Micro-irrigation at a glance
Systems and scheduling with drippers and micro-sprinklers.
This article is based on technical information compiled by irrigation specialist Karen van der Westhuizen from Agrimotion. Dr Johan van Zyl of the Department of Soil Science at Stellenbosch University provided additional inputs.
Micro is often used to refer to micro-sprinklers, but both drippers and micro-sprinklers are types of micro-irrigation. The aim is to reduce water use by controlling the wetted area while limiting run-off and deep drainage.
The hardware
Micro-sprinklers have two parts: one affects the delivery rate, and the other affects the spreading pattern. Both depend on system pressure. Micro-sprinkler systems are designed to accommodate 10%–15% pressure variation within a block, which still translates into a 10%–15% variation in water volume applied in the block.
Too-high pressures can cause misting or increase run-off, whereas too-low pressures lead to insufficient water delivery. Both will result in poor water distribution within the orchard.
Drip emitters are integral to drip lines. Modern drip emitters are pressure-compensated to deliver a constant water volume as long as system pressure remains within the tolerance of the dripper.
Drip lines drain after each irrigation event. Consequently, lower-lying orchard sections may become too wet, especially when irrigation events are scheduled to be shorter and more frequent — unnecessary pulsing should be avoided.
Drip irrigation systems operate at lower pressures and require smaller pumps and thinner pipes than micro-sprinkler systems. This can reduce installation and running costs for drip irrigation. Drip systems require less power per hectare irrigated but longer running times than micro-sprinkler systems — growers should consider the load-shedding interactions of the different systems in their own context.
If growers have access to or plan to install backup generators or alternative energy, drip irrigation systems could be attractive because they require less power. However, if growers depend on Eskom, they may find it challenging to complete their scheduled drip irrigation events during periods of frequent load-shedding.
The system design must factor in soil characteristics. Micro-sprinkler delivery rates must be based on soil infiltration rates. Too-high deliveries will only lead to run-off or deep drainage. Micro-sprinkler spacing is determined by tree spacing. High-density planting can pose the problem of either too many micro-sprinklers or too many tree trunks between them.
The number of dripper lines, delivery rates, and emitter spacing must be based on hydraulic conductivity — how water moves through the soil. Drippers need to be closer together and have a slower delivery in porous soils where lateral water movement is poor.
If a drip irrigation event is long enough to increase moisture in the subsoil, it should also increase soil moisture between drippers. If the soil between drippers remains dry, the dripper lines or drip emitters are too far apart.
When planning a new irrigation system, remember that mixing drip and micro-sprinkler systems on the same main line is a recipe for failure. The two systems require different approaches to scheduling and fertilisation — see below for more details.
The schedule
Micro-sprinklers deliver double-digit litres per hour, whereas drippers deliver single-digit litres per hour. The most common delivery rate for drippers is 1.6 litres per hour. Low-flow drippers emit less than 1.0 litre per hour. It follows that delivering the same amount of water takes longer with drippers than with micro-sprinklers.
Slower water delivery allows more time for infiltration and reduces run-off. Slower delivery can also reduce deep drainage because trees use water as it’s applied, and the irrigation system can be stopped before the water passes below the root zone. Drippers have the further benefit of minimising water lost to wind and evaporation during application.
Appropriate dripper spacing will apply water to only the root zone, whereas micro-sprinklers often apply water to areas beyond the root zone. Drip irrigation can create challenges for growers wishing to maintain cover crops in the work row during dry periods. On the other hand, micro-sprinkler run-off or irrigating too widely can result in soggy work rows, especially on heavier soils.
Irrigation with micro-sprinklers is usually scheduled to top up soil moisture to field capacity — the amount of water the soil can hold against gravity — once moisture levels have dropped to the refill point — the lowest acceptable soil-moisture level. Longer micro-sprinkler irrigation events increase wetted depth, whereas longer drip irrigation events increase wetted depth and width.
In practice, micro-sprinkler irrigation events tend to be days apart, and trees experience larger fluctuations in water availability. Irrigation with drip is usually scheduled to maintain more consistent soil moisture so trees are never exposed to a deficit. However, the soil under the drippers should not remain saturated for extended periods — roots need oxygen.
The menu
Fertigation is the application of fertiliser through the irrigation system. It is usually associated with drip but can also be done with micro-sprinkler systems. Drip fertigation is applied with each irrigation event or at weekly intervals. Micro-sprinkler fertigation tends to be done at weekly or longer intervals.
