Water, electricity and labour. These are among the factors with the biggest impact on the success and profitability of an irrigation farm. On the downside, water use for irrigation is becoming more of an issue and electricity and labour costs are ever increasing. On the upside, the farmer is in a position to control the use of all three.
Eksteen Lindeque, agricultural engineer at MBB Consulting Engineers, says that the irrigation in the sugar industry has gone through various stages, from flood, to sprinkler, to pivot to drip. Each type has its advantages and place. Subsurface Drip Irrigation (SDI) has in recent times become a winner among the numerous options for the following reasons:
- simplified management and control;
- reduced water use;
- reduced electricity consumption;
- reduced labour requirements; and
- increased yields.
Important Planning Elements
Lindeque notes the following points to consider when planning a SDI system:
- Manageability: simplify the manageability of the system wherever possible.
- Ease of operation: the ideal system is a user friendly, simple system that can be operated easily, typically making use of appropriate low maintenance and proven technology.
- Efficiency: high efficiency is one of the biggest advantages of a SDI system. It requires less water and uses less electricity and fertilizers as a result. Due to its higher efficiency (95%) in comparison to other irrigation types, larger areas can be developed.
- Optimising electricity use: to optimize electricity use it is recommended that the development area be subdivided into pressure zones to arrive at standard pumps with high operating efficiencies. Variable speed drives (VSD’s) should be considered where feasible.
- Labour: reduce labour requirements for irrigation.
- Water quality: good water quality is essential for a SDI system. Proper design of the water abstraction system and a correctly sized filtration system are important to ensure good quality water is conveyed to the dripper lines.
- Soils: SDI systems should only be installed on well-drained soils. In addition, it should be remembered that dripper lines are installed subsurface, so soil types that are suitable to this type of installation should take preference.
- Scheduling: simplify irrigation scheduling practices to meet crop water demand as a result of different growth stages and climatic conditions. The use of soil moisture sensors is essential.
- Fertigation: simplify the application of fertilizers. The ideal is to apply fertilizers in the root zone through the system when needed by the plant.
- Security: use appropriate technology and equipment to minimize theft and vandalism.
- Maintenance: maintenance of pumps, filters and dripper lines are important elements to take into account in the design. The aim is to simplify maintenance and operation of the system and therefore selection of the appropriate infrastructure and features to simplify actions is important.
Eksteen says that a correctly designed SDI system puts the farmer in control and offers the option to do proper irrigation scheduling, which will prevent over and under irrigation. “Measuring the soil moisture in the root zone to determine crop water requirements for scheduling purposes is seen as key for optimised irrigation management and higher crop yields. Oxygen in the root zone and the preservation of fertilizers will further contribute to a healthy root system.”
Irrigation Master Plan
It is advisable to start with an irrigation master plan. Eksteen explains that the purpose of the master plan is to have a holistic look at the irrigation development, give the farmer a clear direction of where he is heading and also assist him with planning. The master plan should include the following:
- Demarcation of all development areas (present and future expansions).
- Evaluation of natural resources.
- Calculation of irrigation system demands and bulk water supply capacity.
- Water balance calculations to determine allowable development areas and storage requirements, based on available water rights and allocations, and irrigation system demand.
- Concept design of bulk water supply system and in-field irrigation layout.
- Indication of required electricity supply points.
- Indications of main haulage roads and waterways.
- Budget cost estimates.
Bulk Water Supply
The bulk water supply (BWS) system conveys irrigation bulk water from the water source to the irrigation block valves. It can consist of canals, balancing/storage dams, main/booster pump stations and bulk water mainlines. Eksteen says that the BWS system should be properly designed to optimize all of these for effective water reticulation, simplified management and reduced electricity use. Important elements to consider are the following:
- Pumps to abstract clean water where possible.
- Use of VSD’s switchgear to optimize electricity use.
- Facility to start and stop the pump/s using a cell phone (GSM type system where status signals can be received on the cell phone).
- Bulk water metre for the entire project to monitor water use.
