Irrigation scheduling is one of the cheapest ways to save water and improve plant growth. It is not just about avoiding wilt. It is about managing how a plant grows by controlling how much water is available, and where it is available in the root zone. This guide explains single (deep) cycle and dual cycle irrigation, and a practical “self-learning” method to measure true crop factor and soil water holding capacity for your specific site using evaporation, rainfall, water applied, and simple soil measurements.
Irrigation scheduling: Manage the water, manage the plant
Irrigation scheduling is the most cost effective way of making more effective use of water. It is more than simply ensuring the plant has enough water. It is managing the growth characteristics of plants by manipulating the availability of water.
Vegetative growth (leaf and shoot growth) or reproductive growth (fruit production) can be stimulated by applying surplus water or deficit water to selective sections of the root zone. To do this well, it is essential to know exactly how much water the plant is using.
Many growers use traditional crop factors and evaporation as a guide. These values can help, but actual water use varies significantly from site to site. It changes with the stage of growth of the plant, row spacing or crop density, terrain, and local exposure (sun and wind). That is why this approach focuses on measuring the true water use for a particular site, not relying only on generic values.
Why “true crop factor” matters more than book values
To irrigate correctly we need two things:
- How much water the plant is actually using at your site.
- The effective water holding capacity of the soil for that crop and irrigation method.
Literature values provide a useful starting point. The problem is that every farm is different: topography, slope, north or south aspect, crop density, plant size, and soil type. Even within one farm, values change through the season as the crop develops.
The practical solution is “adaptive” or “self-learning” scheduling. This measures evaporation, rainfall, water applied, and either soil moisture or irrigation depth. The information is analysed and the crop factor is corrected. As the plant grows, the crop factor is continuously updated.
Single (deep) cycle irrigation: The classic approach
Classic irrigation scheduling uses a single cycle (also called deep cycle) principle. In this approach:
- Water holding capacity is estimated using field capacity and a refill point above the wilt point.
- The soil is allowed to dry down to a refill point.
- Enough water is then applied to refill the profile.
Single cycle scheduling does not give optimum growth for many crops. It is mainly recommended for shallow rooted plants such as many vegetables, where managing the top zone is the main priority.
Dual cycle irrigation: Higher production and better water efficiency
Dual cycle irrigation is recommended for deeper rooted plants. It tends to give higher production and is more water efficient when done correctly. The idea is simple:
- Apply a deeper irrigation at intervals to wet the entire active root zone (but not beyond it).
- Between those deep irrigations, apply a series of smaller irrigations to keep the upper soil moist.
Soils dry down progressively from the surface. Dual cycle maintains both shallow and deeper moisture levels without leaving the soil saturated for long periods.
Many plants have two functional root systems. In the nutrient-rich upper layer they develop fine feeder roots. When this upper layer is moist, the plant can extract the food and water it needs and will flourish. If the upper layer dries out, the plant relies on survival roots deeper down. It may survive, but it often grows slowly because key nutrients (such as phosphorus and calcium) are less available deeper in the profile.
How dual cycle works in practice
Dual cycle can be thought of as a repeating pattern:
- A deep irrigation is applied to fully wet out the entire root zone. No water should pass beyond the roots.
- The plant uses water from the upper zone first, where feeder roots and nutrients are concentrated.
- As the upper zone dries, the plant begins to take more water from deeper in the soil.
- A shallow irrigation is applied to re-wet the upper layer only.
- After several shallow irrigations, both upper and lower layers begin to get dry.
- This is the time for the next deep irrigation to refill the whole profile.
An additional benefit is salt management. There is always some salt in irrigation water. In dual cycle, soil below the root zone is allowed to dry out and form a “space” that can later be used for salt flushing. At some point, a flushing irrigation will be needed to move accumulated salt beyond the plant’s active roots.
I-Planner: A practical way to apply the method
A software program called I-Planner was designed to do the calculations needed to implement these ideas. It can be used to calculate the true crop factor and the effective water holding capacity of the soil. The user can then select single cycle or dual cycle mode, and the program calculates when to irrigate and how much water to apply.
