Why do we irrigate when it is raining?

Irrigation systems are designed on a rotational basis. To be cost effective they apply a certain amount of water per area each day. Irrigation aims to manage the amount of moisture in the soil to ensure plant growth is not compromised during periods of low rainfall. An ‘irrigation trigger’ is used to do this – the moisture level in the soil has to reach a certain low before the trigger is reached. During the New Zealand summer plants use an average of 4–5mm of water a day. During peak growth they can use more than 8mm a day. This means a soil with 30mm of water available for plant growth typically has 5–8 days of optimal plant growth before requiring irrigation. Rain events during the summer are frequently only light showers of 5–10mm. Irrigators fine-tune their irrigation as best their system allows. They continue to irrigate but apply less water. When a heavy rainfall event occurs, replenishing the soil’s moisture, irrigators will switch off for a short period until the irrigation trigger is once again reached. Typically this period is 4–8 days, depending on the rotation length.