Drip Irrigation

Overview

Drip irrigation, also referred to as micro irrigation, trickle irrigation or localized irrigation, involves dripping water onto the soil at very low rates (2 - 20 liters/hour) from a system of small diameter plastic pipes fitted with outlets called emitters or drippers. Water is applied close to plants so that only part of the soil in which the roots grow is wetted, unlike surface and sprinkler irrigation, which involves wetting the entire soil profile. With drip irrigation water, applications are more frequent (usually every 1 - 3 days) than with other methods, thereby providing a favorable high moisture level in the soil for the plant. As long as the application rate is below the soil's infiltration capacity, the soil remains unsaturated and no free water stands or runs over the surface.

The highest coverage was recorded in the Americas (1.9 Mio. ha) followed by Europe and Asia (1.8 Mio. ha each), Africa (0.4 Mio ha), and Oceania (0.2 Mio. ha). The top ten countries in drip irrigated areas were USA, Spain, India, China, Italy, Brazil, South Africa, Russia, Mexico, and Saudi Arabia. These countries shared 77% of the total drip-irrigated area of the world. In five countries viz. Austria, Israel, Libya, Slovak Republic and United Kingdom, irrigation is accomplished solely through pressurized systems.

A typical drip irrigation system consists of the following components:

• Pump unit;
  
• Control head;
  
• Mainlines and sub-mainlines;
  
• Lateral lines;
  
• Emitters or drippers.
  

 The system may include additional features, such as reservoir tanks, filters and fertigation devices.

Design

The pump unit takes water from the source and provides the right pressure for delivery into the pipe system.

The control head consists of valves to control the discharge and pressure in the entire system. It may also have filters to clear the water. Common types of filter include screen filters and graded sand filters that remove fine material suspended in the water. Some control head units contain a fertilizer or nutrient tank. These slowly add a measured dose of fertilizer into the water during irrigation. This is one of the major advantages of drip irrigation over other methods.

Mainlines, sub-mainlines and lateral lines supply water from the control head into the fields. They are usually made from PVC or polyethylene hose and should be buried below ground because they easily degrade when exposed to direct solar radiation. Water distribution to the plants is effected through lateral lines hosting the specific drip devices or emitters. In principle, there are two types of drip irrigation:

• Sub-surface drip irrigation - Water is applied below the soil surface.
  
• Surface drip irrigation - Water is applied directly to the soil surface.
  

Sub-surface drip irrigation

Sub-surface irrigation (SDI) is a more sophisticated and hence expensive and rare method, which employs narrow plastic tubes of about 2 cm diameter. These are buried in the soil at a depth between 20 and 50 cm, deep enough so as not to interfere with normal tillage or traffic. The tubes are either porous throughout, or are fitted with regularly spaced emitters or perforations. If porous, the tubes exude water along their entire length. If fitted with emitters, they release water only at specific points. The released water then spreads or diffuses in the soil. The pattern of wetting depends on the properties of the surrounding soil, as well as on the length of the interval between adjacent emitters and their discharge rates.

A potential problem with this technology is that the narrow orifices of the emitters may get clogged by roots, particles, algae or precipitating salts.

Experience in Israel, California and elsewhere has shown that this method of subsurface irrigation is feasible in plantations of fruit trees and other perennial row crops. It may also be applicable to annual crops grown in regular beds when high maintenance intensity can be assured. The employment of modern subsurface drip irrigation technology in developing countries is seldom and is often not feasible due to unfavorable framework conditions.

Surface drip irrigation

Surface drip irrigation is much more common and uses a very large range of drip emitter devices. Lateral lines, supplied from a field main, are laid on the surface. They are commonly 10 to 25 mm in diameter and are either perforated or fitted with special emitters. The latter are designed to drip water on to the soil Manual and Tools for Promoting SPIS – Stocktaking & Analysis Report 20 at a controlled rate, ranging from 1 to 10 liters per hour per emitter. The operating water pressure is usually in the range of 0.5 to 2.5 atmospheres. This pressure is dissipated by friction in flow through the narrow passages or orifices of the emitters, so the water emerges at atmospheric pressure in the form of drops rather than a jet or spray.

Emitters or drippers are devices used to control the discharge of water from the lateral to the plants. They are usually spaced more than 1 meter apart with one or more emitters used for a single plant such as a tree. The basis of design is to produce an emitter which will provide a specified constant discharge that does not vary much with pressure changes, and does not block easily.

