IW/CPE Ratio & When to Irrigate Next
Schedules intervals
Enter the irrigation depth (IW), the IW/CPE ratio and daily pan evaporation to find the cumulative pan evaporation trigger and the days between irrigations.
IW/CPE scheduling
Next: irrigate every 7.8 days — or whenever the open pan has evaporated 62.5 mm cumulatively — applying 50 mm each time.
The IW/CPE ratio is crop- and stage-specific (often 0.6–1.0); a lower ratio stretches the interval and saves water but risks stress. Pan evaporation changes daily with weather.
IW/CPE scheduling — key facts
- CPE trigger
- IW ÷ ratio
- Days interval
- trigger ÷ daily pan
- Higher ratio
- more frequent irrigation
- Lower ratio
- longer interval
- Typical ratio
- ≈ 0.6–1.2
- Typical IW
- ≈ 4–7 cm
- Needs only
- a pan and a fixed depth
- Privacy
- Runs in your browser; nothing uploaded
One pan reading a day tells you when to irrigate
The IW/CPE rule is irrigation scheduling stripped to its essentials: keep a running total of daily pan evaporation, and irrigate a fixed depth every time that total reaches your trigger. The trigger is simply your irrigation depth divided by the IW/CPE ratio you choose — raise the ratio to irrigate more often and keep the soil wetter, lower it to stretch the interval. It needs nothing more than an evaporation pan and a ruler, which is why it has stayed a trusted field rule for decades.
This tool turns your IW, ratio and daily pan evaporation into the cumulative CPE trigger and the approximate days between irrigations, and the interval naturally tightens in hot weather and stretches when it is cool. Use it as your day-to-day scheduling rule, and pair it with the Pan Evaporation, Irrigation Scheduling and Soil Moisture tools for a complete picture.
A simple, robust rule
Schedule from one daily pan reading.
Tune the frequency
The ratio sets how often you irrigate.
Weather-aware timing
Interval tightens in heat, stretches when cool.
No sensors needed
Just a pan and a fixed irrigation depth.
Frequently Asked Questions
What is the IW/CPE approach?+
The IW/CPE approach schedules irrigation by watching cumulative pan evaporation (CPE). You irrigate a fixed depth of water (IW) each time the running total of pan evaporation reaches IW divided by your chosen ratio. It turns a single pan reading each day into a clear, repeatable rule for when to irrigate.
How does the calculation work?+
The trigger is the cumulative pan evaporation at which you irrigate: CPE trigger = IW ÷ ratio. If you irrigate 5 cm at an IW/CPE ratio of 0.8, you irrigate every time CPE reaches 6.25 cm. Dividing that trigger by the daily pan evaporation gives the approximate days between irrigations.
What does the IW/CPE ratio mean?+
The ratio sets how aggressively you replace evaporative demand. A higher ratio means you irrigate before as much evaporation has accumulated, so irrigations are more frequent and the soil stays wetter. A lower ratio lets more evaporation build up between irrigations, stretching the interval. Typical values range from about 0.6 to 1.2.
Why use a higher ratio?+
A higher IW/CPE ratio triggers irrigation sooner, keeping more water in the root zone — useful for shallow-rooted, sensitive or high-value crops, light soils, and critical growth stages. A lower ratio suits deep-rooted, drought-tolerant crops and heavier soils that hold more water between irrigations.
What is cumulative pan evaporation (CPE)?+
CPE is the running sum of daily pan evaporation since the last irrigation. Each day you add that day's pan reading to the total; when the total reaches the trigger, you irrigate and reset the count to zero. It is a direct, weather-driven measure of how much demand has built up.
Why is this method popular?+
It is simple, robust and needs only an evaporation pan and a fixed irrigation depth — no soil sensors or full weather data. It has been widely validated for many crops in research and extension, which is why it remains a standard field rule for scheduling irrigation in many regions.
How do I choose the irrigation depth (IW)?+
IW is the depth you apply each time, usually set to refill the readily available water in the root zone for your soil and crop. Common values are around 4–7 cm. A larger IW means a longer interval for the same ratio; a smaller IW means more frequent, lighter irrigations.
Does the days interval stay fixed?+
No — it shifts with the weather. In hot, dry spells pan evaporation is high, so the cumulative total hits the trigger in fewer days and you irrigate more often. In cool, humid or cloudy periods evaporation is low and the interval stretches. The trigger depth stays the same; only the timing moves.
Is this as precise as a full water balance?+
It is a practical scheduling rule rather than a full soil water balance, and it does not directly account for rainfall, crop stage or soil type beyond your choice of IW and ratio. For tighter control, combine it with crop ET and soil moisture tools, but for many crops the IW/CPE rule schedules irrigation reliably on its own.