by Jerry Austen, Netafim South Africa
The installation of a greenhouse project typically takes place in a confined area, in comparison to open-field projects on a farm with many people from several disciplines, all co-ordinated by the project manager who would usually be the grower himself. This paper considers aspects related to the irrigation design that need to be considered long before the project gets underway.
The project life cycle
Like any project, the life-cycle of a greenhouse project undergoes the following overlapping phases:
- Definition – This would include the preliminary irrigation design.
- Planning – This would include the actual irrigation design.
- Implementation – Including procurement, construction and installation.
- Close-out – Including commissioning of the project.
Figure 1: The project life cycle and costs.
The costs increase rapidly as the project progresses through the first stages, peak during implementation and tail off in the close-out. Changes that occur during the project for whatever reason, have an effect that becomes more pronounced the later that they occur into the project.
To this end, the later into the project, the less flexibility there is to make a change. See Figure 2.
Figure 2: The ability/flexibility to make changes during a project.
Any change during a project involves an interaction between the following three factors:
Figure 3: Cost-quality-time triangle.
For instance, if a certain task’s completion date needed to be brought forward (time), it could only be done by increasing resources (cost) or reducing quality. The time cannot be reduced by keeping costs and quality as they were.
The later the changes, the worse the effect on one or more of cost, quality and time. The following looks at the feasibility of changes to the irrigation design during each of these phases.
Changes in irrigation design – Definition phase
The preliminary irrigation design takes place during this definition phase. Changes are very feasible. This phase is earmarked for changes and this is the phase to get a reasonably clear idea of what one wants.
Changes in irrigation design – Planning phase
This is the phase where the irrigation design is formulated. Materials will be procured against this design. Changes are still reasonably feasible during the planning phase.
Changes in irrigation design – Implementation phase
Procurement of materials occurs during this phase. Once procurement has begun, changes are more difficult to make. Irrigation equipment for instance is often only manufactured to order. Construction and installation occurs during this phase. Changes now begin to have a more severe effect with increased costs, time delays or a reduction in quality.
This is the phase in irrigation systems where most changes through inadequate planning come to light.
Changes in irrigation design – Close-out
This is when commissioning takes place and where changes are, for all intents and purposes, not feasible.
The project manager and the integration of the irrigation with the rest of the project
The irrigation part of the project cannot be viewed in isolation at any time during the project life-cycle. The irrigation involves the interaction of the following aspects and individual specialist personnel, which must be catered for, planned and integrated from the outset. The project manager is the key person who must be on the ground from beginning to end, to facilitate interaction of the irrigation designer and irrigation contractor with:
- Agronomy, including:
- Cultural practices
- Water source
- Greenhouse design and installation including:
- Climate control
- Electrical power supply
- Infrastructure, including:
The project manager requires a clear understanding on who is responsible for what.
The following highlights some of the key aspects of a greenhouse irrigation design to be dealt with early on in the project.
A system must be designed to transport water from the source to head control / irrigation pump room, where there is a bulk storage tank. A flooded suction is essential in a greenhouse irrigation system. This supply to the bulk tank is not necessarily carried out by the irrigation designer.
This water supply includes:
- Irrigation water – involving the agronomist.
- Service water – involving the grower.
- Cooling water – involving the greenhouse designer.
Once the total flow rate from these three is decided, it cannot feasibly be changed later.
Water from the head control to the greenhouse
Having received the relevant information from the grower and the greenhouse designer for the service water and cooling water respectively, the systems to transport this water from the head control to the greenhouses are normally designed by the irrigation designer.
The norms for the irrigation water supply are usually based on a flow rate of 1,2ℓ/m²/h (mm/h) comprising:
Crop water requirement = 0,8ℓ/m²/h
Leaching = 0,4ℓ/m²/h (33%)
Irrigation flow rate = 1,2ℓ/m²/h
The irrigation flow is probably the largest single contributing factor to the size and cost of the irrigation design. It must be decided with reasonable accuracy as it cannot feasibly be increased later.
Agronomy – Nutrition – Dosing unit
The most common pitfall is failing to get the right fertiliser mixture into the irrigation water using the dosing unit. The agronomist knows what fertiliser is needed and the irrigation designer knows what a dosing unit is capable of. Together they need to come up with the correct formulation of fertiliser solution that is capable of being injected, and the correct selection of the dosing unit that is capable of injecting the fertiliser recipe.
