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Minor Irrigation --- Drip Irrigation Part 2 --- www.nabard.org

Minor Irrigation

Drip Irrigation: Part 2


3.0 TECHNICAL ASPECTS

3.1  Design Parameters

The design of a drip Irrigation system involves estimation of the following parameters.
  1. Area to be irrigated, type of plants, their spacing and numbers per hectare.
  2. Peak water requirement of a plant per day. For estimation of total water requirement for a given area, the number of emitters required per plant, amount of water discharged per hour through each emitter and the total number of hours water is available should be known/estimated.
  3. Design of Main and Lateral Drip Lines. This depends upon friction head loss which in turn is governed by the type of plantation/crop and field configuration.
  4. Water required to be pumped from the well. This depends upon hydrogeological conditions in the area and water requirement of plants/crop.
  5. Horse Power of Pump set depends upon discharge and total head including friction losses over which water is to be lifted/pumped.
  6. Unit cost.

3.1.1   Command Area

A command area map giving systems layout is necessary to plan and design a drip irrigation system. It may not be necessary to have a detailed contour plan but it is helpful if a plan showing the highest and lowest points along with well location is given in the scheme. This enables proper design of main line and laterals to suit the spacing and number of plants.
The recommended spacing and population of some of the important plants/crops are given in the Table 1 below.

Table 1 : Spacing and Plant Population of Important Plants/Crops


Sr.No
Crop
Spacing (m)
Plant Population (Nos/ha)
1
Grapes
3.0x3.0
1,100
2
Mango
10.0x10.0
100
3
Oranges
5.0x5.0
400
4
Lime
6.0x6.0
270
5
Coconut
7.5x7.5
175
6
Banana
1.5x1.5
4,400
7
Cotton
1.3x1.3
5,900
8
Tomato/Brinjal
1.0x0.5
20,000
9
Sugarcane
1.0x0.3
33,000
10
Litchi
6.0x8.0
208


3.1.2   Water Requirement of crops/plants

Water requirement of crops (WR) is a function of surface area covered by plants, evaporation rate and infiltration capacity of soil. At first, the irrigation water requirement has to be calculated for each plant and thereafter for the whole plot based on plant population for the different seasons. The maximum discharge required during any one of the three seasons is adopted for design purposes.
The daily water requirement for fully grown plants can be calculated as under.

WR = A X B X C X D X E.................Equation (1)

Where : WR = Water requirement (lpd/plant)

A = Open Pan evaporation (mm/day)
B = Pan factor (0.7)
C = Spacing of crops/plant (m2)
D = Crop factor (factor depends on plant growth for fully grown plants = 1)
E = Wetted Area (0.3 for widely spaced crops and 0.7 for closely. spaced crops)
The total water requirement of the farm plot would be WR x No.of Plants
The daily water requirement pf various crops per plant for different pan evaporation readings are given in Table 2.


Table 2 : Water requirement of Crops/Plants on the Basis of Pan Evaporation Data


Crops
Spacing
(m)
Pan Evaporation ( mm/day )
2
4
6
8
10


Water Requirement( lpd /plant)
Grapes
3.0x3.0
3.7
7.6
11.3
15.1
18.9
Mango/Sapota
10.0x10.0
42
'84.0
'126.0
'168.0
'210.0
Oranges
'5.0x5.0
10.5
'21.0
31.5
'42.0
52.5
Coconut
6.0x6.0
15.1
30.2
45.4
60.5
75.6
Banana
7.5x7.5
24.2
48.5
72.8
'97.0
121.3
Cotton
1.5x1.5
1.7
4.4
6.6
8.8
'11.0
Tomato/Brinjal/Chillies
1.3x1.3
'0.5
3.3
'5.0
6.6
8.3
Sugarcane
1.0x0.3
'0.3
'1.0
2.5
'2.0
2.5
Litchi
6.0x8.0
35
42
65
69



The water requirement for different seasons can be calculated using Equation 1 given above. The maximum discharge required during any one of the three seasons is adopted for design purposes

