How would you determine irrigation run time using a simple constant-rate system?

• A. Guess based on plant height
• B. Run each zone until water is visibly present on mulch
• C. Measure zone output in inches per hour, then apply the needed soil water deficit, adjusting for rainfall and plant type.
• D. Use the rainfall total for the month

Answer: (C)

When sizing run time for a simple constant-rate irrigation system, you compare how much water the zone needs with how much water the system can apply each hour. First, identify the soil water deficit for the zone—how many inches of water are needed to meet the plants’ current moisture needs, taking into account soil type, current moisture, and the plants being grown. Next, determine the zone’s output rate in inches per hour by doing a straightforward test: run the zone for a known period and measure how much water was applied in inches. With those two pieces, you calculate the run time as deficit divided by the output rate, giving you the number of hours to run. Adjust this runtime for rainfall (which reduces the deficit) and for plant type (which influences how much water the plants actually require). For example, needing 1.5 inches of water and a zone delivering 0.5 inches per hour means 3 hours of runtime. This approach is precise and avoids relying on guesswork, visible wetting, or monthly rainfall alone, which don’t reflect actual soil moisture or plant water demand.