Good irrigation is not guesswork. It is about matching water supply to what your plants actually use, and what your soil can safely hold. Published “average” values help, but every farm differs—slope, wind exposure, row spacing, canopy size, and soil type all change the result. This guide explains a practical, do-it-yourself method to measure crop factor and water holding capacity on-site, then use adaptive learning to irrigate to the right depth—efficiently, reliably, and with less stress on plants and soil.
Why On-Farm Measurement Matters
To irrigate correctly we need two key numbers: how much water the plants are using, and how much water the soil can hold before plants begin to stress. Many values are published in books and research papers, but there is a major problem: every farm is different. Topography, hills, north- or south-facing slopes, wind exposure, row spacing, plant size, and soil type can all shift water use and storage.
Literature values are still useful as a guide, but what growers really need is a simple way to measure accurate values for their own site—without needing a research team. This article shows how to do that using practical measurements and a self-correcting approach.
Menu: Four Parts of The System
This guide is split into four sections:
- Basic Scheduling Theory — how plants use water and nutrients, and why irrigation depth matters.
- Measuring Crop Factors — how to measure plant water use relative to evaporation, directly on your farm.
- Measuring Water Holding Capacity — how to measure the usable water stored in your soil profile before stress begins.
- Practical Scheduling — the simple method for using these measurements day to day.
Basic Scheduling Theory
Plants use water as a transport system for nutrients. Sunlight provides the energy, and water evaporates from the leaves. This creates water tension (intermolecular forces) that lifts water and dissolved nutrients up through the plant. The nutrient solution enters the roots 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, the roots cannot extract water. Water then becomes limiting even when the soil appears wet. Most of the water the plant “uses” is not consumed in tissue. It is lost to the air by evaporation. In a closed system, like a terrarium, water is not used up—it is recycled.
The weaker the nutrient solution, the more water must pass through the plant to deliver enough nutrients. Plants often respond by increasing leaf area. If the nutrient solution is stronger (within safe limits), less water is needed to meet nutrient requirements, and more plant energy can be directed to fruit and carbohydrate production.
So the aim of irrigation is not just “enough water.” It is also about managing plant growth by controlling water availability and nutrient concentration in the root zone.
Two Root Zones and The Logic of Dual-Cycle Irrigation
Some plants, such as lettuce, have small root systems. Many larger plants have two distinct root zones: fine feeder roots in the nutrient-rich upper zone, plus deeper roots (including tap roots). While moisture is available in the upper zone, plants grow rapidly using those fine feeder roots.
When the upper zone dries out, plants can survive on deeper roots, but growth slows markedly. That is why irrigation should be applied regularly to the upper zone, with less frequent deeper irrigations to build reserves—especially ahead of hot spells. This is often called dual-cycle irrigation or, when coupled with weather forecasts, anticipatory irrigation.
The key scheduling task is to control irrigation depth. To do that, we must know how much water to apply.
Measuring Crop Factors
Because weather changes daily, we cannot say a plant uses “1.5 litres per day” as a general rule. Instead we use a ratio called a crop factor. A crop factor is the ratio of the water used by the plant to evaporation.
Evaporation can be measured cheaply and reliably. A simple manual gauge can be as easy to read as a rain gauge. It works like a plant: water in a container (like water in soil) travels up a wick (like a trunk) and evaporates from a disc (like leaves). Automatic versions and data from conventional evaporation pans are also widely available, and in many areas evaporation data is inexpensive or free.
The Crop Factor Measurement Problem: Area
Crop factors are often measured by growing a plant in a pot and weighing it to calculate water use. That gives litres. But evaporation is measured in millimetres. To convert litres to millimetres, we need an area, because 1 litre per square metre equals 1 mm.
So what is the correct area? The pot area? The canopy area? The row spacing area? Each choice gives a different crop factor. In the field the problem grows: row crops, variable spacing, and canopy differences mean the “area” is not obvious. If the wrong area is chosen, irrigation can be seriously wrong.
Slopes, shelter, and wind also change crop factor. So the real goal is to measure the crop factor directly on-site in a way that automatically accounts for these variations.
Why Soil Moisture Alone Does Not Solve It
Soil moisture sensors are accurate in their local “sphere of influence” (often around 300 mm), but there is a major catch: wetted volume. In a tank, litres convert neatly to depth. In soil, water does not wet uniformly, and we cannot easily know how far it penetrates or how wide it spreads. Penetration depends on pore space and existing moisture.
Drip systems wet only a small volume, but all irrigation methods create uneven distributions. Plants also do not extract water uniformly: they usually pull from surface layers first where feeder roots are strongest. Water does not move through soil by miracle. Variable water distribution is a reality we must design around.
Using many sensors in a 3D grid could measure total soil water volume, but that is impractical. We need a different strategy.
