Evapotranspiration is the water a field loses to the atmosphere, combining what evaporates from the soil surface with what the crop transpires through its leaves, and it is the demand side of every irrigation decision. If you know how much water the crop is losing each day and how much rain or irrigation is coming in, you can keep a running account of the soil's water and irrigate exactly to replace what is used. This guide defines reference ET and the crop coefficient, shows how a soil-water-balance model turns weather data into a daily irrigation requirement, and explains how a control platform automates ET-based scheduling instead of relying on fixed timers.
Evapotranspiration (ET) in one line: Evapotranspiration (ET) is the total loss of water from a cropped field to the atmosphere, the sum of evaporation from the soil and transpiration from the plants, usually expressed as a depth of water per day. Scheduling uses a reference value, ET0, computed from weather data for a standard reference surface, then multiplies it by a crop coefficient, Kc, to get the actual crop's water use. Feeding that daily crop ET into a soil water balance tells an irrigator how much water the crop has used and therefore how much to apply.
Because measuring the exact water loss of every field is impractical, ET scheduling starts from a standardized reference. Reference evapotranspiration, written ET0, is the water use of a well-watered standard reference surface, conventionally a short grass, under the given weather. It is calculated from weather-station measurements of solar radiation, air temperature, humidity, and wind using a physically based equation, most authoritatively the Penman-Monteith formulation, which combines the energy available to evaporate water with the drying power of the air. ET0 captures the weather's demand for water on a common yardstick, so a hot, dry, windy, sunny day yields a high ET0 and a cool, humid, still day yields a low one, independent of what crop is actually growing.
To get from that reference to a specific crop, ET0 is multiplied by a crop coefficient, Kc, which scales the reference demand to the crop's actual water use at its current stage of growth. A bare or newly planted field with little leaf area has a low Kc and uses less water than the reference grass, a crop at full canopy has a Kc near or above one and uses as much or more, and a maturing or senescing crop's Kc falls again. Kc therefore tracks the crop through its season, and the product of ET0 and Kc, the crop evapotranspiration ETc, is the daily depth of water the crop is actually losing. This two-step approach is powerful because ET0 depends only on weather and can be computed anywhere with a weather station, while Kc carries the crop-specific and seasonal knowledge, so one weather feed serves many different crops and fields by pairing it with the right coefficients.
Knowing daily crop ET becomes actionable when it is run through a soil water balance, a simple accounting model of the water stored in the root zone. The root zone is treated like a bank account: crop ET withdraws water each day, while rainfall and irrigation are deposits, and drainage past the root zone or runoff are losses that do not count as usable storage. Starting from a known soil water content, the model subtracts each day's ETc and adds any rain or irrigation, tracking how the stored water rises and falls. The account has a useful working range between field capacity, the full point above which water drains away, and a lower limit set to avoid crop stress, and the difference between them is the water the crop can draw on before it needs replenishing.
The irrigation requirement falls straight out of this balance. As crop ET depletes the root zone day by day, the model shows the stored water declining toward the stress threshold, and the irrigation requirement for a given day is the depth needed to refill the account back toward field capacity, less whatever rain has contributed. This is fundamentally smarter than a fixed timer because it responds to the actual weather and crop stage: a stretch of hot, windy weather draws the account down fast and calls for irrigation sooner and in larger amounts, while a cool, cloudy, rainy spell slows the depletion and may defer irrigation entirely. The balance also naturally credits rainfall, so the system does not irrigate ground that a storm has already refilled, which is exactly the waste a calendar-based schedule cannot avoid.
Running an ET-based schedule by hand means gathering weather data, computing ET0, applying Kc, updating the balance, and translating the result into run times every day, which is exactly the kind of repetitive data work a control platform can automate. A weather station feeds its measurements in, the platform computes ET0 and multiplies by the crop's current Kc to get daily crop ET, maintains the running soil water balance including logged rainfall and the water each irrigation actually applied, and produces the day's irrigation requirement. That requirement is then turned into pump run times and zone durations without a person redoing the arithmetic, and the schedule adjusts itself as the weather and the crop stage change through the season.
On a cloud SCADA and monitoring platform such as Merobix, this closes into a full loop because the same system that computes the requirement also commands and confirms the irrigation. The weather feed, the computed ET and balance, and the pump and valve states live together as tags, so the platform can schedule irrigation from the balance, then verify from flow and run-time feedback that the intended depth was actually delivered and fold that back into the water account, rather than assuming an open valve means water reached the field. An operator sees the ET trend, the soil water account, and the resulting irrigation events on one screen, gets alarms if the weather station drops out or an irrigation fails to run, and can override the schedule when needed. This is the ordinary SCADA discipline of measure, compute a setpoint, actuate, and verify, applied to the weather-driven demand of a growing crop instead of a fixed clock.
ET0, reference evapotranspiration, is the water use of a standard reference surface such as short grass under the given weather, computed from radiation, temperature, humidity, and wind. Crop ET, or ETc, is the water use of the actual crop, obtained by multiplying ET0 by a crop coefficient Kc that scales the reference demand to the crop and its growth stage. So ET0 depends only on weather, while crop ET adds the crop-specific and seasonal information.
The balance treats the root zone like an account, subtracting each day's crop ET and adding rainfall and irrigation. As crop ET depletes the stored water toward a lower stress threshold, the model signals that irrigation is needed, and the amount required is the depth to refill the account toward field capacity, minus any recent rain. This responds to real weather and crop stage rather than a fixed calendar, so it irrigates sooner in hot weather and defers after rain.
A fixed timer applies the same water on the same days regardless of the weather, so it over-waters in cool, rainy periods and under-waters in hot, dry ones. ET-based scheduling tracks how much water the crop actually loses each day and credits rainfall, so it applies only what is needed to replace the loss and refill the root zone. This saves water and reduces both drainage losses and crop stress compared with a calendar schedule.
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