Anticipatory irrigation is a practical, common-sense approach to watering crops more efficiently by working with rainfall, soil behaviour, and evaporation rather than against them. Instead of reacting too late or overwatering, this method focuses on getting water deep into the soil when conditions are right. The result is better use of small rainfalls, lower evaporation losses, healthier plants, and far more reliable irrigation scheduling using simple measurements and feedback.
What Is Anticipatory Irrigation?
Anticipatory irrigation is a simple method designed to achieve two main goals at the same time: making use of smaller rainfall events and minimising evaporation losses.
The basic aim is to get water deep into the soil profile where it is protected from evaporation and available to plant roots over a longer period.
Thresholds in Soil and Water
Just as there is a threshold level for runoff in dams, there is also a threshold amount of irrigation
water that must be applied before water penetrates into the deeper layers of soil.
The surface soil always has an insulating crust. This crust must be wetted out first, and all the water used to wet this surface layer will usually be lost to evaporation within a few hours.
Experienced irrigators know that enough water must be applied to fill the soil profile. Doing this extends the time between irrigations and reduces the repeated losses associated with wetting the surface again and again.
Why Timing Matters
What is less obvious is that the best time to irrigate is often just after a rainfall event. When rain has already wetted the soil surface, a much smaller volume of irrigation water is needed to push moisture deeper into the soil profile.
In this way, even small rainfalls become useful. Instead of evaporating quickly, they act as the first stage of filling the soil, allowing irrigation to complete the job efficiently.
Different Situations, Different Responses
There are times when rain is expected, but plants need water immediately. In these cases, the goal is not to fill the entire soil profile, but simply to apply enough water to meet short-term plant needs.
At other times, extreme heat may be forecast. In these conditions, it can be far better to irrigate ahead of time rather than irrigate during peak heat when evaporation losses are highest.
Common Sense Meets Measurement
All of this sounds like common sense, and it is. However, applying it consistently requires knowing how much water is needed to fill the soil profile.
The Limits of Soil Moisture Probes
Soil moisture probes are widely used to measure moisture levels, but they have two major limitations.
First, they only measure moisture content in the immediate area around the probe. Moisture varies widely throughout the root zone, so readings can change significantly depending on where the probe is placed. Experts attempt to position probes in an “average” location, but in practice this is far more difficult than it sounds.
The Bigger Problem: Wetted Volume
A more serious issue is knowing how much of the root zone has actually been wetted. Irrigation systems never apply water uniformly. Only part of the root zone is wetted at any one time.
This makes it extremely difficult to calculate total soil water content from a few sample points.
An Apparently Impossible Problem
This leaves us with what appears to be an impossible challenge: how do we calculate the total amount of water in the soil using only a small number of measurements?
The Simple Jar Analogy
The solution turns out to be surprisingly simple. Consider a jar filled with stones that is already partially filled with water. The problem of soil water measurement is exactly the same as this jar.
A water expert might be tempted to use probes, estimate the water between the stones, and calculate how much more water is needed to fill the jar.
The real solution is almost childlike: simply measure how much water is required to fill the jar. That measurement tells you exactly how much water was missing.
Applying This Idea to Irrigation
We apply the same idea to irrigation scheduling. First, we fill the soil profile and use soil moisture probes
to confirm when the profile is full. More specifically, we determine how much water must be applied
for moisture to reach the bottom of the root zone. We do not need to know how much water is in the soil at that point; we simply define that condition as “full”.
Letting the Plants Do the Measuring
Next, we allow the plants to use some water. Again, we may not know exactly how much water they have used. But we can measure it indirectly by refilling the soil profile.
The Practical Snag
There is one practical difficulty. It can take a long time for water to soak down to the base of the root zone. We cannot simply keep applying water until the profile is full, as this would introduce large errors.
Making an Educated Guess
The solution is to make an initial estimate. We guess how much water has been used by estimating a crop factor and multiplying it by evaporation. We then apply that estimated amount of water.
Refining the Estimate
We do not even need to start with a full soil profile. We begin with a guessed crop factor, apply the estimated water, and measure the irrigation depth. We then adjust the crop factor until, after applying the estimated water, the profile is again full. Once this is done, we know at any point how much water is needed to fill or partially fill the profile simply by looking at evaporation data.
Predictor–Corrector: Making Guessing Efficient
Guessing alone would be inefficient, but the process becomes very effective when combined with a mathematical technique known as a predictor–corrector method.
This method is built into a simple software program that continually refines estimates based on measured results.
Basic Theory
To manage irrigation effectively, we need to know two things: how much water plants are using, and the maximum allowable deficit in the soil.
These values are site-specific and must be learned through monitoring.
Learning From the Site
We cannot measure these values directly, but we can learn them over time by observing how the soil and plants respond. We start by making cautious estimates of crop factor and allowable deficit.
Daily Decision Making
Evaporation is measured, and the current deficit is estimated using evaporation and the current crop factor. This is compared with the allowable deficit to decide whether irrigation is needed.
Adjusting After Irrigation
After irrigation, soil moisture or irrigation depth is measured once water levels have stabilised. This information is then used to adjust the crop factor.
Stabilising the System
When the crop factor becomes stable, the onset of plant stress can be observed to determine the allowable deficit.
Why This Approach Works
Anticipatory irrigation avoids the trap of chasing exact soil moisture values. Instead, it focuses on refill amounts, timing, and feedback. The result is better use of rainfall, lower evaporation losses, healthier plants, and more reliable irrigation decisions.
Conclusion
Anticipatory irrigation combines practical observation with simple measurement and feedback. It recognises that timing matters as much as volume.
By filling the soil profile efficiently and adjusting decisions based on real outcomes, growers can reduce water waste, make better use of rainfall, and irrigate with confidence rather than guesswork.