The advantage of fertigation is the steady provision of nutrients in the root zone, which should improve uptake and reduce waste. Uneven nutrient distribution due to drier pockets between drippers is a risk. Continuous fertigation requires sophisticated equipment and monitoring but lends itself to fine-tuning nutrient provision and can save labour.
Soil nutrient levels fluctuate more with the surface application of granular fertilisers than with fertigation because surface applications occur at far longer intervals. Granular fertiliser programmes are consequently less adaptable to real-time tree needs.
Granular fertilisers can be washed into the soil by micro-sprinklers or rain and are suitable for wet or dry periods. Fertigation during wet periods can lead to overwatering and may increase stress on cold, wet soils.
Managing soil pH and fertilisation in different zones within a block can be done with variable spreaders and granular fertilisers but not with fertigation. Growers may need to resort to spreading before rains if variable applications in drip-irrigated blocks are necessary. Some growers fertigate with calcium nitrate to combat soil acidification under drippers.
The management
Both drip and micro-sprinkler systems require maintenance. The risk of blockages due to poor water quality is greater with drippers, and regular line and filter maintenance is essential. Drip systems need chemical treatments to prevent blockages, whereas blocked micro-sprinklers must be manually cleaned or replaced. Blocked drippers can be harder to spot than blocked micro-sprinklers.
Dripper lines must be securely positioned — and re-positioned when displaced. Micro-sprinklers are less easily disturbed, provided they are suspended. Suspending dripper lines from wires risks the water running down and dripping from the lowest point on the wire. Dripping from a height also increases evaporative and dispersion losses.
Other advantages of drip are that orchard activities are not interrupted by irrigation events, and the spread of water isn’t affected by low-hanging branches or weeds.
The management of salinisation differs between drip and micro-sprinkler irrigation. Leaching salts so that they drain away is easier with micro-sprinklers. Keeping salts out of the root zone is easier with drip because water is applied more continuously. However, salts accumulate on the outer perimeter of the wet zone and can move back into the root zone during rain. Farmers in dry areas counter this problem by irrigating when it rains during the growing season.
Due to past failures, drip irrigation acquired a bad reputation among especially apple growers. But the adoption of drip is ticking up thanks to better technology, more experience, and increased pressure for greater water productivity.
Drip irrigation generally requires 20%–40% less water than micro-sprinkler irrigation, although mileage will vary for any given orchard and grower. A Hortgro-funded project assessing different irrigation systems, including low-flow drip, in a commercial apple orchard is currently in its second season — read more in the research summary in this issue.
Table 1 Some of the main differences between drip and micro-sprinkler irrigation
Drip irrigation | Micro-sprinkler irrigation | |
System | Pressure-compensated emitters are less affected by pressure fluctuations. | Delivery is pressure dependent and delivery rate more variable. |
Runs at lower pressures and flows. | Runs at higher pressures and flows. | |
Spacing of emitters and lines determined by hydraulic conductivity of soil. | Spacing of emitters determined by tree spacing. Delivery rate of emitters determined by water infiltration rate of soil. | |
Scheduling | Losses minimised because water is applied directly to soil. | Greater losses due to evaporation, dispersion, and when watering beyond the root zone. |
Easier to control run-off. | Harder to control run-off. | |
Easier to maintain soil moisture in soils with low water-holding capacity. | Harder to maintain soil moisture in soils with low water-holding capacity. | |
Easier to maintain zero-deficit soil-moisture levels. | Soil moisture fluctuates between refill point and field capacity. | |
Nutrition | Usually fertigation. | Usually granular fertiliser. |
Nutrient application directly to root zone should reduce waste. | Uniform nutrient application to planting row should result in uniform soil nutrient levels. | |
Harder to manage soil pH. | Easier to manage soil pH. | |
Easier to manage salinisation but harder to leach salts. | Easier to leach salts but harder to manage salinisation. | |
Nutrient application problematic when soils are wet and cold. | Granular fertilisers can be applied during wet periods. | |
Zone management is difficult. | Zone management is easier especially with variable spreaders. | |
General | Good for high-density plantings. | Less good for high-density plantings. |
Spread of water not blocked by vegetation. | Vegetation can block spread of water. | |
Difficult to maintain cover crop but easier to control weeds. | Easier to maintain cover crop but more difficult to control weeds. | |
Hard to spot blocked emitters. | Easy to spot blocked emitters. | |
Poor water quality can increase blockages. | Less chance of blockages due to poor water quality. | |
Dripper lines easily displaced. | Suspended micro-sprinklers not easily disturbed. |