Agronomy – Nutrition – Dedicated mainlines
A dosing unit can only inject one fertiliser recipe at a time. Only plants that require that recipe can be irrigated at that time. The effect of a five-minute irrigation pulse is that fertiliser that is injected into the irrigation water at the head control / pump room may not necessarily reach the plants during those five minutes. This means that each block must have its own mainline downstream of the point of fertiliser injection. Irrigation blocks may only share a mainline if they share the same fertiliser recipe.
Agronomy – Nutrition – Areas planted according to age or cultivar
Plants that require a different recipe due to age or cultivar need to be irrigated separately.
This affects the size of the area of one crop planted at one time, and the area planted according to one type of crop or cultivar.
Agronomy – Crop type and plant population density
It is important to establish exactly which crops will be grown: all of them, now and in the future. Different plant population densities that are multiples of the other can use the same irrigation equipment. For instance, a crop that is planted at a density of two plants/m² can share the same irrigation equipment as one that is planted at four plants/m². However, cucumbers for instance, at 2,1 plants/m², and peppers, at 3,5 plants/m², cannot use the same irrigation equipment.
Drainage systems must be designed to cater for the following three aspects:
Drainage in the head control / pump room
This is designed by the irrigation designer but must be built by the builder. The pump house / head control floor needs to be sloped. In addition, it needs elevated equipment and concrete trenches. See Figures 4 and 5. This in turn requires co-ordination between all persons who will be involved in the head control, including the irrigation designer, the builder and the electrician.
Figure 4: Elevated irrigation equipment in the head control.
Figure 5: Pipe work installed in covered concrete trenches.
Rain water can be drained away or harvested. For instance, 750mm annual rainfall on a 1ha roof equals 7 500 000ℓ per year of gross irrigation water. This is not necessarily designed by the irrigation designer but must nevertheless be catered for.
Drainage within the greenhouse
Irrigation of 9mm/day with 33% leaching means that 3mm/day drainage or 30 000ℓ of leachate per hectare needs to be removed from the site or recycled each day. This aspect is crucial from the outset, as it involves both ground slopes and greenhouse gutter slopes. It requires a meeting of minds between the greenhouse designer, the land leveller, who will cut and fill and the irrigation designer.
The norms for the ground slopes are as follows:
- Parallel to the dripper lines (gutters) and crop row direction. (1,0% < slope < 2,0%)
- At right angle to the dripper lines (parallel to the gable side). (0,25% ≤ slope < 2,0%)
The irrigation design needs to fit in with the greenhouse design: inside and outside, which requires co-ordination between the irrigation designer and the greenhouse designer. Buried pipes, for instance, cannot be laid within 300mm of concrete foundations. Multiple dedicated mainlines outside the greenhouse need room to be laid, generally 300mm between mainlines.
Climate control design and planning are undertaken by the greenhouse designer, but the actual climate controller is often selected and installed by the irrigation designer.
Electrical power is supplied to irrigation and non-irrigation equipment. An integrated electrical supply system must be designed.
Irrigation equipment includes:
- Irrigation, service water and cooling systems.
- Fertiliser equipment.
- Automation and communication
Non-irrigation equipment includes:
- Greenhouse motors – fans, vents, curtains, screens, pad pumps.
- Lighting – head control, greenhouse, outside.
- General supply.
- Automation and communication.
This equipment shares an electric supply with common wire ways and cable sleeves. There must be a common surge and lightning protection programme. It is essential that different voltages are kept separate. For instance, sensor cables (extra low voltage), and particularly communication cables, should not share wire ways / sleeves with 230/400V (low voltage). It must be decided whether to use aerial or underground cables. This involves co-ordination between the irrigation designer, greenhouse designer, electrical supplier and the builder.
The heating layout should fit in with the irrigation layout, and involves co-ordination between the irrigation designer, the greenhouse designer and the heating contractor.
Irrigation pipes lie underneath roads. It must be planned which comes first, the pipe or the road? If the road, then sleeves will be necessary through which to lay pipe. If the pipe, then that pipe would need to be made of steel. The same would apply to cables: over or under roads? If under, which comes first: the cable sleeve or the road?
All parties and their equipment criss-cross each other in a confined space. Communication between the parties cannot take place spontaneously. A full-time project manager is therefore essential.
This paper is aimed at emphasising the need for thorough planning of a greenhouse irrigation system.
It has highlighted, from experience, certain technical aspects that have become pitfalls through inadequate planning. Not all aspects have been covered herein, however. A project manager needs to carefully consider each and every aspect for success in the long run.