The micro irrigation system is classified based on the installation method, emitter flow rate, wetted soil surface area or the mode of operation.
Types of the micro irrigation systems are briefly described below.
2.1.1 Drip Irrigation
Drip irrigation applies water directly to the soil surface and allows the water to dissipate under low pressure in a form of drops. A wetted profile develops in the plant’s root zone beneath each dripper. The shape depends on soil characteristics, but often it is onion-shaped.
Ideally, the area between rows or individual plants remains dry and receives moisture only from incidental rainfall. In this system, the emitters and laterals are laid on the land surface. It has been primarily used on widely spaced plants, but can also be used for row crops.
Generally, discharge rates are less than 12 lph for single outlet point-source emitter and less than 12 lph per meter of lateral for line-point-source emitters. Advantages of this system include the ease of installation, changing and cleaning the emitters and measuring individual emitter discharge. Often the terms drip and trickle irrigation are considered synonymous. It is suitable for establishing the forestry plantations under wasteland development program. Still we are applying drip irrigation to water scarce area to grow the crops.
2.1.2. Sub-surface System
It is a system in which water is applied slowly below the surface through the line-source emitters. The water is applied through emitters with discharge rate generally in the range of 0.6 to 4 lph. A thin walled drip line has internal emitters glued together at pre-defined spacings within a thin plastic distribution line. The drip line is available in a wide range of diameters, wall thickness, emitter spacing, and flow rates.
The emitter spacing is selected to closely fit plant spacing for most row crops. Drip lines are buried below the ground and therefore called sub-surface drip irrigation systems. Burial of the drip line is preferable to avoid degradation from heat, ultraviolet rays and displacement from strong winds. These systems are used on small fruits and vegetable crops. Advantages of subsurface system include freedom from
Fig. 2.1: Emitter connected with the lateral (Jain Irrigation Systems)
Fig. 2.2.: Sub-surface drip irrigation systems (source: Peter Thorburn, www.askgillevy.com)
8 AN INTRODUCTION TO DRIP IRRIGATION SYSTEM
anchoring of the lateral lines at the beginning and removing them at the end of the growing season, little interference with cultivation and possibly a longer operational life.
2.1.3. Bubbler
Bubblers are very similar to the point source on-line emitters in shape but differ in performance. In this system, the water is applied to the soil surface in a small stream or fountain from an opening with a point-source. The discharge rate is usually greater than surface or subsurface drip irrigation but usually less than 225 lph. A small basin is required to control the distribution of water. Advantages of bubbler system are reduced filtration, maintenance or repair and energy requirements as compared with other micro irrigation systems. Larger size lateral is used with this system to reduce the pressure loss associated with a high discharge rate. The bubbler heads are used in planter boxes, tree wells, or specialized landscape applications where deep localized watering is preferable. High irrigation application efficiency up to 75%
can be achieved with total control of the irrigation water. Another advantage is that the entire piping network is buried, so there are no problems in field operations. Associated disadvantages are not supplying the small water flows as in other micro-irrigation systems.
It is not possible to achieve a uniform water distribution over the tree basins in sandy soils with high infiltration rates.
2.1.4. Micro-sprinkler
This is a combination of sprinkler and drip irrigation. Water is sprinkled around the root zone of plants with a small sprinkler working under low pressure. Water is given only to the root zone area as is in the case of drip irrigation but not to the entire ground surface as done in the case of sprinkler irrigation method. Depending on the water throw patterns, the sprinklers are referred to as mini-sprays, micro-sprays, jets, or spinners. The sprinkler heads are external emitters individually connected to the lateral pipe typically using spaghetti tubing, which is very small (1/8 inch to 1/4 inch) diameter tubing. The sprinkler heads can be mounted on a support stake or connected to the supply pipe. Micro-sprinklers are desirable because fewer sprinkler
Fig. 2.3: Bubbler placed near the tree and fitted with underground pipelines
Fig. 2.4.: Micro-sprinkler spraying the water (source:www.agricultureinformation.com)
10 AN INTRODUCTION TO DRIP IRRIGATION SYSTEM
heads are necessary to cover larger areas. Micro sprinklers require 35 kPa to 300 kPa of pressure for operation. Discharge rates usually vary from 15 lph to 200 lph. Micro sprinkler system is less likely to clog than a drip irrigation system, but water losses due to wind drift and evaporation are greater.
2.1.5. Pulse System
Pulse system uses high flow rate emitters and consequently has a shorter water application time. Pulse systems have application cycles of 5, 10, or 15 minutes in an hour, and flow rates for pulse emitters are 4 to 10 times larger than the conventional surface drip irrigation system.
The primary advantage of this system is a possible reduction in the clogging problems.