By Rivulis
The challenge of optimizing water use and yield in horticulture is something that growers contend with every season. When deciding on their irrigation strategy, a traditional approach taken when growing plants with drip systems has been to use emitter spacings between 20 and 30 cm (8 to 12”), no matter what the crop.
Grower experience and recent research have shown, however, that one size does not fit all, and that there is a better approach when it comes to using drip irrigation for germinating seeds and for crops that are closely planted, e.g. 10 to 15 cm (4 to 6”) apart.
In this article, we examine important factors for growers to consider to maximize production and use irrigation water as efficiently as possible. We look at three specific situations:
- when plant spacing is denser than every 20 cm
- when a solid wetted band is needed to germinate seeds
- when there is a risk of deep drainage occurring with irrigation applications of longer duration.
In these situations, which often apply to crops such as onions, carrots, strawberries, celery, garlic and lettuce, it is more efficient to use drip tape with emitter spacings of 15 cm (6”) or less, with the key factors to consider, in combination, being dripper flow rate, wetted zone and soil type.
Dripper flow rate
A study conducted to evaluate the time taken for individual wetted zones to merge for different emitter spacings and flow rate configurations showed that a combination of lower emitter flow rates and closer emitter spacings can result in wetted zones merging up to many hours faster than comparable products with wider spaced emitters and higher flow rates (see Figure 1).
Figure 1. The effect of emitter spacing and flow rate on how quickly wetted zones merge. In this example, for 0.5 L/hr emitters at 15 cm spacings, it took six hours for the wetted zones to merge, while for emitters at 30 cm spacings, it took 10 hours.
The implication of these results is that drip tape with close emitter spacings will lead to an overall reduction in the duration of irrigation events needed to create a full band of moisture suitable for germinating seeds and plants sown every 20 cm (or less). For growers, this means that:
- irrigation events will be shorter
- the risk of deep drainage of water will be reduced
- seedlings will germinate more evenly, which will improve the uniformity of the crop at harvest
- multiple irrigation applications during the germination period will ensure soil surface remains friable minimizing the risk of surface crusting.
Of course, agronomic factors as well as production aims need to be considered when choosing emitter spacing for a particular drip tape flow rate. Key agronomic factors include:
- infiltration rate of the soil
- soil wettability (hydrophobic characteristics)
- required width of spread
- required depth of moisture
- depth of installed drip tape.
As with most things in horticulture, there is no one-size-fits-all answer, rather solutions must be tailored to suit each situation. For example, in soils where there is a wettability issue (hydrophobic soil) and the requirement is for deep irrigation, low flow rate or ultra-low flow emitters may work well as they will give the soil time absorb the water. In this case, longer irrigation cycles will be needed to allow the required depth of irrigation to be reached.
Soil type
It is no secret that soil type affects the pattern of the wetted zone and how deep irrigation water will travel (see Figure 2). In general, clay soil will have a wetted zone that spreads wide and is relatively shallow in the profile, sandy soils will have a wetted zone that is more restricted but much deeper in the profile, and loams will be somewhere in between these extremes.
This is why soil type is a key factor when designing an irrigation system and choosing emitter spacings, flow rates and how long irrigation water is applied for.
Figure 2. The influence of soil type on the pattern of the wetted soil zone.
Case study: Germinating carrot seeds in clay soil
Situation. A carrot grower converted from overhead to subsurface drip tape with a key aim being to achieve an even strike of seed by ensuring good lateral spread of irrigation water. The tape chosen to meet this requirement had an emitter application rate of 0.25 L (0.07 gal)/hr with emitters spaced at 15cm (6”).
Results. According to the grower the close spaced drip tape resulted in less soil surface crusting during the warmer months improving germination rate. As well, water use efficiency was better than with overhead irrigation, there was less inter-row weed management and carrots harvested were more even in size.
The wetted zone
For good, uniform germination, even distribution of water is critical, and for crops that are closely planted or where seed germination is required, a continuous wetted zone along the bed is essential.
Applying this knowledge to choosing dripper spacings means that one size does not fit all, rather the decision must be based on emitter spacing, soil type and wetted pattern at the surface of the seedbed.
In the above image, the green arrows represent plants. The wetted zone of each emitter is clearly visible and the potential for dry soil zones in the bed are shown as red arrows. In this situation, up to 50% of the plants can be impacted by having some of their effective root system in dry or drier than average soil. This can result in variability in plant growth and negatively impact crop productivity.
The point about avoiding deep drainage is a critical one. While some growers might believe that it is possible to achieve a continuous wetted zone with wider spaced emitters simply by increasing the length of irrigation events, the risk is that this will result in deep drainage of irrigation water.
Research by Rohan Prince at Western Australia’s Department of Primary Industries and Regional Development (2016) confirmed this effect by injecting die into drip line with different emitters and analyzing the subsequent cross-section of the soil.
In the above test, you can see in Application 2 that using higher flow drippers and wider spacing between drippers only results in a 5 cm (2”) wider spread of water, despite applying four times the volume of water.
Wider spaced drippers that are traditionally used in horticulture can result in applying a lot more water and fertilizer than is necessary. With closer drip spacings, less water and fertilizer can be applied with the better results.
Case study: Dual row strawberries on sandy soil with a clay loam subsoil
Situation. A strawberry grower established several varieties in dual rows with a single row of drip tape between. There were six plants per metre (six plants per 39”) and plants were placed 16 cm (6.3”) on either side of the tape. The tape chosen to meet this requirement had an emitter application rate of 0. 5 L (0.13 gal)/hr with emitters spaced at 15 cm (6”).
Key considerations for this arrangement were to ensure the full bed would evenly wet up and that peak water demand would be met. Another consideration was that irrigating after the first pick then letting the profile dry down before the next pick enabled the grower to maximise fruit quality.
Results. The combination of close-spaced emitters and plant configuration ensured that the full bed was evenly and effectively wet up after each three to four day irrigation cycle. The flow rate of 0.5 L/hr enabled lateral movement of water to maintain a full width bed of moisture and avoided deep irrigations, which could have caused water to build up on the subsoil loam layer. As a result the risk of nutrient leaching and EC building up was reduced.
The bottom line
While there are many variables that growers cannot control, there are some that can be which will contribute significantly to optimizing crop quality and yield and ensuring efficient water use. On-farm practice and research have shown that optimizing drip emitter spacings, with careful consideration of factors such as soil type, ensuring an even wetted zone and emitter flow rate can make the difference between an average crop and an excellent one.
