DRIP IRRIGATION SYSTEM

INTRODUCTION

Irrigation is the watering of land by artificial methods. Without irrigation, agriculture is limited by the availability and reliability of naturally occurring water from floods or rain. Drip irrigation is widely accepted as the most efficient irrigation technique as it allows high uniformity of water and nutrient applications.

A drip irrigation system comprises many components, each one of them playing an important part in the operation of the system. The aim of this chapter is to provide an overview of the drip irrigation system components, their functions and properties.

1. Water source
2. Pumping station
3. Air valve
4. Pressure gauge
5. Check valve
6. Shock absorber
7. Manual valve
8. Main filtration unit
9. Main filtration automatic drainage valve
10. Water meter
11. Hydraulic valve
12. Secondary filtration unit
13. Dosing unit
14. Fertilizer tank
15. Irrigation controller
16. Mainline
17. Sub mainline
18. Distribution line
19.  Kinetic valve (vacuum breaker)
20. Dripperline
21. Flushing valve
22. Flushing manifold
23. Fertilizer filter

Water source

 There are basically two main types of water sources: groundwater and surface water: 

Many existing and potential water supply sources for irrigation systems are derived from surface water, which does not tend to have high levels of salts (with the exception of some coastal areas), and thus systems are usually less prone to the formation of precipitates in drippers when using a surface water source. 

Surface water, however, tends to introduce biological hazards. If wastewater is being considered as a source, quality and clogging potential will vary depending upon the extent of treatment. 

Groundwater is generally of higher quality than surface water. However, iron and manganese levels should be measured, as high levels may lead to dripper clogging, and treatment may be required. 

Pumps & pumping stations

 Unless the water at the source is supplied at an adequate flow rate and pressure (by municipal or other entity supply, a pre-existing pump upstream from the irrigation system or gravitational pressure*), a pump will be needed to push water from the source through the pipes and drippers. Most irrigation systems include pumps as an integral part of the drip irrigation system. 

*Gravitational pressure (also known as hydrostatic pressure) is the pressure at a point in a fluid at rest due to the weight of the fluid above it. If the water source is at a higher elevation than the drippers in the field, the elevation difference between them will determine the gravitational pressure in the system (e.g. the water level in a tank is 5 meters above the elevation of the pump's axis, the gravitational pressure is 5 meters = 0.5 bar = 7.25 PSI). 

Selecting a pump for an irrigation system requires an understanding of the water conditions and local system requirements. 

Poor pump selection can lead to high operating costs and shortened pump life; this in turn impacts on the performance and reliability of the whole irrigation system. 

When a pump site is selected it is necessary to consider a range of factors, including availability of power, proximity to the development site and water quality issues. 

Power source for the pump 

The power source for the pump will depend on the availability and accessibility of the energy resource in the local area.

 In most instances, electricity is preferred because of reduced labor requirements and higher efficiency, resulting in lower energy costs. Three-phase power is usually required to operate over 10 horsepower (hp) irrigation pumps.

 If electricity is not available, alternative power sources such as diesel, gasoline, or solar may be used. The most common alternatives are gasoline engines for small pumps and diesel engines for larger pumps.

Pump types

In most irrigation applications, centrifugal pumps are used.

 A centrifugal pump is a rotodynamic pump that adds energy to the water using a rotating impeller. It may be either horizontal-shaft or vertical-shaft (including submersed pumps). 

Horizontal pumps are more frequently used to pump water from surface sources such as ponds. 

Horizontal-shaft pump 

Vertical-shaft pump 

Vertical-shaft submerged pump 

Pump capacity 

When selecting a pump, four basic factors must be considered: 

• Pump discharge (flow rate) defines the quantity of water supplied by the pump during the 1-time unit (units: m3/hour, liter/second or gallons/hour).

 • Pressure (pressure head) defines the internal energy of a fluid due to the pressure exerted on its container's walls (also known as static pressure head or static head) (units: bar or psi. 1 bar = 14.5 psi).

 • Net Positive Suction Head (NPSH) is the required head value (suction lift) at the inlet of a horizontal pump enabling it to pull water upwards while keeping the water from cavitating* (inherently limited to 0.8 barnets). 

*Cavitation - The formation of vapor cavities ("bubbles" or "voids") in a liquid. It usually occurs when a liquid is subjected to rapid changes of pressure that cause the formation of cavities where the pressure is relatively low. When subjected to higher pressure, the voids implode and can generate an intense shockwave causing significant damage to the pump's impeller and chamber.

 • Friction head - Head loss caused by the friction between the fluid and the inner walls of the shaft the enclosure of a vertical pump (or in the outlet pipe of a vertical submerged pump) which pulls the water upwards. Friction loss increases with run length and by the square of the fluid velocity. It affects the required pressure and flow rate.

