Which emitter flow rate and spacing should I choose for my field?

One of the most frequently asked questions in terms of drip irrigation systems design, is how should the driplines be located along with the plantation and which model of dripline regarding the flow rate and spacing of the emitters (or drippers).

In many cases, the selection of the dripline model is conditioned by what the neighbors have got for their plots, or what is most commonly used in the area as per installation material availability. But it is important to highlight that:

"The proper dripline layout selection will determine the success of the crop's productivity".

The main target is to supply the crop water requirements ensuring that the maximum yield will be obtained if any other limiting factor is not missed; while achieving the most competitive irrigation systems according to the initial installation costs and to the operational expenses (energy, maintenance, labor...) of the system.

As important as the above points, is to know which crop will the irrigation system be designed for, as the root system of lettuce crop is completely different from a banana one.

By knowing the above, the objective depth that the wet-bulb shall reach can be determined according to the combination of those elements: plant water requirements, effective root depth, soil texture. This point is crucial to ease the plants' water uptake while avoiding in-depth water losses (deep percolation) and plant hydric stress.

Those are generally known under the Agronomic Design name: to know the flow rate, the number of emitters (per plant) and their spacing within the irrigation installation, according to the irrigation water requirements, soil texture and targeted soil depth.

• Emitter flow rate
• Emitter spacing
• The number of emitters (per plant, per meter...)
• Irrigation application time

It is critical when performing an irrigation application design, to select the right emitter flow and spacing, in order to achieve a continuous wet stripe along the crop row, applicable, in general, to all kind of open-field agriculture. This is a must, in order to provide an optimum wet volume in the soil, and consequently, continuous moisture and water supply to the crop’s roots.

The configuration of the emiters (number, spacing and flow rate) should be able to supply the crop water requirements while ensuring a minimum wet area wihtin the plant shaded area.

The wetted volume within the soil is primarily depending on the soil texture, emitter flow rate & application time. By knowing the shape and size of the wet bulb for the selected emitter in a certain soil, we can make the following calculations.

The most convenient way to determine the wet bulb area is to perform an on-field test! For many occasions, that won't be possible, so, fortunately, there are some other ways, as estimative equations such as the following:

DRIPPERS' LAYOUT

1.- How many emitters?

The sum of all the emitters multiplied by the flow rate of each one, and by the irrigation time, shall meet the Crop Water Requirements during the peak water demand of the crop.

One of the advantages of drip irrigation systems is the water savings by partially wetting the planted area with precisely the required amount of water. Then, the number of emitters per plant, can be reduced if the flow of each emitter is augmented (so saving in installation cost), but we need to ensure that a minimum of the crop area (where roots are located) is being wetted for optimal crop development.

On the other hand, the wider spreading of the wetted area, the larger the wet volume within the soil that the roots are able to explore, which will consequently bear in more canopy growth. But, as said before, there must be a minimum wetted soil percentage in relation to the total cropped area (planting distance), for an adequate crop growing. These depend on the crop type and climate, and is it know as "Percentage of wetted area".

The minimum % area (related to the horizontal wetted area at the effective crops' root depth [10-15 cm (horticulture) and 30-35 cm (fruit/nut trees)] for each conditioning, are:

1. Horticultural/field crops:

• For any climatic region >85%

2. Fruit/Nut crops:

• High rainfall areas (tropical / sub-tropical climates): >20%
• Low rainfall areas (arid / semi-arid climates): >33% (50-60% advisable)

Then, according to the wetted area of the given emitter, the calculations can be made to determine the minimum number of emitters by dividing those.

Minimum number of emitters per plant = Crop spacing (m2) * Min percentage of wetted are / wetted area by selected emitter (m2)

2.- Which flow?

There are different emitter models regarding the flow rate, depending on the manufacturer. In regards to the type of emitter discharge, the flow can vary depending on the pressure (turbulent flow) or supplying a constant flow regardless of the pressure (known as PC emitters: pressure-compensating. See here how it works). But we'll carefully review this matter in further articles.

As above pointed, the emitter flow rate is one of the factors that determine the shape of the wetted bulb. For a given soil characteristic and irrigation time, the bigger the flow, the greater the saturated soil volume. There are many options available of emitter flow rate, but those regarding integrated emitters usually bear from 1 to 4 l/h (check it here for PREMIER PC models).

3.- Which spacing?

When approaching the irrigation design, it must be ensured a continuous wet strip along the crop row. For that, at the targeted soil depth, the emitter max distance shall be that one provided by the wetted bulb diameter at that depth. Using the previous equation for wetted diameter calculation, the distance can be estimated according to each soil texture and flow rate.

Important that the overlapping shall be created around the targeted soil depth (where highest root concentration is). To get a minimum of 10% of overlapping would be advisable.

The wet bulb diameter can be used for the optimal determiantion of the emitter spacing within the irrigation layout.

Hence, what should we consider for the right emitter layout selection? Is it better to increase the flow rate by distancing more the emitter spacing, or reduce both flow and spacing?

The higher the number of emitters per crop, the more guaranteed water supply is and more water spread over the crop area, but the installation cost will increase as well; and vice versa. So a balance between spacing, flow and number of emitters shall be achieved for an optimal purchase cost, performance, and matching the crop needs and client requirements.

Always ensuring that the configuration is able to supply the Crop Water Requirements within a reasonable application time (3 hours is a good number for fruit/nut trees, indeed) to avoid deep percolation water and nutrients loss and extra energy expenses.

For further technical advice within your upcoming irrigation design, PM me or freely?contact us here! ??♂?