The Answer: Adaptive Learning (Predictor–Corrector)
The solution is an adaptive learning approach—also called an error correction or predictor–corrector method. It is how people learn complex skills. We do not learn to walk by modelling physiology. We try, we fall, we correct, and we improve. Irrigation can work the same way.
You start with your best estimate of crop factor. You irrigate. You measure a result. The software calculates the error and adjusts the crop factor. Over time, the crop factor becomes correct for your farm and continues to adjust as plants grow and seasons shift.
Method 1: Using Soil Moisture Sensors at Target Depth
In this method, place a soil moisture sensor at the target irrigation depth. If you use dual-cycle irrigation, you will need one sensor for each depth.
Irrigate using your current best crop factor estimate: multiply the accumulated evaporation since the last irrigation by the crop factor, convert to litres using the anticipated wetted area, then convert to run time. This is typically done with software.
Record soil moisture before irrigating and again a short time after irrigating.
Enter the evaporation, irrigation, and “before/after” soil moisture readings into a program such as IIS-Planner. The program calculates the error, corrects the crop factor, and—subject to your approval—updates the database. With repeated irrigations, the crop factor becomes stable and remains adaptive to site and season.
The major advantage is that it does not matter if your row spacing, slope, or wetted area are unusual. The adjustment process automatically accounts for real field behaviour.
Method 2: Using Irrigation Depth (Auger or Soil Sampler)
The second method uses depth measurement. After irrigation and once water has had time to soak, measure how deep the wetting front has reached using a simple auger or soil sampler. Enter the measured depth and the known irrigation amount into the software.
Applying a known amount of water and measuring how far it penetrates is one of the simplest and most reliable ways to interpret soil moisture in practical terms. It focuses on what matters most for efficiency: irrigation depth.
Measuring Water Holding Capacity
Water holding capacity is the amount of water available between field capacity (soil “full” after drainage) and the point where the plant just begins to go into stress.
This depends on soil type, plant species, root volume, and wetted volume. Different plants extract water very differently. A tomato plant may show stress while the soil still seems moist. A grape vine may show no stress while soil looks dry and crumbly. Vines can pull water from drier soil and often have far larger root systems, so their effective water holding capacity is much higher.
Irrigation method also matters: drip systems usually wet a smaller volume (lower effective water holding), while sprinklers and flood can wet a larger volume (higher effective water holding). As plants grow, roots spread and water holding capacity increases.
Step-By-Step: Measuring Water Holding Capacity on Your Site
1) Irrigate to target depth. Apply irrigation until water reaches the target depth, checking with a depth method or a sensor after allowing time for soaking. At this point the wetted volume is at field capacity. You may not know the exact litres stored in the soil, and that is fine—treat it as “full” and use it as your reference point.
2) Measure evaporation. Record local evaporation and calculate accumulated evaporation since the last irrigation (daily readings summed, or directly from a manual gauge).
3) Monitor the plant for stress. A plant moisture sensor is best. Otherwise, visual signs such as leaf droop can be used, but these often occur after stress has already reduced production.
4) Estimate the deficit at first stress. Multiply accumulated evaporation by the site crop factor (which must be accurate). The deficit when the plant first begins to stress is your effective water holding capacity for that crop, soil, and irrigation method.
Practical Scheduling: The Simple Day-To-Day Method
First decide the irrigation depth. Digging and observing roots can help, but current root patterns can reflect past irrigation habits, not necessarily what is ideal.
The topsoil horizon—often about 300 mm—is usually the most important zone. This is where nutrients concentrate and where you should regularly irrigate. Deeper irrigation can be used less often to build reserves for heat or dry spells using a dual-cycle approach.
Use “Millimetres Below Full” Not Just “Soil Moisture %”
A key practical output is a graph of mm of water below full. This is often more useful than a raw soil moisture graph, because it shows how much water you need to refill the profile to your chosen target depth.
Starting Point If You Do Not Know Your Crop Factor
If you do not have an estimate, keep irrigating as you normally would, but record evaporation and irrigation and enter them into the software. The program can estimate the crop factor and water holding capacity implicit in your current practice.
Use a soil moisture or depth check after irrigating and enter the results. The software will recommend a crop factor. You may choose to wait for several irrigations before locking it in. Once stable, you can safely increase the water holding capacity setting (within reason) to extend the time between irrigations—without risking stress.
How Often Do You Need to Check Sensors?
You do not need to constantly inspect soil moisture to decide when to irrigate. With the correct crop factor, evaporation data tells you how much water has been removed since the last refill. The practical method is to measure only before and after each irrigation to keep the model accurate and self-correcting.
You can irrigate any time up to the maximum deficit you have set, and apply water equal to that deficit. This approach reduces waste, improves control of irrigation depth, and allows plants to grow with fewer shocks from wet–dry extremes.
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For more information email: colinaus@bigpond.net.au
Colin Austin — © Creative Commons. Reproduction permitted for private use with source acknowledgment; commercial use requires a license.