The output pressure of a pump is dependent on pressure head and flow rate (a higher flow rate causes a lower pressure and vice versa, all other variables being unchanged). Make sure the pump is able to deliver an adequate flow rate and pressure for the application. Obtain a performance curve for the pump and have modifications made if it is not adequate - the energy savings alone will easily pay for any upgrades required, which will also improve system operation and crop production, resulting in a shorter ROI. 

Pump selection

 The irrigation system design will specify the required pump duty (flow rate and pressure head). The best pump choice is the pump in which the Best Operating Point (BOP) occurs at this flow rate and pressure head and that can operate at the available suction head.

The pump's performance curve 

Each pump must be supplied with its performance curve, as an integral part of the product and the supplier/manufacturer must commit to the data presented in it.

 It is very important to keep the pump data documentation available for the whole lifetime of the pump. 

The performance curve of the pump (flow rate/pressure range) is indispensable for the design and the construction of the entire irrigation system. 

The pump outlet pressure is related to the discharge rate. A change in the flow rate will cause a change in the working pressure. Changes in the flow rate and pressure may be critical when considering the relationship between the flow rate, the working pressure and the pump's efficiency curve in the planning process. 

The steeper the pump's operating curve, the more a change in flow rate will affect the working pressure.

Filtration 

Filtration is critical in any drip irrigation system. Effective filtration is essential for proper irrigation system operation and long-term performance, as it prevents the irrigation water from clogging the drippers.

 Water quality 

The concept "water quality" relates to the variety and concentration of the dissolved and suspended components in the water. 

Water requirements for drip irrigation

The quality of water for irrigation relates to the parameters required to maintain the crop's health and the integrity of the irrigation system. Every type of pressurized irrigation system requires attention to the water quality to avoid clogging of the irrigation components in order to enable orderly long-term irrigation according to the irrigation program. 

Water quality will dictate filtration requirements, chemical injection requirements, and management of the irrigation systems to prevent dripper clogging. 

Causes of dripper clogging in systems may be chemical (precipitates or scale), physical (grit or particulates such as sand and sediment) or biological (such as algae or bacteria). 

The water’s chemical characteristics are influenced by the variety and concentration of the substances dissolved in it. These dissolved substances include ions of dissolved salts such as chloride, sodium, and nutrients (nitrogen, phosphorus, potassium, and others). Calcium and magnesium influence the hardness of the water, iron, and manganese are liable to be found either dissolved or as a residue, along with other dissolved organic compounds and even poisonous substances. 

The biological characteristics of the water quality include a variety of living organisms such as microorganisms, including bacteria, viruses, single-celled entities, algae and zooplankton, which develop in open water along with creatures developing within the water transport system itself. 

The water quality is expressed by the physical conditions and the variety and concentration of its constituents.

Types of filters

 The types of filters used most often in drip irrigation systems are: 

Media filters (gravel or sand) are necessary for any surface water source and especially so for wastewater. They consist of a metal or plastic enclosure incorporating small gravel stones or sand, which traps the dirt. This filter includes a flushing system for washing the gravel or sand and returning the dirt to the water source. 

ATTENTION It is highly recommended to install a screen filter downstream the media filter in order to prevent infiltration of the filter medium into the system in the event of a malfunction of the media filter. 

Disk filters are used with surface water systems, wells or municipal water sources. These filters are comprised of a series of grooved plastic disks stacked together with a total equivalent screen size ranging from 40 to 400 mesh. These filters enable deep three-dimensional filtering (e.g. allow entrapping of more particles as water passes through the pores created by the grooves in the surfaces of the filtering disks stacked together in the filter). 

Having more surface area than screen filters, disk filters are better suited for higher flow rates. 

Screen filters are used mainly as secondary filters with surface water systems or as primary filters with well or municipal water sources. A screen filter is comprised of a cylinder with a net that traps the dirt. This filter is intended for relatively clean water; its use is less common with water from a reservoir or pumped water. 

ATTENTION In any type of filter, the dirt returned to the water source should be discharged as far as possible from the suction point. In a streaming source (e.g. a river) the discharge the point should be downstream from the suction point. 

Hydrocyclone sand separators are used as a preliminary stage of filtration in the presence of sand or other heavy particles (50 microns or bigger) in the source water. It utilizes centrifugal force to separate the particles from the water. The separated material drops down into a tank or reservoir where it can be removed later. 

It is not a true filter since there is no physical barrier to separate out the particles, but it is often used before a filter to first remove the bulk of the contaminant, where the filter does the final cleaning. This type of design reduces the time required to flush and clean the main filter. Each hydro cyclone model has its specific operation flow rate range, it will not perform outside this range.

REFERENCE 

OTHER TYPE OF IRRIGATION

SPRINKLER IRRIGATION

SURFACE IRRIGATION

FURROW IRRIGATION