Determining the correct run time for a sprinkler system focuses on delivering water deep into the soil to promote a healthy root system. Effective irrigation requires watering infrequently but deeply, encouraging plants to establish robust roots that can withstand dry weather. There is no universal setting for how long a sprinkler should run in a zone. The required time depends entirely on local variables like soil composition, plant type, and the specific application rate of your sprinkler heads.
Key Factors Determining Water Needs
The first step in setting a sprinkler timer is understanding the volume of water required, which is dictated by the soil and the plants. Soil type directly influences the infiltration rate—how quickly water soaks in before running off. Sandy soil absorbs water rapidly but drains quickly, requiring more frequent, shorter watering sessions. Clay soil absorbs water very slowly, making it prone to runoff, but it retains moisture for a much longer period. Loam soil offers a balanced mixture, providing a moderate absorption rate and good water retention.
Beyond the soil, the type of plant determines the weekly water volume needed, often expressed as evapotranspiration. Turfgrass typically demands a higher weekly volume of water than drought-tolerant shrubs or groundcovers. This plant-specific water need is the target depth, often ranging from 1 to 1.5 inches of water per week for a healthy lawn during peak season.
Calculating Sprinkler Run Time
Converting the desired water volume into a precise run time requires measuring the actual output of your system. This is accomplished by performing a simple “catch-can test.” The test involves placing several straight-sided containers, such as tuna cans, evenly throughout a zone and running the sprinklers for a set period, often 10 or 15 minutes. By measuring the average depth of water collected, you can calculate the zone’s precipitation rate in inches per hour.
This rate measures how quickly your sprinklers apply water to the landscape. Once the precipitation rate is known, the run time is calculated using the formula: (Desired Depth / Precipitation Rate) multiplied by 60. For instance, applying 0.5 inches of water with a precipitation rate of 1.5 inches per hour results in a 20-minute run time. This calculation provides the total minutes needed to deliver the required water depth for deep saturation.
The Cycle-and-Soak Technique
Running the total calculated time all at once, especially on clay soils or sloped areas, often leads to significant water waste through runoff. This occurs because the soil’s infiltration rate is frequently slower than the sprinkler’s application rate. The cycle-and-soak technique solves this by splitting the total run time into multiple short sessions separated by a rest period.
This method allows the water from the first cycle to fully soak into the root zone before the next cycle begins, preventing surface runoff. For example, a total calculated run time of 30 minutes might be split into three 10-minute cycles. The soak period between cycles should be between 30 and 90 minutes, allowing sufficient time for the water to penetrate the soil profile. This approach ensures maximum absorption and promotes deeper root growth.
Adjusting Timings for Different Zones
The run time calculation must be modified based on the unique characteristics of each irrigation zone. Several factors necessitate adjustments to the base run time:
- Slope: Zones with a noticeable slope should use the cycle-and-soak technique with shorter cycles to prevent runoff. Lower sections of a slope often receive runoff from upper sections, meaning their run time can be reduced.
- Shade: Areas receiving partial or full shade require significantly less water volume due to reduced evaporation. A shaded zone’s run time might be reduced by 30% to 50% compared to an identical sunny zone.
- Sprinkler Hardware: Fixed spray heads apply water quickly and need shorter cycles. Rotor heads apply water more slowly and must run for longer periods to deliver the same volume.
- Drip Irrigation: Drip zones are timed by gallons per hour rather than inches of water. They typically run for hours, not minutes, to slowly saturate the soil around individual plants.