Minor Irrigation

Drip irrigation, also known as "trickle" irrigation, is the latest method of water management. Under this system, water is carried to the plant under low pressure, through small diameter plastic pipes and delivered at the root zone, drop by drop through drippers. Drip irrigation is widely practised and established method of irrigation in developed countries and is slowly gaining popularity in India. It is most suited for horticulture crops, vegetables etc. and finds applicability in hard rock areas where groundwater is scarce and helps in optimisation of the limited water resources. The system has its advantages and limitations. Its advantages are in terms of savings of water (50-60%) of that required for flow irrigation, effective use of fertilizers, less labour and energy cost. The limitation for adopting of this method is its high initial cost which is beyond the purchasing capacity of small and marginal farmers and thus mainly adopted by large farmers.

As a policy to encourage use of such systems, the Govt. of India under Centrally sponsored Scheme for small and marginal farmers to increase irrigation, provides subsidy to the extent of 50% of the cost of the equipment, the balance is available by institutional credit. Bankable schemes have to be formulated for availing bank loans. This booklet gives broad guidelines for scheme formulation by banks for financing drip irrigation systems.

SCHEME REQUIREMENTS

Scheme formulation for installation of drip irrigation systems against bank loans require both technical and financial details. The important items that should be included in a scheme for drip irrigation system are given bellow :

Introduction

This should briefly give the command area, type of plant/tree, required spacing between plants, land scope etc. and general topographic features.

Soil

The general nature of the soil and its characteristics. Soils have a bearing on the water requirements of crops and setting up the irrigation schedule.

Climate and Rainfall

The climatic condition and rainfall of the area governs the irrigation requirements of the crops. The evapo - transpiration data is also important. The normal monthly evaporation data as per Indian Meteorological Department (IMD) should necessarily be given which would greatly help in determining the daily water requirements and irrigation needs in different seasons.

Groundwater quality

Groundwater quality in the scheme area should be given. Its suitability for irrigation may be indicated in sodium absorption ratio, total dissolved solids etc.

Designs of Drip System

The designs of the drip system especially the layout, size and length of mains, sub-mains, laterals etc. based on land slope and field plot layout should be given in the scheme.

Emitter selection, number of emitters to the plant, water discharge through the emitter and total pumping schedule should be indicated.

Well Capacity

The source of water should be indicated. If the source of water is a groundwater structure, the diameter, depth and well yield together with HP of the pump set already installed may be given. This is necessary to decide the discharge available from the well and its optimum utilisation.

Economics

The economics of investment should be given in detail to justify the loan. The scheme should also give details about repayment period, rate of interest, subsidy available etc.

Basic Data Information

A drip irrigation system requires certain basic data information to plan its layout and ensure trouble free operation. A format for the required information is given in the Annexure I which necessarily should be provided in the scheme.

TECHNICAL ASPECTS

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 This depends upon discharge and total head including friction losses over which water is to be lifted/pumped.
   
   
6. Unit cost.

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

Table 1

Spacing and Plant Population of Important Plants/Crops

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.

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

DESIGN AND PERFORMANCE OF EMITTERS

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.

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

PERFORMANCE OF EMITTERS

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 type of soils and the surface area wetted due to drippers at various flow rates are given in Table 4&5.

A drip system is not suitable for clayey or gravely soils as would be seen from table 4. Best results with this system are obtained with medium textured soils.

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.

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.

LAYOUT OF DRIP SYSTEM

The main Line in a drip system should follow land contour as closely as possible. If there is a slope, should be made for pressure differences due to change in elevation. A fall of 1 m in elevation is equivalent to an increase in pressure of about 0.1 atmosphere. Where main lines are laid down on a slope, the increase in pressure due to elevation change may partly compensate the friction head loss. To provide nearly uniform pressure at each emitter, the tubing should be of sufficient diameter to avoid excess friction losses. The water delivered in the supply line is released through emitters spaced along the supply line. The total friction head loss due to lateral openings can be calculated by multiplying the head loss over the total length by a factor F given in Table 6. However, the additional head loss on account of diversion of flow from the man/laterals into the emitters has to be separately added while estimating the total head for purpose of calculating hp of the pump set. Friction head loss for various flow rates in plastic tubing of different sizes are given in Table 7.

The allowable pressure drop in mainline and laterals depend upon the operating pressure required at emitters. The pressure difference between the proximate and distant point along the supply line should not exceed 20% which will keep the variation of discharge within 10 of its value at the first emitter.

Mainline

To design the main line the pressure required at proximate end of laterals and the maximum friction loss at that point should first be determined. Friction losses due to valves, risers, connectors, etc., should be added to this. sometimes, two or more laterals simultaneously operate from the mainline and these have to be properly accounted for in the design.

The friction head loss in mains can be estimated by Hazen-williams formula given bellow.

hf = 10.68x(Q/C) xD x(L+Le)

Where : hf = Friction head loss in pipe (m)

Q = Discharge (M /sec)

C = Hazen Willian constant (140 for PVC pipe)

D = Inner dia of pipe (m)

L = Length of Pipe (m)

Le = Equivalent length of pipe and accessories

Laterals

The design of lateral pipe involves selection of required pipe size for a given length which can required quantity of water to the plant. This is the most important component of the system as large amount of pipe per unit of land is required and the pipe cost is such that system is economically viable.

In designing the lateral, the discharge and operating pressure at emitters are required to be known and accordingly, the allowable head can be determined by the same formula as the main line.

Design Criteria

The pressure head of emitter of any lateral should be calculated based on discharge requirement of each emitter (Table 3).

1. It should be ensured that the head loss in the lateral length between the first and last emitter is within 10% of the head available at the first emitter.
   
   
2. The friction head loss in the mainline should not exceed 1m/100m length of the mainline.

Friction head loss for various discharges is given in table 8 and equivalent lengths of straight pipe in meters giving equivalent resistance to flow in pipe fittings in Table 9.

The unit cost of Drip Irrigation system depends upon the shape and size of command area, spacing and number of plants and their water requirement. The unit cost should include the cost of following main items.

1. Mainline/Submain
2. Laterals
3. Drippers/micro-tubes
4. Lateral connectors
5. Straight connectors
6. Filters (Screen or Gravel)
7. Bends/end plugs, couplers, joint, tees
8. Pressure gauge, water meters
9. Water regulators
10. Installation charges

The following estimations are made for designing a suitable drip irrigation system on the farm.

1. Basic data - land slope, plant spacing, length of main line and laterals.
2. Irrigation water requirement
3. Emitters - number and spacing
4. Size and length of Main line and Laterals, manifold etc.,
5. HP of pump set
6. Unit cost
7. financial Analysis

Basic Data Analysis 1. No. of plants Area = 1 ha = 100x100m

Spacing (m) = 6x6

No.of plants = (100x100) / (6x6) = 277

2. Estimation of Water Requirement

The irrigation water requirement is determined using IMD pan evaporation data. The average monthly pan evaporation data for the area is given below.