Techniques of Water Distribution and Quality of Irrigation Water

Techniques of Water Distribution and Quality of Irrigation Water

Techniques of Water Distribution

1. Free/wild/ordinary flooding:

a) Ditches are excavated in the field in this case, and water from these ditches are allowed to flow across the field without any restriction by opening the field regulators.

b) In case of controlled free flooding, surplus water flows through the wastewater channel and is discharged into the drainage or river.

c) The cost of land preparation is low, but the cost of labour is high.

d) The main disadvantage is that the water application efficiency is low, especially when flooding is not controlled.

e) In this method, the series of field channels is connected to the main channel (supply channel).

NOTE

Free flooding is suitable for close growing crops as losses in this case are comparatively less and where the land profile is steep.

2. Border flooding:

a) In this case, the land is divided into a series of long, uniform graded, narrow strips separated by low levees.
These levees guide the flow of water down the field.

NOTE

Levees are small embankments which don't permit the flow of water across them.

• The length of the strips is in the range of 100 to 400 m, whereas the width of the strip is in the range of 10 to 20 m.
• This method is suitable when the area is levelled in a direction perpendicular to the flow in order to prevent water from concentrating on either side of the border.
• Water is allowed to flow from supply ditches into each strip, and during its travel, water gets infiltrated into the soil.

A relationship between the discharge through the supply ditch (Q), the average depth of water flowing over the strip (y), the rate of infiltration of the soil (f), the area of the land irrigated (A), and the approximate time required to cover the given area with water (t), is given by the equation:

t = 2.3(y/f)*log₁₀ {Q/(Q-f*A)}

where

Q = Discharge through the supply ditch

y = Depth of water flowing over the border strip.

f = Rate of infiltration of soil.

A = Area of land strip to be irrigated.

t = Time required to cover the given area A.

The above equation can be obtained by considering small area (dA) of the border strip of area A.

It can also be concluded that after this maximum area is irrigated, losses will take place. The border strip's discharge per unit area (Q/A) should be adjusted according to the field's infiltration capacity (f), or else excess water would be lost, or the entire area would not be irrigated.

3. check flooding:

a) It is identical to ordinary flooding; however, the water is regulated by a check area surrounded by low, flat levees.

b) Levees are usually built to follow the contours of the land.

c) The check is filled with water at a rapid rate and left to stand until the water seeps through.

d) Crops that grow close together, such as jowar and paddy, are the best for this technique.

4. basin flooding:

a) It is a special type of check flooding, generally used for orchard plants.

b) Similar to check flooding, here, the basin is flooded having one or more trees in it.

5. furrow irrigation:

a) Furrows are narrow field ditches which are excavated between the rows of plants to carry water through them.

b) Depth of furrows varies from 8 to 30cm, while the lengths of furrows are around 400m.

c) Water infiltrates the soil by moving vertically and then laterally across the wetted perimeter of the furrow.

d) This method is most suitable for the crops like sugarcanes, groundnuts, potatoes and tobacco etc.

NOTE

In furrow irrigation, water covers the entire area, and only one-fifth to one-half of the land surfaces is wetted by water. As a result, there is less evap-oration, less soil puddling, and the soil can be cultivated sooner after irrigation.

6. sprinkler irrigation:

a) Sprays of irrigated water are applied to the field in this situation.

b) A network of main pipes, sub-main pipes, and lateral pipes is used to spray water.

c) Lateral pipes may be perforated on the top and sides, or they may have a number of nozzles that produce a fountain effect.

d) It is suitable for all types of soil and for widely different topographies and slopes.

The following conditions encourage the use of this method:

• When the land terrain is uneven, making surface irrigation impossible.
• When there is a steeper land gradient and the soil is easily erodible.

• When the ground soil is either excessively porous or highly impermeable, preventing good water distribution via surface irrigation.
• When the groundwater table is high.
• When the water is available with difficulty and is scarce.

Sprinkler System

Advantages & disadvantages of Sprinkler irrigation

7. drip irrigation:

Note

Drip Irrigation can also be termed as trickle irrigation.

• It is the latest method of irrigation.
• In this method, water and fertiliser are slowly and directly applied to the root zone of the plant in order to minimise the evaporation and seepage losses
• Specially designed emitters and drippers are used for this purpose.
• This method is best suited for row crops and orchards.
• Example: Tomato, grapes, cauliflower, cabbage, etc.

Advantages & disadvantages of drip irrigation

It is the latest field irrigation technique in which water is slowly and directly applied to the root zone of the plants, thereby minimising the losses by evaporation and percolation.

NOTES

• Free flooding is also termed as ordinary/wild flooding.
• The length of the strip in the border flooding is in the range 100 to 400 m whereas width of strip is in the range of 10 to 20 m.
• Depth of furrows varies from 8 to 30 cm while the length of furrows is around 400 m.
• Sprinkler irrigation is used for all types of soil and for widely different topographies and slopes.

quality of irrigaton water

1) Irrigation water is harmful to the growth of plants, so there should be a minimum amount of impurities in water.

