Components of Drip Irrigation Systems
3.2. Distribution Network
3.2.3. Drippers/Emitters
The emitters are connected to the laterals and control the flow of water coming out of laterals. The design of single outlet emitter is based on the principle of energy losses by sudden change in velocity of fluid flow in the emitter path. The successive change in velocity occurs due to sudden enlargement and contraction in the designed flow path of the disc element. The necessary arrangement for expansion of flow is done by making larger cross sectional area in the shape of circle and contraction of flow by smaller cross-sectional area in the shape of
52 AN INTRODUCTION TO DRIP IRRIGATION SYSTEM
rectangular channel. The bottom of the path is made flat to increase the wetted perimeter of the flow passage, which leads to decrease the hydraulic radius and velocity of flow in the flow passage (Fig. 3.9).
The energy losses occur when the flow channel suddenly expands to a larger diameter circular path. The energy loss is accomplished by two ways – first due to the impulse momentum which takes place as the water flows from narrower rectangular passage to the wider circular passage and secondly when the water flows from wider circular section to the narrower channel section. In this process, a lot of eddies are formed which causes a considerable dissipation of energy. The high pressure from the lateral line transmitted into the emitter is greatly reduced and controlled flow of water emits in discrete drops almost at atmospheric pressure. Emitters are made from poly-propylene or polyethylene and available in the market in different types and designs.
Fig. 3.9: Components of a point-source online pressure compensating dripper.
Point-source and line-source emitters operate either above or below the ground surface. Most of the point source emitters are either on-line or in-line emitters. The primary difference between on-line and in-line emitters is that the entire flow required downstream of the emitter passes through an in line emitter. There is more head loss along a lateral with on-line emitters than one with in-line emitters because of obstruction created by the barbs of on-line emitters. The percentage area wetted, and the reliability of the emitters against the clogging and malfunctioning are two important aspects of quality and safety of drip irrigation systems. An ideal set of emitters should have the attributes such as durability, low cost, reliable performance with a relatively low rate of uniform discharge and relatively large and/or self-flushing passageway to reduce or prevent clogging.
3.2.3.1. Types of Drippers/Emitters
(a) In-line Emitters: In-line emitters are fixed along with the lateral line. The pipe is cut and dripper is fixed in between the cut ends, such that it makes a continuous row after fixing the dripper.
They have generally a simple thread type or labyrinth type flow path (Fig. 3.10).
(b) On-line Emitters: These are fixed on the lateral by punching suitable size holes in the pipe. These are of the following types:
i) Simple Type/Laminar Flow: In this type of dripper, the discharge is directly proportional to the pressure. They have simple thread type, labyrinth type, zig-zag path, vortex type flow path or have float type arrangement to dissipate energy.
ii) Turbo Key Drippers: These are made of virgin and stabilized polymers and are available in 2, 4 and 8 lph discharge. They provide resistance to blockage and are pressure compensating.
iii) Pressure Compensating Drippers: This type of dripper gives a fairly uniform discharge within the pressure range of 0.3 atmospheres to 3.5 atmospheres. They are provided with a high quality rubber diaphragm to control pressure and are most suitable on slopes and difficult terrain.
54AN INTRODUCTION TO DRIP IRRIGATION SYSTEM
Fig. 3.10: In-line dripper
iv) Built-in Dripper Tube: In this system, polyethylene drippers are inseparably welded to the inside of the tube during extrusion of polyethylene pipes. They are provided with independent pressure compensating water discharge mechanism and extremely wide water passage to prevent clogging. Other accessories include take out/ starter, rubber grommet, end plug, joints, tees and manifolds.
3.2.3.2. Emitter discharge exponent
The flow from the emitter used in drip irrigation system is expressed by the following relationship:
q = Kd Hx ...…... (3.5) where, q is discharge rate of emitter in lph, Kd is discharge coefficient which is a constant of proportionality, H is the pressure head at emitter in meter, and x is emitter discharge exponent. The value of x governs the flow regime, and discharge and pressure relationship of an emitter.
The pressure variation will less affect the discharge with lower value of x. Normally x is taken as 0.5 for non compensating simple orifice and nozzle emitters. For fully compensating emitter, x = 0.0. The exponent of long path emitter is normally varies from 0.7 to 0.8. The values of x usually lie between 0.5 and 0.7 for tortuous path emitters.
3.2.3.3. Selection of an emission device
The selection of an emission device such as line-source and point-source emitters, micro- sprinklers and bubblers depends upon crop to be irrigated, filtration requirement and soil type. The required emitter flow rate can be computed as follows:
where,
Qr = required emitter flow rate, l/h di = water requirement of per plant, l/d Ii = irrigation interval, day
56 AN INTRODUCTION TO DRIP IRRIGATION SYSTEM
It = irrigation time per set, h Ea = application efficiency, fraction N = number of emitters per plant
The crop water requirement under drip irrigation is different from the crop water requirement under surface and sprinkler irrigation primarily because land area wetted is reduced resulting in less evaporation from soil surface. The capacity of emission device can be computed by using equation
…... (3.7) where,
Q = capacity of emission device, l/h
A = area irrigated by the emission device, m2 d = depth of applied water, mm
H = hours of application of irrigation water Tm = off time for maintenance, h
Ea = application efficiency, %
The area irrigated by an emission device is computed by the equation
…... ... (3.8) where,
A = area irrigated, m2,
L = spacing between adjacent plant rows, m S = spacing between emission point, m Wp = percent of cropped area being irrigated
Ne = number of emission devices at each emission point.
The value of Wpvaries from 30 to 100%, depending upon the type of
crop and its age. For widely spaced horticultural fruit tress (vine, bush, mango etc.), Wp varies between 30 to 60 % of area of each tree and for close growing crops such as vegetables, the value of Wp is considered as 100%. The number of emission devices per emission point,required for desired wetting pattern is determined on the basis of horizontal and vertical movement of water through the soil. The soil moisture movement studies are required to be conducted at several representative sites around the field to obtain horizontal and vertical spread data.
3.2.3.4. Emitter spacing
As we have discussed in Chapter-2 that wetting pattern of the emitter determines the spacing and number of emitter required per plant. To determine the wetted width and depth of water front, Mohammed (2010), Schwartzmass and Zurr (1985) and Wetup model can be used.
United States Soil Conservation Services (1984) has provided an estimate of maximum horizontal wetted width from a single point source emitter and this can be used where field data are not available (Table 3.3). For line-source emitter, this value must be multiplied by 0.8.
Table 3.3: Wetted width or diameter of wetted circle of a point source emitter of 4 lph capacity (United States Soil Conservation Services, 1984)
Depth of root zone and Homogeneous Soil profiles of varying
soil texture soil layer, m textural group
Low density, m Moderate density, m Root zone depth, 0.75 m
Coarse soil 0.45 0.75 1.05
Medium soil 0.90 1.20 1.50
Fine soil 1.05 1.50 1.80
Root zone depth, 1.5 m
Coarse soil 0.75 1.40 1.80
Medium soil 1.20 2.10 2.70
Fine soil 1.50 2.00 2.40