- Mainlines optimally designed, with electricity cost in mind and protected against water hammer as a result of power failures.
- The design should provide for efficient draining and scouring facilities of all mainlines.
- Primary filtration (automatic back flush filters with a 100 micron filtration grade).
- Air valves strategically positioned to deal with air in the system.
Design Parameters and In-field Layout
The irrigation design parameters and in-field layout are key elements determining the final product, and are the result of a number of interacting variables, of which the following are the main drivers:
- Cane row spacing (suitable for harvesting equipment, optimum cane production, tramline or single row planting).
- Type of drippers available on the market (pressure compensated or not, in-line dripper spacing and dripper flow rate (ℓ/h)).
- Plant water requirements based on climatic conditions and yield target.
- Effective rooting depth.
- Soil type (soil moisture holding capacity, easily available water and lateral movement from dripper).
- Irrigation efficiency.
- Available daily hours to irrigate, taking power outages into account.
- Adequacy of bulk water supply and balancing dams.
- Use of existing infrastructure (balancing dams, canals and mainlines).
- Harvesting roads (maximum and ideal road lengths and widths, turn circles and slope).
- Plant direction of cane rows, normally at a downhill slope of 1% – 2% to drain surface water out of the field and improve efficiency of dripper line flushing.
- Maximum allowable dripper line lengths, based on dripper flow rate and spacing, and required minimum flush velocities at dripper line ends.
- Flushing requirements of dripper lines.
- Block sizes for harvesting purposes.
Eksteen says that the trend is to move towards pressure compensated drippers with lower flow rates of 1ℓ/h, 0.7ℓ/h or 0.6ℓ/h. “These have the advantage of greater horizontal soil moisture movement from the dripper, resulting in a wider possible dripper spacing, which in turn results in larger irrigation blocks and thus less valves to open and close during an irrigation cycle. It further results in longer dripper line lengths, which means less roads and larger possible production areas.”
He says the ideal is to have a one shift irrigation system, meaning irrigation scheduling is done by only starting and stopping the pump/s. “It should be noted that this does not mean there is only one irrigation block. There can be many blocks with varying sizes, determined by farming practices, soil types, or topography, but all of them will be open. This allows flexibility in terms of irrigation standing time and fertigation of individual blocks.”
Irrigation block valves are typically grouped together in a cluster, either in the pump station, or in a cluster building. The cluster building usually contains the following:
- Secondary filters (100 micron filtration grade) if located far from the primary filters, which are usually located at the pump station.
- Pressure reducing valves for block inlet pressure control.
- Water metres, one per block, for management and monitoring purposes.
- Butterfly isolating valves, one per block.
- Anti-vacuum valves, one per block.
Sub-mainlines are installed from the valve clusters to the irrigation blocks and each block has a dedicated sub-mainline. Eksteen says that although this complicates installation, since it requires more pipelines, it simplifies management because control is done from a central point. “Automation, if opted for, is also easier, since all the valves are grouped together, which eliminates the need for expensive radio controlled field units.”
He adds that fertigation and drip irrigation goes hand in hand. “It is another advantage of SDI, since fertilizer is applied in the root zone for easier uptake by the plant. There are many options available when it comes to fertigation, ranging from a simple system where fertilizer is mixed in one tank and gravity fed into the pump suction pipe, to a complex system with holding and mixing tanks, fertilizer pumps and electronic fertilizer injection controllers. Ultimately the decision lies with the farmer. The preferred system should fit in with his farming practices and management style.”
Eksteen says a facility to inject hydrogen peroxide (cleaning) and treflan (prevention of root intrusion) into the system is also important to maintain the integrity of the filters and drippers, and should be used as a preventative measure against clogging of the drippers.
Also a SDI system has many advantages, Eksteen says. “However it must be properly planned and designed, correctly installed and properly maintained to ensure its success. If all of these are adhered to a SDI system will simplify irrigation management on a sugar cane farm considerably. It will also increase yields and address the three important factors of water use, electricity consumption and labour costs, thereby achieving the ultimate objective, which is more money in the pocket of the farmer.”
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