A successful tunnel irrigation system must be designed to deliver the correct amount of water to crops in an agriculturally and technically sound way. In most cases, micro-irrigation (drippers or micro-sprinklers) is used to achieve this outcome.
Modern micro-irrigation has evolved from a simple water source management system to a complex, knowledge-intensive management system that manipulates the chemical, physical and biological environment in which the plant functions.
Types of tunnels and irrigation
Two types of tunnels are mainly used in South African agriculture. The first is a large, multi-span tunnel of between 1 000m² and 10 000m² or 0,1 to 1ha, and the second is a small tunnel of 300m² or 0,03ha. Growth mediums consist of the natural soil on which the tunnel stands or soilless mediums such as coconut fibre, wood chips, gravel or polystyrene.
The reviewed and updated SA irrigation design and user manual
Several technologically advanced and reliable micro-irrigation systems are available on the market. In addition to technologically advanced products, such as filters, control equipment, drippers and micro-sprinklers, this technology also includes automation systems that incorporate precision farming principles in water application to provide precise fertiliser application as and when the plant needs it.
Three types of irrigation can be distinguished. Drip irrigation is used for crops grown in soil or in soilless mediums such as troughs, where integral dripper lines are used, and bags, where separate button drippers are used. Overhead mist sprinklers are commonly used for the cultivation of cuttings, and overhead micro-sprinklers are used for germinating seedlings in seed trays or, often, also for dense plantings grown in soil.
This article focuses on drip irrigation for soil or soilless conditions. Crops require water that is applied accurately and evenly throughout the tunnel to each plant. The system must deliver a consistent amount of water and the right concentration of nutrients. Table 1 shows the basic properties of drip irrigation.
Table 1: Properties of drip irrigation systems.
| Characteristic | Description |
| Use | Row crops (both permanent and annual crops) |
| Application method | Point application by means of the drip action and horizontal distribution through the soil or growth medium |
| Potential system efficiency | More than 95% |
| Mounting of drippers related to the pipe | On the pipe wall inside the pipe or inline as an integral part of the pipe, either directly or indirectly on the outside wall of the pipe |
| Dripper interval | If externally connected: At random distances All other couplings: At fixed intervals ranging between 0,3 and 1,25m |
| Delivery | 1,05 to 12ℓ/hour |
| Working pressure | 50 to 200 kPa |
Critical considerations
Regardless of its geographical location in South Africa, a tunnel aims to optimise the environment so that the temperature is approximately 25°C and the relative humidity is approximately 70% under optimum light conditions.
The following critical factors must be considered to ensure a successful system:
- Growth medium: Clay loam and clay soil laterally spread the water better than sandy soils, and it is necessary to test whether drip irrigation will be effective. With regard to soilless mediums, use coconut fibre, wood chips, gravel, polystyrene or blends and seek advice on the water-holding capacity of each of these growth mediums.
- Water quality: Test the water quality in order to make informed decisions regarding filtering requirements and the possible preventative treatment of water.
- Dripper type: Differentiate between pressure-sensitive, pressure-compensated and button drippers. Each dripper type’s resistance to clogging differs.
- Wall thickness: Pipes with thin or thick walls can be used. However, lifespan and cost differ.
- Pipe diameter: Different sizes are available, and the size used is determined by the flow rate it should be able to handle in order to stay within the friction limits.
- Spacing: Soil and crop type as well as cost are determining factors. Clay soil can handle larger spacings than sandy soils, and small spacings are more expensive.
- Filtration: Filtration is extremely important, and sand filtration should preferably be used.
- Design: Drip irrigation is a specialised field and requires good evaluation, planning and design.
- Management and maintenance: Drip irrigation requires careful management. Table 2 indicates the basic aspects that need to be monitored to ensure that the system functions correctly.
Table 2: Maintenance schedule for drip irrigation systems.
| Monitoring action | Each cycle | Monthly | Annually |
| Examine the system for leaks | X | ||
| Check system pressure and flow | X | ||
| Rinse laterals (depending on water quality) | X | ||
| Service air and pressure control valves | X | ||
| Check hydraulic and electrical connections | X | ||
| Check hydraulic valve operation on filter bank and inspect working parts | X | ||
| Chlorine treatment (depending on water quality and method of application) | X | ||
| Take water samples at the system’s endpoint and evaluate for changes in water quality | X |
The merits of drip irrigation
Drip irrigation has been very successful as a permanent irrigation system, and it has made huge strides in technological development over the past four decades. Worldwide the concept has been known for 60 years; however, South Africans have only been using it for 50 years. Simcha Blass from Israel developed the first commercial drippers in 1959 and the first drippers were installed in South Africa in 1969.
This technology has a high degree of automation, the labour demand for its operation is low, and a high degree of uniformity of application is achieved. This is possible because drip irrigation applies water at a slow, controlled rate to the growth medium or directly into the root zone.
Tunnel irrigation ensures successful production of various crops, provided that it is properly selected, designed, maintained and managed.
For more information, phone the author on 012 842 4009 or send an email to reindersf@arc.agric.za. – Felix Reinders, ARC Institute for Agricultural Engineering
