Evapotranspiration vs Soil Moisture Monitoring: Which Should You Use for Irrigation Scheduling?
Two Approaches to Irrigation Scheduling
Irrigation scheduling is one of the most important decisions a grower makes throughout the season. Apply water too infrequently and crops experience moisture stress that affects yield and quality. Apply it too often and water is wasted, input costs rise, and leaching of nutrients through the profile becomes a risk.
Two monitoring approaches are commonly used to support irrigation scheduling decisions: evapotranspiration-based scheduling and soil moisture monitoring. Both are legitimate and widely used across irrigated agriculture, but they work in fundamentally different ways and each has distinct strengths and limitations depending on the farming context.
Understanding the difference between these approaches — and knowing when to use one, the other, or both together — is an important part of getting the most from modern precision irrigation systems.
How Evapotranspiration-Based Scheduling Works
Evapotranspiration, commonly referred to as ET, is the combined process of water evaporating from the soil surface and transpiring through plant leaves into the atmosphere. It represents the primary pathway by which water is lost from the crop system between irrigation events, and calculating it accurately gives growers an estimate of how much water needs to be replaced to keep crops adequately supplied.
ET is calculated using weather data including temperature, solar radiation, humidity and wind speed. The standard reference ET value, known as ETo, represents the water demand of a reference grass surface under measured atmospheric conditions. This reference value is then adjusted using a crop coefficient, known as Kc, which varies depending on the crop type and its current growth stage, to produce an estimate of actual crop water demand.
Irrigation scheduling based on ET works by tracking cumulative water demand since the last irrigation event and triggering a new irrigation once that demand reaches a defined threshold. The approach is entirely driven by atmospheric conditions and does not directly measure anything happening in the soil or root zone.
The primary strength of ET-based scheduling is that it responds to actual environmental demand on the crop. During hot, sunny and windy periods, ET rates increase and the system responds accordingly. During cooler or overcast periods, demand falls and irrigation frequency reduces.
How Soil Moisture Monitoring Works
Soil moisture monitoring takes a different approach. Rather than calculating estimated water demand from atmospheric conditions, it measures what is actually happening in the soil profile directly. Sensors installed at multiple depths in the root zone track volumetric water content or soil water tension continuously, giving growers a real-time picture of current moisture availability to plant roots.
Irrigation scheduling using soil moisture data works by defining upper and lower reference thresholds for the root zone — field capacity as the target end point for irrigation, and the refill point as the trigger for starting a new irrigation event. When sensor readings indicate that soil moisture has depleted to the refill point, irrigation is initiated. When readings confirm the profile has returned to field capacity, irrigation stops.
The primary strength of soil moisture monitoring is that it reflects actual conditions in the root zone rather than estimated demand. Soil variability, drainage behaviour, root distribution and profile characteristics are all captured implicitly in the sensor readings. Growers can see directly whether applied water is reaching the target depth, whether drainage is occurring below the root zone, and whether the soil is holding water as expected.
Where ET Scheduling Works Well
ET-based scheduling is particularly effective in situations where crop water demand is the dominant variable driving irrigation decisions and where soil conditions are relatively uniform and well understood.
It works well for crops with well-defined growth stages and established crop coefficients, where seasonal water demand patterns are reasonably predictable. Broadacre irrigated crops, pasture systems and some vegetable production systems have benefited significantly from ET-based scheduling approaches.
ET scheduling is also useful in situations where soil moisture sensors are not yet installed but a weather station is already available. Growers can begin making more informed irrigation decisions using ET data alone while building familiarity with environmental demand patterns before investing in soil sensing infrastructure.
Because ET is calculated from weather data, it responds immediately to changing conditions without any lag. On days of unexpectedly high temperatures or strong drying winds, ET-based systems will reflect increased demand straight away.
Where Soil Moisture Monitoring Works Well
Soil moisture monitoring is particularly valuable where soil variability is high, where understanding actual root zone conditions is important, and where growers want direct confirmation that irrigation is being applied and retained effectively.
It is especially useful in horticultural production systems — tree crops, vines, vegetables and other high-value crops — where maintaining precise moisture conditions throughout the root zone is critical to fruit quality, yield and consistency. In these systems, the cost of moisture stress or over-irrigation can be high, and the direct feedback that soil sensors provide is genuinely valuable.
Soil moisture monitoring also helps identify problems that ET scheduling cannot detect. Blocked emitters, soil compaction layers, preferential drainage paths and irrigation system inconsistencies all show up in sensor data in ways that atmospheric calculations would never reveal.
For growers managing irrigation across varied soil types within a single property, sensors installed in representative locations across different soil zones provide a practical way to account for variability in scheduling decisions.
The Limitations of Each Approach
ET-based scheduling has a fundamental limitation in that it estimates demand without confirming supply. It assumes that applied water is being retained in the root zone and taken up by the crop as modelled, but it has no way of detecting when this assumption is incorrect. If irrigation is being lost to deep drainage, surface runoff or distribution problems, ET scheduling will continue calculating demand as if water is being delivered effectively.
Crop coefficients also introduce uncertainty. Standard Kc values may not accurately represent actual crop behaviour under local conditions, across different varieties, or at non-standard planting densities. Errors in crop coefficient selection can lead to systematic under or over-irrigation relative to what the ET calculation suggests.
Soil moisture monitoring has its own limitations. Sensors measure conditions at specific points in the soil, and soil variability means that readings at one location may not represent the full range of conditions across a paddock or irrigation zone. Sensor calibration is important and needs to account for site-specific soil characteristics to produce accurate volumetric readings. Installation depth and placement relative to emitters also significantly influences what the sensors actually capture.
Neither approach provides a complete picture on its own.
Using ET and Soil Moisture Together
The most effective irrigation scheduling systems use both approaches in combination. ET data provides a forward-looking estimate of daily crop water demand, helping growers anticipate when irrigation will be needed and how much water to apply. Soil moisture data provides real-time confirmation of what is actually happening in the root zone, validating whether applied water is being retained effectively and whether the crop is accessing moisture as expected.
Together, they address the core limitations of each individual approach. ET tells you what the atmosphere is demanding from the crop. Soil moisture tells you what the soil is delivering to the roots. When the two datasets align, confidence in irrigation decisions increases significantly. When they diverge, it flags a problem worth investigating — whether that is a sensor fault, a soil behaviour that differs from expectations, or an irrigation system performance issue.
Modern farm monitoring platforms are increasingly capable of displaying both ET calculations and soil moisture sensor data within a single dashboard, making it practical to use both data streams simultaneously without needing separate systems for each.
What Equipment You Need
Implementing a combined ET and soil moisture monitoring approach requires two main components. A weather station capable of measuring temperature, relative humidity, solar radiation and wind speed provides the data needed to calculate reference ET. Soil moisture probes installed at multiple depths in representative soil zones provide the root zone monitoring data.
Both systems can transmit data to a cloud platform via cellular or LoRaWAN telemetry, allowing readings to be accessed remotely and reviewed alongside each other in real time.
For growers already operating a farm weather station, adding soil moisture probes is a relatively straightforward step toward a more complete irrigation monitoring capability. For those starting from scratch, deploying both systems together from the outset delivers the most complete irrigation scheduling foundation.
Conclusion
Evapotranspiration and soil moisture monitoring are complementary tools rather than competing alternatives. ET scheduling provides an atmospheric demand signal that drives proactive irrigation planning, while soil moisture monitoring provides direct feedback on root zone conditions and irrigation effectiveness. Used together, they give growers the most reliable foundation for irrigation decisions — reducing water waste, protecting crop performance and building a more complete understanding of how water moves through the farming system throughout the season.