The question of how long to run an irrigation system often results in guesswork, leading to wasted water or unhealthy plants. Effective irrigation timing requires establishing a precise weekly water volume needed by the landscape, moving beyond simply estimating minutes. The goal is to provide enough water to encourage deep, resilient root growth while avoiding the shallow, frequent watering that promotes weak surface roots. Calculating the required water volume and matching it to the system’s output capability determines a precise and efficient runtime.
Determining Plant Water Requirements
The first step in setting an irrigation schedule is identifying the volume of water plants lose over a given period. Water loss occurs through evapotranspiration (ET), which is the combination of water evaporating from the soil and transpiring through the leaves. Factors like temperature, wind, humidity, and sunlight influence the rate of ET, meaning plant water requirements change daily.
Local weather stations publish reference ET rates, typically measured in inches per week, representing water loss from standard turfgrass. To find the specific need for a particular plant, this reference ET is multiplied by a crop coefficient, which represents the plant’s unique water use relative to the reference grass. For example, warm-season turfgrass may require 1 to 2 inches of water per week in summer, while established, drought-tolerant plants may only need 0.5 inches every two weeks.
This calculation provides the target water depth that must be replenished by the irrigation system. Focusing on the volume (inches of water) rather than the time shifts the approach from a fixed schedule to a needs-based system. This ensures water is only applied to replace what the plant has used.
Translating Water Needs into System Run Time
Once the target water volume is established, the next step is determining how long the irrigation system must run to deliver that amount. This depends on the system’s precipitation rate, which is the speed at which sprinklers apply water, measured in inches per hour. For a fixed spray head system, this rate can be found using a catch-can test, where containers measure the average depth of water collected over a short period within a zone.
The run time in minutes is calculated by dividing the required water volume (in inches) by the system’s precipitation rate (in inches per hour) and then multiplying by sixty. For instance, if the weekly need is 0.8 inches and the system applies water at 0.4 inches per hour, the total run time is 120 minutes. This calculation provides the theoretical time required to deliver the moisture.
The total calculated run time must be tempered by the soil’s ability to absorb water, known as the infiltration rate. If the system’s application rate exceeds the soil’s absorption rate, water will pool and run off the surface before reaching the root zone. Sandy soils absorb water quickly (around 2 inches per hour), while heavy clay soils absorb water slowly (as low as 0.2 inches per hour).
The final runtime for any single watering session must be limited by the slower rate: either the time to deliver the volume or the time until runoff begins. In clay soils, the run time is often restricted by the low infiltration rate, meaning the total required time must be delivered in multiple, shorter intervals. Observing the zone for surface pooling or runoff determines the maximum runtime before the soil is overwhelmed.
Optimizing Watering Through Cycle Soaking
Cycle soaking overcomes the limitation of a low soil infiltration rate, especially in clay soil or on sloped landscapes. Instead of running the irrigation system for the full calculated time continuously, the total duration is broken into two or three shorter intervals. This technique is sometimes referred to as split application.
The first short cycle runs just long enough (often a few minutes) to wet the surface and break the soil’s initial surface tension. A rest period follows, allowing the water to soak deeply into the soil profile without creating surface runoff. A typical soak time is 30 to 60 minutes, permitting the initial application to infiltrate below the surface.
Subsequent watering cycles apply the remaining water volume, which is absorbed more effectively into the pre-moistened soil. This method ensures the entire calculated water volume reaches the root zone while eliminating wasteful runoff that occurs when the application rate is faster than the absorption rate.
Adjusting Run Times for Seasonal Changes
The total run time established for the peak growing season must be modified throughout the year. The primary driver for this adjustment is the seasonal variation in Evapotranspiration (ET), which increases with summer heat and decreases in cooler months. Watering the same amount year-round results in significant waste and potential overwatering.
Many modern irrigation controllers include a “seasonal adjust” or “water budget” feature, allowing the user to scale all programmed zone run times simultaneously by a percentage. If the initial run time represents 100% for the hottest month, a user can reduce this setting to 70% in the spring or 20% in the winter with a single adjustment. This removes the need to manually recalculate and reprogram every zone individually.
In the spring and fall, the seasonal adjust percentage should be reduced to reflect the drop in the ET rate as temperatures and wind speeds are lower. Conversely, during periods of unusually high temperatures or dry, windy conditions, the percentage can be temporarily increased above 100% to compensate for higher water loss. This percentage-based modification ensures the system dynamically adapts to current weather conditions without altering the established base schedule.