2) The following are examples of impurities that make water inappropriate for irrigation:

a) Sediment concentration in water:

• The effects of sediments present in irrigation water depend upon the type of irrigated land and the type of soil.
• When fine sediments from water (having N, P, and S in it and obtained from erosion of sedimentary rocks) are deposited in sandy soil, their fertility is improved.
• However, if coarse sediments from water (having deficiency of N, P and S and obtained from erosion of igneous rock) are deposited in soil, it may reduce the fertility of the soil.

b) Total soluble salt content in water:

I) Salts of calcium, magnesium, sodium and potassium are present in irrigation water that proves to be harmful for the growth of crops; as they are in excess, they lead to:
1) Injury to the crops.
2) Reduction in osmotic activity.
3) Choke the voids in roots; hence affects aeration.

c) The concentration of salts in soil solution has a serious effect on plant growth as these salts get deposited on soil with the passage of time.

d) Concentration of the salts in soil solution is given by:

C_s = (C * Q ) / (Q + R_e - C_u)

Where, C, = Salinity concentration of soil solution.
Q = Water quantity applied
C_u = Consumptive water use
R_e = Useful or effective rainfall
C = Concentration of salts in irrigation
water.

S. NO	Types of Water	Concentration of Salts (mmho/cm) @ 25o C and use in irrigation
1	Low Salinity Water (C1)	100-250 It can be used for irrigation of almost all type of crops and for almost all kind of soils except for soil of extremely low permeability.
2	Medium Salinity Water (C2)	250-750 It can be used, if a moderate amount of leaching is provided or normal salt-tolerant plants can also be grown.
3	High Salinity Water (C3)	750-2250 It cannot be used on soil with restricted drainage.
4	Very High Salinity Water (C4)	>2250 It is not suitable for irrigation.

NOTES

If total concentration of salts is more than 700 mg/l it is harmful for some plants, but if its concentration is more than 2000 mg/l it is harmful to all the crops.

e) The proportion of sodium ions in relation to other cations.

• Generally, soils contain Ca²⁺, Mg²⁺: and Na⁺ ions.
• If the percentage of Na⁺ ions is less than 5% of total exchangeable cations, it is tolerable.
• If the percentage of Na⁺ ion increases to about 10%, the following changes are induced in the soil
  • 1) It increases the pH of soil.
  • 2) It reduces the permeability o the soil.
  • 3) It increases the plasticity of the soil; hence it becomes stick when wet and crusts when dry

Sodium Absorption Ratio (SAR) represents the sodium ions concentration in the soil.

SAR is given by following expression:

SAR = Na⁺/√'( Ca²⁺, Mg²⁺ ) / 2

Here concentration of these cations is in terms of 'meq/l'.

S. NO	Types of Water	SAR and its Uses
1	Low Sodium Water (S1)	0-10 It can be used for irrigation of almost all roots and for all crops except those which are highly sensitive to sodium Example: Stone fruit trees
2	Medium Sodium Water (S2)	10-18 It is not suitable for fine textured soil, which may require gypsum prior to its use
3	High Sodium Water (S3)	18-26 It proves harmful for almost all types of soil and require good drainage for leaching
4	Very High Sodium Water (S4)	>26 It is not suitable for irrigation

S.NO	Classifications	EC	ESP	pH
1	Saline Soil (White Alkali)	>4000	<15	<8.5
2	Alkaline Soil / Non- Saline Soil / Sodic Soil / Black Alkali	<4000	>15	8.5 - 10
3	Saline Alkaline Soil	>4000	>15	<8.5

1 Concentration of potentially toxic elements present in water:

a) Toxic elements like boron, selenium are present in irrigation water (soap water consists of excess of boron).

b) Concentration of boron above 0.3 mg/l may prove to the toxic for certain plants, and its concentration above 4 mg/l is toxic for all the plants.

2 Bicarbonate concentration in relation to calcium and magnesium concentration.

3 Bacterial contamination.

NOTES

• The salt concentration is also measured in terms of electrical conductivity of water.
• If the percentage of Na⁺ ion is less than 5% of total exchangeable cations, it is tolerable.
• SAR is defined as: SAR = Na⁺/√'( Ca²⁺, Mg²⁺ ) / 2

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Summary

• Irrigation is the science of artificially applying water to land in accordance with the 'crop requirement' throughout the 'crop season' for full-fledged crop feeding.
• There are two types of irrigation:
  • a) Flow irrigation
  • b) Lift irrigation
• Free flooding is also termed as ordinary/wild flooding.
• Levees are the small embankments, which don't permit the flow of water across it.
• Check flooding is similar to the ordinary flooding except the water is controlled by surrounding check area with low and flat levees.
• Basin flooding is a special type of check flooding and is adopted specially for orchard trees.
• Furrow irrigation is suitable for the row crops like sugarcanes, ground-nuts, potato and tobacco etc.
• Sprinkler irrigation can be used for all types of soil and for widely different topographies and slopes.
• Drip Irrigation is also known as trickle irrigation.
• The effect of sediments presents in irrigation water depends upon the type of irrigated land and type of soil.
• If total concentration of salts is more than 700 mg/l it is harmful for some plants, but if its concentration is more than 2000 mg/l it is harmful to all the crops.
• If the percentage of Na⁺ ion is less than 5% of total exchangeable cations, it is tolerable.
• Concentration of boron above 0.3 mg/L may prove to the toxic for certain plants and its concentration above 4 mg/ l is toxic for all the plants.
• The salt concentration is also measured in terms of electrical conductivity of water.

Important keywords

• Irrigation.
• Surface Irrigation.
• Sub - Surface Irrigation.
• Methods of Water Distribution.
• Quality of Irrigation Water.