Making informed irrigation scheduling decisions

Water Balance

Soil water content can be described on three levels. The soil can either be saturated, reach field capacity (FC) or reach a permanent wilting point (PWP). Between field capacity and the permanent wilting point lies plant available water capacity (PAWC) - the water available for plant growth.

Broadly speaking, there are two approaches to open-field irrigation, for example. Full-surface wetting, as used with pivots or open-field sprinklers, or approaches used with methods such as drip irrigation where a smaller percentage of the soil area is wetted. In the case of full-surface wetting, a farmer would allow water extraction of up to 50% of field capacity before refilling the water reservoir. When only 40 – 80% of the soil area is wetted and water is applied with higher efficiency, the farmer can adjust his approach based on the water demand of the crop at certain growth stages. The most common approach would be to allow between 20 - 30% extraction before refilling the reservoir. This allows better soil-water management as the soil status is always kept close to the ideal soil-water balance. The result will be a better-managed root zone and a happier plant.

Efficient water use is about preventing water loss on the one hand and ensuring optimal water intake by the roots on the other hand. Water loss can occur through evaporation, deep percolation, subsurface outflow, run-off, and transpiration. The latter is the only positive water loss as water moves through the plant.

The main concern is with evaporation and deep percolation. If these two culprits can be limited, we can ensure highly efficient water use. Evaporation is limited by wetting a smaller soil area and deep percolation is limited with proper irrigation scheduling through which we can ensure we do not irrigate beyond the active root zone.

Irrigation Scheduling

Your knowledge of soil composition and attributes, how water moves through the soil and the size of the root zone is critical to proper irrigation scheduling.

A long list of factors influences scheduling calculations:

• Plant spacing
• Row spacing
• Continuously updated data of all variables affecting evapotranspiration (ETo)
• Soil moisture levels
• Crop type (Will determine the crop coefficient)
• Dripper/Sprinkler spacing
• Flow rate/Delivery rate of drippers/sprinklers
• Rainfall (Calculation of water penetration to the active root zone)
• Soil type
• Age, health and growth stage of plants
• Size of the active root zone

The three most important questions to answer are:

• For how long should I initially irrigate to ensure that I start with a wet soil profile?
• How should I irrigate in the initial crop stages while roots are shallow?
• How should I irrigate in later stages when the root zone is well-established?

Remember, our aim is to not irrigate past the active root zone, while ensuring sufficient water availability.

Digging for knowledge

As mentioned, it is extremely important to know your soil. Without knowledge about how water moves through the soil in question, all the calculations in the world will be futile. This is why we must get our hands dirty. It is necessary to dig a profile pit, and/or do a water distribution test, to determine how long it will take to fill the entire soil profile. You must know your soil so well that you know exactly what two hours, three hours or four hours’ worth of irrigation with your system will do. Click here to read more about, profile pits, soil water distribution tests and irrigation scheduling.

System Application Rate

The application rate of an irrigation system is determined by the emitter flow rate and spacing. For a drip irrigation system, for example, the application rate can be determined by the following calculation:

Dripper Flow Rate ÷ Dripline Spacing ÷ Dripper Spacing

Consider a system with a 1 ℓ/h flow rate, 1,5 meter dripline spacing and 0.3 m dripper spacing:

1 ℓ/h ÷ 1,5 m ÷ 0.3 m

= 2,22 mm/h

As soon as the application rate is calculated, it can be combined with the water demand calculations described below and expert soil and crop knowledge to determine the necessary irrigation duration and plan irrigation scheduling.

Water Demand

It is not only important that we understand the intricacies of soil water balance, but also the factors that determine crop water demand.

Consider the following calculation:

ETo x Kc = ETc

Be sure to source accurate and relevant ETo data. ETo values will differ from day to day and updated values must be used for calculations. Experts suggest that we use weekly ETo predictions to plan an irrigation schedule for the week ahead. Remember, this is a guideline and you must also measure soil moisture and other factors determine the best irrigation schedule for the circumstances.

The Kc value, or crop coefficient, is a fixed value for each crop type that represents the ratio of evapotranspiration of the crop in question compared to that observed for a reference crop under the same conditions.

The ETc value represents predicted crop evapotranspiration. Simply put, the ETc value suggests how much water the plant is likely to use. This is the value is used for accurate irrigation scheduling.

Take note that the crop coefficient (Kc) will differ for the same crop between different phenological stages, which will of course, impact the water demand. The table below is based on data collected over five years from a tomato field in Komatipoort. It explains differences in water demand during different phenological stages.

Doing the math

Let’s consider ETc calculations for the vegetative growth staged showed in the table above. The crop crop coefficient (Kc) is 0.4 and the ETo is 4,8 mm per day.

ETc = ETo x KC

       = 4,8 x 0,4

       = 1,92 mm/day

The predicted crop water demand is therefore 1,92 mm per day.

Weekly Irrigation Planning

As mentioned, it is recommended to use ETo predictions for the week ahead to calculate daily ETc when planning irrigation. If you add up each day’s ETc, you can determine the predicted water demand of the crop for the full week. This will allow you to calculate the necessary active irrigation hours for the week.

For illustration purposes, let’s imagine the ETo, and as a result the ETc, have remained constant for seven days.

Using the ETc value calculated above, we can calculate:

Predicted weekly water demand: 13,44 mm/week (1,92 mm/day x 7 days)           

Required active irrigation duration: 13,44 mm ÷ 2,22 mm/h (application rate) = 6.1 hours

The crop requires 6 hours of active irrigation during the week in question. These active irrigation hours must be scheduled in the most efficient way possible. Remember the goal – to not irrigate past the active root zone, while ensuring sufficient water availability.  Now, your knowledge of the soil, how water moves through it and the size of the root zone becomes important to determine how the six hours must be distributed throughout the week. Will it be better to have shorter and more frequent irrigation sessions, or longer and less frequent sessions? Will you irrigate for two hours every second day, or three hours only twice per week? If you know how long it takes for the active root zone to be filled, and how fast water extraction takes place – it is easy to decide. With the necessary knowledge, the right calculations and by involving the right experts you can make an informed decision to ensure that the crop receives optimal irrigation while you make extremely efficient use of the available water.

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