Anticipatory irrigation is a practical and commonsense way to use water better by working with rainfall, evaporation, and soil behaviour rather than fighting them. This article explains how irrigation can be timed and measured so water is driven deep into the soil where it is protected from evaporation. By focusing on irrigation depth, learning from site measurements, and using simple adaptive methods, growers can reduce water losses, improve plant performance, and make far better use of small and unpredictable rain events.
Anticipatory irrigation
Anticipatory irrigation is a simple but powerful way of achieving two important goals at the same time: making use of smaller rainfalls and minimising evaporation losses. The central idea is to get water deep into the soil where it is protected from rapid evaporation and can be accessed by plant roots over a longer period.
Just as there is a minimum threshold of rainfall required before water runs off into a dam, there is also a threshold of irrigation water that must be applied before water penetrates into the deeper soil layers. Below this threshold, water only wets the surface and is quickly lost.
There is always an insulating crust at the soil surface that must be wetted first. All the water used to wet this crust is normally lost to evaporation within a few hours. Experienced irrigators understand that they must apply enough water to fill the soil profile. This extends the time between irrigations and reduces these repeated surface losses.
What is less obvious is that one of the best times to irrigate is just after rainfall. When the surface is already wet, a much smaller volume of irrigation water is needed to fill the soil profile. This allows growers to take advantage of rain events that would otherwise be wasted through evaporation.
There are times when rain is expected but plants still need water immediately. In these situations the aim is to apply just enough water to satisfy short-term plant needs without filling the entire profile. At other times, extreme heat may be forecast. In these cases it is better to irrigate ahead of time rather than apply water under conditions of very high evaporation.
All of this is largely common sense. The difficulty lies not in understanding the ideas, but in knowing how much water is needed to achieve the desired outcome.
Why soil moisture alone is not enough
Soil moisture probes are widely used to measure soil water content, but they have two major limitations. First, they only measure moisture in a small region around the probe. Moisture levels vary widely throughout the root zone, so readings depend heavily on probe placement.
Even experts struggle to position probes in a truly representative location. What appears to be an “average” position often turns out to be misleading once root distribution and wetting patterns are considered.
A more serious problem is the issue of wetted volume. Irrigation systems never apply water uniformly. Drip systems wet small volumes, sprinklers wet unevenly, and flood irrigation produces highly variable patterns. This means we rarely know how much of the root zone has actually been wetted.
This leads to what seems like an impossible problem: how do we calculate the total amount of water in the soil from a few scattered measurements?
A simple way of thinking about the problem
The solution is surprisingly simple. Consider a jar filled with stones that is already partially full of water. One approach would be to try to measure how much water is present, estimate the empty spaces between the stones, and then calculate how much water is needed to fill the jar.
A far simpler approach is to just pour water into the jar until it is full and measure how much water was added. That tells you exactly how much water was required.
The same idea can be applied to soil. We fill the soil profile with water and use sensors to detect when water reaches the base of the root zone. We do not need to know how much water is in the soil, only that it is “full” for practical purposes.
We then allow the plant to use some water. Again, we may not know exactly how much water has been used, but we can determine it by refilling the profile and measuring how much water is required.
There is a practical complication. Water takes time to move down through the soil, so we cannot simply keep adding water until the profile is full. That would lead to large errors.
Learning by adjustment
The way around this is to make an initial estimate of how much water has been used. This can be done by guessing a crop factor and multiplying it by measured evaporation. We apply that amount of water and then measure how deep the water has penetrated.
If the water does not reach the desired depth, the estimate was too low. If it goes too deep, the estimate was too high. By adjusting the crop factor and repeating the process, we quickly converge on the correct value.
This approach does not require starting with a full soil profile. We simply guess, measure, and adjust. Over time the system becomes accurate and reliable.
While guessing may sound crude, the process can be made very efficient using a mathematical technique known as a predictor–corrector method. This approach is built into simple software tools designed for irrigation planning.
Basic theory behind the method
Effective irrigation scheduling requires two key pieces of information: how much water plants are using and how much water the soil can hold before plants begin to suffer stress.
These values are site specific. They depend on soil type, root depth, crop type, irrigation method, and local climate. They cannot be taken reliably from books or tables.
Instead, we learn them by monitoring the site over time. We begin with conservative estimates and refine them as data is collected.
Evaporation is measured daily. Using the current crop factor, we estimate the soil water deficit and compare it to the allowable deficit. This tells us whether irrigation is required.
After irrigation, we measure either soil moisture or irrigation depth and use this information to adjust the crop factor. When the crop factor stabilises, we can determine the allowable deficit by observing when plants begin to show signs of stress.
Using software to manage the process
In practice, software simplifies this process. Weather and irrigation data are entered, including evaporation, rainfall, and irrigation volumes. Predicted evaporation is replaced with measured values to maintain accuracy.
The system tracks water use by block. A full soil profile is treated as zero deficit. Negative values indicate how much water is needed to refill the profile. When a threshold is reached, the system indicates that irrigation is required.
Predicted irrigation dates are displayed, allowing growers to consider weather forecasts and decide when and where to irrigate. After irrigation, soil moisture or irrigation depth is measured once the water has stabilised.
Crop factors are then recalculated. Viewing how the crop factor changes over time helps confirm that the system is behaving sensibly. Once confirmed, the updated value is recorded and used for future scheduling.
This process is repeated regularly. Over time, the system learns the true behaviour of the crop and soil without wasting water or damaging plants.
Managing salinity and water availability
When irrigation water is saline, applying only enough water to refill the profile can lead to salt accumulation. In these cases, occasional flushing irrigations may be required.
Irrigation scheduling is not only about plant needs. It is also about managing limited water supplies across a season. Annual water use must be monitored and balanced against availability.
Anticipatory irrigation helps by reducing unnecessary losses and making better use of rainfall. It allows growers to stretch limited supplies while maintaining productivity.
Learning in the real world
The key to this approach is learning from the site itself. Rather than relying on fixed rules or book values, growers observe, measure, and adjust.
Over time, the correct crop factor and water holding capacity emerge naturally. The result is an irrigation system that responds to weather, soil, and plant behaviour in a balanced and efficient way.
Anticipatory irrigation is not complicated technology. It is a structured way of applying common sense, supported by simple measurements and gradual learning.
By understanding thresholds, irrigation depth, and evaporation, growers can dramatically reduce water waste while improving plant health and resilience.
