Evapotranspiration vs Soil Moisture Monitoring: Which Should You Use for Irrigation Scheduling?
Irrigation scheduling comes down to one core question: how do you know when to turn the water on? Two data-driven approaches dominate modern farming, and they answer that question in very different ways. Evapotranspiration (ET) modelling estimates how much water a crop is losing to the atmosphere. Soil moisture monitoring measures how much water is actually left in the root zone. Both are legitimate, both are widely used, and most growers eventually want to know which one deserves their investment first.
How ET-based scheduling works
Evapotranspiration is the combined water loss from soil evaporation and plant transpiration. ET models start with a reference figure, ETo, calculated from weather station data such as temperature, humidity, wind speed and solar radiation. That reference is then adjusted using a crop coefficient (Kc), which accounts for the specific water demand of a crop at its current growth stage. The result is an estimate of how much water the crop is using on a given day, which growers use to replace what has been lost.
The strength of ET scheduling is that it is forward-looking. It tells you what the crop is likely to need based on atmospheric demand, which makes it well suited to fixed irrigation schedules and large, relatively uniform blocks.
How soil moisture monitoring works
Soil moisture sensors take a different approach entirely: rather than estimating water loss, they measure water content directly in the root zone. Growers typically track two reference points. Field capacity is the maximum water the soil can hold after drainage. Refill point is the threshold at which irrigation should begin to avoid crop stress. Watching moisture levels move between these two points gives a real-time, ground-truthed picture of what the crop actually has available.
Because it measures outcomes rather than predicting them, soil moisture monitoring tends to catch what ET models can miss, such as soil variability, drainage issues, or an irrigation system that isn't delivering water evenly.
Where each approach earns its keep
ET scheduling suits large, uniform blocks on a fixed rotation, situations where soil probes are impractical to install at scale, and farms that already have reliable weather station coverage.
Soil moisture monitoring suits high-value crops where over- or under-watering is costly, fields with variable soil types or drainage, and operations fine-tuning irrigation timing down to the day.
Both together suit growers who want ET as the forecast and soil moisture as the reality check, which is increasingly the standard for precision irrigation programs.
The limitation each approach shares
ET models assume a "typical" crop and field, so they can drift from reality when soil types vary, when a canopy hasn't fully developed, or when microclimates within a farm behave differently to the reference weather station. Soil moisture sensors, meanwhile, are only as good as their placement. A probe in an unrepresentative spot will report unrepresentative data, no matter how accurate the sensor itself is.
ET tells you what the atmosphere is demanding from the crop. Soil moisture tells you what the soil is delivering to the roots.
Using both together
In practice, the two methods complement each other well. ET data sets the baseline expectation for water use across a block, while soil moisture sensors confirm whether that expectation is holding true in specific parts of the field. When the two disagree, that gap is often the most useful signal a grower gets all season, pointing to drainage problems, sensor placement issues, or genuine soil variability worth investigating.
What equipment you need
An ET-based approach requires a weather station capturing temperature, humidity, solar radiation and wind speed, plus crop coefficient data for the varieties being grown. A soil moisture approach requires probes placed at representative points and depths across the monitored area. Many growers now run both from a single connected platform, so ET calculations and soil moisture readings sit side by side rather than in separate systems.
The bottom line
Neither approach is strictly better. ET scheduling gives you a reliable forecast at scale; soil moisture monitoring gives you ground truth where it matters most. The right starting point depends on crop value, field variability and how tightly you need to manage irrigation timing, but the growers getting the most out of their water are usually the ones using both.