Commercial emitters are either in-line (spliced into the lateral supply tubes), or on-line (plugged on to the tubes through a hole punched into the tubing wall). Commercial emitters are usually precalibrated to discharge at a constant rate of 2, 4, 8 or 16 liters per hour. The discharge rate is always affected by changes in pressure, but less so in the case of pressure-compensated emitters. The frequency and duration of each irrigation period are controlled by means of a manual valve or a programmable automatic valve assembly. Metering valves are designed to shut the flow automatically after a pre-set volume of water is applied.

Types of Drip Irrigation Emitters

Emitters (also referred to as “drippers”) are classified into groups based on their design type and the method they use to regulate pressure. Emitters are installed on the pipe and act as small throttles, assuring that a uniform rate of flow is emitted. Some are built into the pipe or tubing, others attach to it using a barb or threads. The emitter reduces and regulates the amount of water discharged.

Drip irrigation emitters are offered in two basic categories: Pressure compensating and non-pressure compensating. Generally spoken, all drip irrigation emitters are pressure compensating to some degree and most are designed to work best at 1,5 to 2,0 bars of pressure.

Pressure compensating (PC) emitters are designed to discharge water at a very uniform rate under a very wide range of water pressures; they would give the same flow under varying input pressure and landscape conditions. PC emitters are best used on plots that have drops in elevation which would then cause an increase in pressure. For pressure compensation, diaphragm type emitters are employed - a silicone diaphragm inside the emitter flexes to regulate water output.

Non-pressure compensating emitters output varies with changes in elevation and pressure. As pressure increases, the more flow the drip emitter emits. These drip emitters are best used where the landscape terrain is flat and level with very little elevation changes and consistent pressure. Non-pressure compensating emitters use an internal labyrinth design to reduce the velocity in the flow of water over a very short distance.

In response to the vulnerability of emitters to clogging due to particles transported with the irrigation water), some drip irrigation emitters are built with a self-flushing (self cleaning) mechanism reducing the clogging risk. These are usually PC diaphragm or turbulent flow emitter types.

Advantages and Disadvantages

Drip irrigation provides a large number of advantages over other irrigation methods:

• Extensive land leveling and bunding is not required, drip irrigation can be employed in all landscapes;
  
• Irrigation water can be used at a maximum efficiency level and water losses can be reduced to a minimum;
  
• Soil conditions can be taken into account to a maximum extent and soil erosion risk due to irrigation water impact can be reduced to a minimum;
  
• Fertilizer and nutrients can be used with high efficiency; as water is applied locally and leaching is reduced, fertilizer/nutrient loss is minimized (reduced risk of groundwater contamination);
  
• Weed growth is reduced as water and nutrients are supplied only to the cultivated plant;
  
• Positive impact on seed germination and yield development;
  
• Low operational costs due to reduced labor requirement, in particular energy cost can be reduced as drip irrigation is operated with lower pressure than other irrigation methods.
  

Disadvantages of drip irrigation include the following:

• High initial investment requirements;
  
• Regular capital requirement for replacement of drip irrigation equipment on the surface (damage due to movement of equipment, UV-radiation);
  
• Drip irrigation emitters are vulnerable to clogging and dysfunction (water filters required, regular flushing of pipe system);
  
• High skill requirements for irrigation water management in order to achieve optimal water distribution;
  
• Soil salinity hazard.
  

Irrigation water tends to spread sideways and downwards in the soil from the point where it is dripped. The fraction of the soil’s total volume that is actually wetted depends on the density of the drip points (the grid) as well as on the rate of application and the internal water-spreading properties of the soil. The wetted zone, and hence the active rooting volume, is usually less than half of what would be the normal root zone if the entire soil were wetted uniformly.

Under frequent drip, the wetted portion of the soil is maintained in a continuously moist state, though the soil is unsaturated and therefore well aerated. This creates a uniquely favorable soil moisture regime. Drip irrigation thus offers a distinct advantage over flood irrigation and even over less-frequent sprinkler irrigation, especially for sandy soils of low moisture storage capacity and in arid climates of high evaporative demand. In contrast to sprinkler irrigation, drip irrigation is practically unaffected by wind conditions. Compared to surface irrigation, it is less affected by soil texture, topography or surface roughness.