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Irrigation Scheduling: Choosing the Best Method for Your Farming Operation

How to Choose the Best Method for Your Farming Operation?

Water is one of your most valuable resources, and getting irrigation right can mean the difference between a great harvest and a struggling crop. But how do you know exactly when to turn on the water—and when to hold off? That’s where irrigation scheduling comes in.

Irrigation scheduling helps you decide when and how much water your crops need. While there are many approaches, let’s focus on the most common methods farmers use—and when each method works best.

Irrigation scheduling can be divided into four main categories: the traditional method, weather-based or ET-based scheduling, plant-based scheduling, and soil-based scheduling.

The Traditional Method

The traditional method is the oldest and simplest approach, which includes:

  • Irrigating based on how the crop looks.
  • Checking soil moisture by feeling it by hand.
  • Watering according to a personal calendar.

Traditional methods are simple and familiar, but they rely on personal judgment, which can lead to inconsistent results. Because of this, results can vary significantly from one farm to another, and they might not always give your crops the right amount of water at the right time.

Weather-based or ET-based Scheduling

Weather-based, or ET-based scheduling uses evapotranspiration, or ET, which is the combined water loss from evaporation and plant transpiration. Weather-based irrigation scheduling relies on daily weather information to estimate how much water your crops use. 

This method requires local weather data—such as temperature, humidity, wind speed, and sunlight—to calculate reference ET. Currently, the easiest way to get the reference ET is to retrieve it from the nearby CIMIS weather station, which is available online free of charge.

The reference ET value should be used with a crop-specific factor called the “crop coefficient,” which accounts for different crop types and growth stages.

The daily irrigation requirement is then calculated by multiplying reference ET by the crop coefficients. 

The ET-based method provides valuable information. Combining ET-based scheduling with soil water balance calculations can improve irrigation precision, conserving water while boosting yields.

While weather data is useful, some farmers prefer a more direct approach — measuring the plants. 

Plant-Based Irrigation Scheduling

Plant-based irrigation scheduling involves monitoring your crops directly to decide when to irrigate. This method is more suitable for high-value crops, orchards, and vineyards. 

Plant-based scheduling uses tools to measure plant water status, and irrigations are planned when measurements are at or below certain reference baselines. 

The most common tool for plant-based irrigation is the pressure chamber, also known as the pressure bomb. Farmers take stem water potential readings around midday and compare them to reference baselines. These baselines vary by crop type, growth stage, and environmental conditions, like vapor pressure deficit. While the pressure chamber is the gold standard for plant-based irrigation, it requires manual operation, making it time-consuming and difficult to automate.

Other plant-based tools that can provide real-time measurements are based on advanced technologies and include:

  • Mini-tensiometer, which measures tree water potential inside the tree’s woody tissue.
  • Dendrometer, which measures fluctuations in the main stem or tree trunk diameter.
  • Sap flow, which measures the movement of water from the root to the leaves.
  • Infrared thermometer or camera, which measures the canopy or leaf temperature.

Soil-based Irrigation Scheduling

Soil-based irrigation scheduling measures soil water potential or water content to determine irrigation needs. Soil water potential sensors tell you how easily plants can uptake the water, and soil water content sensors tell you how much water is in the soil. You can start irrigation when the soil water content or potential is at or below certain levels, which vary by soil type, crop, growth stages, and evapotranspiration demands.

Both water potential and water content sensors can provide real-time data. Knowing exactly how much water your soil holds and how easily plants use it helps you decide when and how much to irrigate. 

The soil water potential devices include tensiometers and resistance sensors such as gypsum blocks and granular matrix.

The soil water content devices include neutron probes and electromagnetic-based sensors, also known as dielectric sensors.

  • The neutron probe is highly accurate and considered the gold standard. However, it requires a license, must be operated manually, and cannot be left in the field due to safety concerns.
  • The dielectric sensors include frequency domain and time domain sensors. The frequency domain sensors mostly include capacitance sensors, which are the most common soil moisture sensors available on the market. The capacitance and time domain sensors require field calibration to account for soil salinity, temperature, and clay content to give high accuracy. 

Combining Methods

The best irrigation strategies often combine weather-based, plant-based, and soil-based scheduling for greater accuracy. Multiple methods provide a more complete and accurate picture of crop water needs.

For example:

You might use weather data to predict irrigation needs, confirm soil moisture conditions with sensors, and check plant stress indicators as a final guide to fine-tune irrigation decisions.

Combining methods leverages the strengths of each technique, helping you achieve greater irrigation efficiency. It helps reduce the risk of mistakes, conserve water, and prevent both under- and over-irrigation.

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