IW/CPE Ratio & When to Irrigate Next

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.

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.

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.

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.

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.

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.

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.

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.

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.