In dual cycle mode, the entire profile is filled with a major irrigation. Then, when the top layer has dried down to the refill point, a smaller irrigation is applied to wet the top layer without reaching the still-moist lower layer. At this stage, only about half of the total water holding capacity has been used.
The smaller irrigations are not intended to fully refill the whole profile. Typically, only about 40% of the full water holding capacity is applied, leaving a deficit of around 10%. This is repeated several times (often around five shallow irrigations) so the upper soil stays moist while the deeper soil progressively dries. Then a larger irrigation is applied again to refill the entire profile.
For practical irrigators, the key point is that the program does the calculations. You do not need to be a mathematician. The theory is useful, but the method is designed to be used in the real world.
Basic scheduling theory: Why plants “use” water
Plants use water as a way of transporting nutrients from the soil. Sunlight provides energy. Water evaporates from the leaves and creates water tension (intermolecular forces) that lifts water and dissolved nutrients. The nutrient solution enters the root system by osmosis: water moves from the weaker solution in the soil into the stronger solution in the roots.
If the nutrient solution in the soil becomes too strong, roots cannot extract the solution properly. After water reaches the leaves, it evaporates and leaves nutrients behind for plant growth. The amount of water held in plant tissue is tiny compared with the amount lost by evaporation.
This is why water is not “consumed” in the usual sense. In a closed container like a terrarium, water is not used up; it is recycled.
The nutrient strength also affects plant behaviour. If the nutrient solution is weak, the plant tends to use more water and respond with more leaf area. If the nutrient solution is stronger, the plant may need less water to meet nutrient needs, and can shift more energy into fruit and carbohydrate production.
So irrigation is not only about meeting minimum water needs. It is also about managing growth by controlling how much water is available and the nutrient strength in the root zone.
Why irrigation depth is the key control point
Many larger plants have two distinct root zones: fine feeder roots in the nutrient-rich upper zone and deeper tap roots. While the upper zone is moist, the plant extracts water and nutrients efficiently and grows rapidly. When that upper zone dries out, the plant survives using deeper roots, but often does not flourish.
This is why dual cycle irrigation works well. Regular water is applied to the upper zone, with less frequent deeper irrigations (often timed ahead of hot spells) to ensure the plant can survive and keep producing.
The key aspect of irrigation scheduling is control of irrigation depth. In practice, that means knowing how much water to apply to reach the target depth—deep enough to wet the active roots, but not so deep that water is lost below the root zone.
Measuring crop factors: Why it is harder than it looks
The amount of water a plant uses depends on weather, so we cannot say a plant uses a fixed number of litres per day. Instead we use a ratio called crop factor: the ratio of water used by the plant to evaporation.
Evaporation can be measured cheaply and reliably. A simple manual gauge can work like a rain gauge. It mimics a plant: water in a container travels up a wick and evaporates from a disc surface. Automatic versions and data from conventional evaporation pans are also available, and often free.
Traditional crop factor measurements often use pots and weighing. The problem is the unit conversion. Water use is measured in litres, evaporation in millimetres. One litre per square metre equals one millimetre—but what is the relevant square metre? Is it the pot area, the canopy area, or the ground area? Different choices give different crop factors.
In field crops the issue becomes even bigger. Row spacing and crop density change water use per area. Slopes, shelter, sun exposure, and wind also affect crop factor. If we choose the wrong “area” assumptions, we can apply the wrong irrigation volumes.
The wetted volume problem
It would be ideal to measure crop factor directly in the field, but there is a catch: wetted volume.
If you irrigate into a tank, you can convert litres to millimetres and know the depth. In soil, you do not know how far water will penetrate. It depends on pore spaces and how much water is already present.
No irrigation method wets perfectly uniformly. Drippers, in particular, wet a small volume, but every irrigation method has uneven distribution. Plants also do not take up water uniformly. They tend to extract water first from surface layers where roots are strongest.
So the soil moisture field is not uniform. Water does not magically spread through soil to create even moisture everywhere. This is one of the realities of irrigation.
Adaptive learning: A simple idea that solves the hard parts
Modern soil moisture sensors can give accurate readings—but only within their “sphere of influence” (often around 300 mm). In theory you could use many sensors to measure the whole soil volume, but that is not practical for most growers.
