillary forces and is called capillary water.
Capillary water is the major source of water available for the plant
Hygroscopic Water
Soil moisture further reduced by ET until no longer moves because of capillary forces. The remaining water which is held on particle surfaces so tightly is called hygroscopic water.
the water is held by adhesive force. And therefore, it is unavailable to the plant.

55. Available water
Water may also be classified as unavailable, available and gravitational or superfluous.
Such a grouping refers to the availability of soil water to plants.
Gravitational water drains quickly from the root zone under normal drainage conditions.
Unavailable water is held too tightly by capillary forces and is generally not accessible to plant roots.
Available water is the difference between gravitational and unavailable water.

56. Available water cont..
Sandy soils drain readily,
clay soils drain very slowly.
one day after irrigating a sandy soil, most of the gravitational water has drained out of the soil,
where as clay may require 4 or more days for gravitational water to drain.

59. Fig. Soil moisture levels and available water ranges

61. Saturation Capacity: -
Soil Water Constants
The following soil moisture contents are of significance importance in agriculture and are termed soil moisture constants.
Saturation Capacity: -
When all the micro and macro pore spaces are filled with water, the soil is said to have reached its saturation capacity.
At field capacity the water is held loosely and tensions are almost nil.
Thus, plants will not have any difficulty in extracting moisture from soil.

62. Field Capacity (FC):
This is the amount of water a well-drained soil contains after gravitational water movement has materially ceased.
It is taken as the water content after 48 hours the soil has been subjected to heavy rainfall or irrigation sufficient to cause saturation.
Field capacity can also be determined by finding the moisture content when suction is 1/3 bar for clay and 1/10 bar for sand.
At field capacity, the macro pores are filled with air & capillary pores filled with water.
Field capacity is the upper limit of available soil moisture.

63. Field Capacity (FC): It is often defined as:
moisture content in a soil two days (light sandy soil)
or three (heavy soil), days after having been saturated and after drainage of gravitational water becomes slow or negligible and moisture content has become stable.

64. Some factors which influence FC.
- Soil texture
Presence of impending layer (soil profile), arise from plaguing the same depth yearly
hard pan.
The volumetric moisture content at field capacity is given by

65. Field Capacity determination
Field capacity can be determined by:
ponding water on a soil surface
in an area of about 2 to 5 m2 and
allowing it to drain for one to three days preventing surface evaporation.
Then soil samples are taken from different depths and the moisture content is determined as usual, which gives the field capacity.

66. Permanent Wilting Point (PWP):
Permanent Wilting Point: - is the moisture content beyond which plants can no longer extract enough moisture and remain witted unless water is added to the soil.
The plant will wilt and may die later if water is not available.
The water beyond the permanent wilting point is tightly held to the solid particles that plants cannot remove moisture at their normal rate to prevent wilting of the plants.
Water tension of soil at PWP is generally taken as 15 bars.

67. Permanent Wilting Point (PWP) cont..
The soil moisture tension at PWP ranges from 7 to 32 atm. depending on the soil texture, kinds of crops and salt content in the soil solution.
For field estimation, a crop is planted and when it wilts, the moisture content is the PWP.
This technique requires personal judgment and prone to mistakes.
Since the change in moisture content (∆θ) is insignificant for changes in SMT from 7 to 32 atm.
Hence, 15 atm. is taken as SMT at PWP.

68. Permanent Wilting Point (PWP) cont…
At PWP the plant starts wilting, and if no water is given to the plant, then it will die.
(volumetric moisture content at Permanent wilting point
N.B. Θv (PW) = ρb * θm(WP)

69. Total Available Water (TAW): Soil moisture range
The soil moisture b/n field capacity and permanent wilting point is called available water.
It is the water content between field capacity and permanent wilting point.
Where, D =root depth of the crop
TAW = FC – PWP
TAW = (Θv (FC) - θv(WP) ) D

70. Total Available Water (TAW): Soil moisture range
This is the water available to crops.
Fine grained soils generally have a wider range of available moisture than course textured soil.

71. Depth of water per unit depth (cm/m depth)
Soil type
Moisture content (%)
Available water(%)
Depth of water per unit depth (cm/m depth) FC
PWP
Fine sand
3 to 5
1 to 3 2
2 to 4
Sandy loam
5 to 15
3 to 8
2 to 7
4 to 11
Silty loam
12 to 18
6 to 10
6 to 8
6 to 13
Clayey Loam
15 to 30
7 to 16
8 to 14
10 to 18
Clay
25 to 40
12 to 20
13 to 20
16 to 30

72. Management allowed deficit, MAD.
The degree to which the volume of water in the soil is allowed to deplete before the next irrigation is applied.
That is portion of the available moisture which is easily extracted by the plant roots.
It is commonly 60 – 80 % of the available water.
P depends on type of crop and crop growing stages

73. Soil moisture deficit, SMD
The depletion of soil moisture below field capacity at the time that particular soil moisture content, θv , is measured.
That is the amount of water required to bring the soil moisture back to the field capacity.
Deficit = Fc – soil moisture at that instant.

74. Readily Available Water (RAW):
This is the level to which the available water in the soil can be used up without causing stress in the crop.
For most crops, 50 to 60% available water is taken as readily available.

77. Soil Water Movement

80. Thank You