How long and how often should we irrigate?

As important as knowing "How crops should be watered", it is to determine when, for how long (application time) and how much land should be irrigated at the same time.

The starting point here is, again, the Gross Irrigation Water Requirements parameter (GIWR), which can be easily calculated, as explained in previous articles, depending on the local climatic conditions of the irrigation project in hands, type of crop & growth stage, and expected efficiency of the selected irrigation systems. Remember that the GIWR is always a number expressed in millimeters per day (equivalent to l/m2 per day), week, decade or month...

Once we have determined how much water the plant need to "drink", the way we are feeding the roots is thereafter selected, by choosing the dripline layout: emitter's discharge rate, spacing and number of driplines per crop row, always bearing in mind the principles of wet-bulb shape, % of the wetted area, the minimum number of emitters and overlapping (remember those concepts here).

The irrigation system needs to be able to supply that maximum GIRW value within a certain irrigation time.

Let’s illustrate this with an example. Imagine that we need to supply 30 liters per plant each day at the beginning of the irrigation season, and 80 l/pl/day during the most demanded period of the Summer. It will be determinant that the system has been capacitated to supply a maximum of 80 l per plant and per day, hence the plants won't suffer hydric stress anytime during its growing period, avoiding the consequent depletion in commercial yield.

IRRIGATION WATER APPLICATION RATE

The planting distance must be known (distance between each consecutive plant, and distance between each row) while the emitter disposition (spacing, flow rate, and the number of driplines per row) -for a dripline model available on the market- need to be selected according to the crop water needs, and the overlapping and wetting pattern criteria) for a given soil, local weather, and type of crop.

Once the above is determined, use the following equation to calculate the application rate:

Several estimative calculations will be done: changing the flow, spacing and number of driplines, to finally select the most appropriate solution.

Let's imagine now a ?? Lemon orchard where trees are planted every 4 m within the row and 6m between consecutive rows, in a loamy soil. The GIWR at the most demanded period is 4.1 mm per day = 4.1 l/m2 per day, we need to be able to supply that amount of water by using:

GIWR (max) = 4.1 l/m2 * 6 * 4 = 94.7 liters per plant

We get many different combinations of emitter rate, spacing...., for example:

......etc..........etc........but, how long should we irrigate our crops for?

APPLICATION TIME

How long should the irrigation system be running to supply the required water volume to the crop?

If we have selected low emitters' flow rate and a little number of emitters (per plant <> per m2), the irrigation time required to supply all the volume of water to the crop could be quite high. This could lead to an increase in the operational costs (energy of running the pump for longer) and the impossibility to irrigate the whole area of our plot within a single day.

It is advisable to set the irrigation application time -at the design stage- within a certain range of time, typically around 3-4 hours for permanent crops, and below 3 hours, but this may change. The point is, it is important to optimize the irrigation operational cost ($/ha) but it will finally depend on how the irrigation system is being operated!

The final number of irrigation blocks will depend on the selected application time.

We could consider the following alternatives to adapt the irrigation time to our target:

? Increase the number of emitters (shortening the emitter's spacing), and/or;

? Select a higher emitter's flow rate;

? Set an extra dripline in parallel for the same crop row.

Again, always bearing in mind that the continuous wet strip will be formed along the dripline and certain overlapping will be achieved!

In the previous example:

? x1 dripline per crop row: 1.6 l/h at 0.75 m ---- ? 11 h (3.7 h with 3 driplines per row)

? x1 dripline per crop row: 3.0 l/h at 1.00 m ---- ? 8 h (4 h with 2 driplines per row)

? x1 dripline per crop row: 2.3 l/h at 0.60 m ---- ? 4.7 h (2.4 h with 2 driplines per row)

IRRIGATION EVENTS FREQUENCY

Finally, the designed irrigation system will be able to supply the required volume of water in amount, quality, and time, while being as cost-effective as possible.

Remember that the goal of drip irrigation is to supply the water the crop has uptaken the previous day. It is possible to make the design of the system with the capacity to supply the water consumed in the previous day, two days before, three, etc.

As a design principle, we must suppose that we will irrigate every day as if it was the most demanded period.

That's why it's best to consider daily irrigation frequency during the most water demanded period.

All the above calculations can be automatically done with a simple excel sheet, so the adequate selection of the dripline disposition is eased. We have developed our own calculation program, which can be freely requested to us here!

Now, the next step is to get all these calculations done and be ready to start with the hydraulic part of the design process! Stay tuned! ??

? For any technical advice or support, feel free to contact us directly, contact us here!