3.1.3   Design and Performance of emitter

The design, number of emitters required for plant and their discharge are important factors in designing a drip irrigation system. Various emitters are designed for controlled release of water to the plants. It is necessary for manufactures of drip system to state optimum operating pressure and discharge and the emitter is so selected that application rate equals to the absorption rate of soil so that no water stagnation takes place on the surface of the soil. In some systems a short length of flexible plastic tubing of small diameter is used as emitter. This tubing is generally of 0.96mm diameter and is inserted through holes in walls of the laterals. This is commonly known as micro tube system. The flow from different lengths of 0.96mm polyethylene tubing under various pressure is given in Table 3.

Table 3 : Flow from polythelene Tube emitters of 0.96 mm diameter(lph)


Length of tubing
Pressure in supply line (Atmosphere)
(mm)
0.1
0.2
0.3
0.5
0.75
1
1.5
7.5
6.1
10.4
13.9
20.2
27.2
33.2
44.7
15.5
4.1
6.7
9
12.8
17
20.7
27.4
25
2.9
4.7
6.3
8.9
11.8
14.4
19
35
2.3
3.7
4.9
7
9.3
11.3
15
50
1.8
2.9
3.8
5.5
7.3
8.8
11.7
75.5
1.4
2.2
2.9
4.2
5.6
6.8
9
100
1.1
1.8
2.4
3.4
4.5
5.5
7.3
125
0.96
1.6
2
2.9
3.9
4.7
6.3
150
0.84
1.4
1.8
2.6
3.4
4.2
5.5
175
0.75
1.2
1.6
2.3
3
3.7
4.9
200
0.69
1.1
1.5
2.1
2.7
3.3
4.4
250
0.6
0.97
1.3
1.8
2.4
2.9
3.8
300
0.53
0.85
1.1
1.6
2.1
2.6
3.4


Another method of releasing water from laterals is through small perforations in the walls which are sometimes called "soakers".

3.1.4   Performance of Emitter

Water from emitters fall on ground and is absorbed by soil. The wetted area depends upon the soil type and rate at which water comes out of emitters. The infiltration rate for various types of soil and the surface area wetted due to drippers at various flow rates are given in Table 4&5.

In orchards having widely spaced plants, two or more line of laterals may be required for each row. Sometimes a loop with 3 to 4 emitters is placed around each plant to provide the required wetted area. This should be away from the plant stem.


Table – 4 : Infiltration Rate of Soil


Sr.No.
Texture
Infiltration Rate (cm/hr)
1
Coarse Sand
2.0 to 2.5
2
Find Sand
1.2 to 2.0
3
Fine Sandy loam
1.2
4
Silty loam
'1.0
5
clay loam
0.8
6
clay
0.5


Table – 5 : Surface Area Flooded by Emitters
Sr.No.
Emitter flow
Soil infiltration rate (Cm/hr)
Rate (lph)
0.25
0.5
0.75
'1.0
1.25
'1.50
Wetted Area (sqm)
1
'1.0
0.4
0.2
0.13
0.1
0.08
0.07
2
'2.0
0.8
0.4
0.27
0.2
0.16
0.13
3
'3.0
1.2
0.6
'0.40
0.3
0.24
'0.20
4
'4.0
1.6
0.8
0.53
0.4
0.32
0.27
5
'5.0
'1.0
'1.0
0.67
0.5
'0.40
0.33
6
'6.0
1.2
1.2
'0.80
0.6
0.48
0.4
7
'7.0
1.4
1.4
0.93
0.7
0.56
0.47
8
'8.0
1.6
1.6
1.07
0.8
0.64
0.53


3.1.5   No. of emitters

The number of emitters is based on the volume of wetting for each plant. Generally, 30-70 percent of the area is wetted dependent upon plant spacing, nature & development of root zone. The number of emitters required per plant is estimated as the ratio of rate of irrigation requirement to the emitter discharge. If single emitter is provided, it must be placed 15-30 cm. from the base of the plant.


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