The alternative is adaptive learning: make an estimate, measure the error, correct it, and try again. This is how humans learn complex tasks without fully modelling the physics. The same idea can be used to learn the true crop factor for a site.
In practical terms, there are two methods:
- Method 1: Soil moisture sensor at target depth. Place a sensor at the target irrigation depth (and if using dual cycle, one probe for each target depth). Irrigate based on your best current crop factor estimate. Record soil moisture just before irrigation and some time after irrigation.
- Method 2: Measure irrigation depth after irrigation. Some time after irrigation, measure how far the water has soaked into the soil using a simple auger or soil sampler. Enter that depth as feedback.
With Method 1, you also record evaporation, rainfall, and water applied. You enter the before-and-after readings into I-Planner. The program calculates the error and corrects the crop factor. With your approval, it stores the updated value. Each irrigation improves the estimate, and the crop factor adjusts as the plant grows and seasons change.
With Method 2, applying a known amount of water and measuring how far it soaked is a simple, reliable way of tracking moisture behaviour. It avoids needing to map the entire wetted volume.
Measuring water holding capacity: The practical definition
Water holding capacity here means the amount of water in the soil between field capacity and the point where the plant just starts to go into stress. It depends on:
- Soil type.
- How effective the plant is at extracting water.
- Wetted volume (which depends on irrigation method and plant size).
Different plants extract water differently. Some plants can still look stressed while the soil seems moist, while others can look fine even when the soil is quite dry. Root system size and plant type matter. Vines, for example, can extract water from drier soil and typically use a larger soil volume than shallow-rooted crops.
Irrigation method matters too. Dripper systems typically have a small wetted area, giving lower effective water holding capacity. Sprinklers and flood irrigation wet larger areas and often have higher effective water holding capacity. As plants grow and access more soil volume, water holding capacity increases.
How to measure water holding capacity step by step
- Irrigate to the target depth. Apply irrigation until water reaches the target depth. Check with a soil moisture or depth sensor after allowing time for water to soak. At this point, treat the soil as “full” (field capacity for the wetted volume).
- Measure evaporation. Measure local evaporation and track accumulated evaporation since the last irrigation.
- Monitor the plant for stress. Ideally use a plant moisture sensor. Alternatively, look for normal signs such as leaf droop, noting that this may occur after productivity has already dropped.
- Read the deficit when stress begins. In this method, water holding capacity is the deficit when the plant just starts to stress. In I-Planner, you read the water deficit and that tells you the effective water holding capacity for your situation.
Practical scheduling: Putting it all together
The first step is to decide irrigation depth. Digging a hole and inspecting the soil is a good start. Be aware that the current root pattern may reflect past irrigation habits, not the ideal arrangement.
The top soil horizon—often around 300 mm—is usually the most critical zone. This is where nutrients are concentrated and where plants should be regularly irrigated. You may also want deeper irrigation at times when water is available, to build a reserve for hot conditions. This is where dual cycle is useful.
When interpreting data, it is often more useful to think in “millimetres of water below full” rather than just a raw soil moisture trace. The practical question is: how far below full is the profile, and how much water is needed to refill to the target depth?
If you do not have a good starting crop factor, you can begin by irrigating as you normally would, while recording evaporation and irrigation amounts. Enter the information into I-Planner. It will estimate the crop factor and effective water holding capacity you are currently applying in practice.
Then, as you collect before-and-after readings (soil moisture or depth), I-Planner recommends improved crop factor values. You may choose to wait for several irrigations before adopting a new factor. Once crop factor stabilises, you can increase the water holding capacity setting (based on real plant response) to increase time between irrigations without stressing the plant.
Importantly, you do not need to be constantly checking soil moisture to decide when to irrigate. With a correct crop factor, evaporation data can tell you the available water in the soil and the current deficit. You only need soil measurements before and after irrigations to keep the model “honest” and updated.
Colin Austin — © Creative Commons. Reproduction permitted for private use with source acknowledgment; commercial use requires a license.
Download “Irrigation Scheduling: Measure Water Use & Control Irrigation Depth” (Full PDF)
