Setting the timer on an irrigation system requires more than guesswork; it directly influences the health of the landscape and the conservation of water resources. Arbitrary runtimes often lead to shallow root growth, plant stress, or excessive runoff, wasting a valuable resource. The goal of any irrigation schedule is to apply the right volume of water deeply into the root zone. Determining the correct duration ensures the landscape receives the necessary moisture without oversaturation or inefficiency. This precision approach balances environmental responsibility with maintaining a thriving outdoor space.
Determining Plant Water Requirements
The starting point for any successful irrigation schedule is understanding how much water the plants require. This need is often expressed as inches of water per week for turfgrass or gallons per week for individual shrubs and trees. The objective is to wet the entire root zone deeply, which encourages roots to grow downward, making the plant more resilient to drought and heat stress.
For a typical established lawn, the desired wetting depth is usually between 6 to 8 inches, while mature trees and shrubs may require moisture penetration up to 12 to 18 inches. Deep watering ensures that the majority of the root mass is adequately supplied. Watering less frequently but for longer durations achieves this depth while allowing the soil surface to dry out slightly between cycles.
Specific water requirements vary significantly based on plant type and local climate conditions, known as evapotranspiration (ET). Warm-season grasses, like Bermuda or Zoysia, typically require about 1 to 1.5 inches of water per week during peak summer heat. Cooler-season grasses, such as Fescue, often demand slightly more, perhaps 1.5 to 2 inches weekly. Landscape beds with mature shrubs and groundcovers may require less, often satisfied with a deep watering every one to three weeks depending on the species.
Measuring Irrigation System Output
Once the required water amount is established, the next step is determining the irrigation system’s delivery speed, known as the Precipitation Rate (PR). This rate measures how quickly the system applies water, typically in inches per hour (in/hr) for sprinkler systems. Using the manufacturer’s specifications can provide a starting estimate, but site-specific measurements are necessary for accurate runtime calculation.
For overhead sprinkler systems, the most reliable method is the “catch can test.” This involves placing several uniform containers, like tuna cans, randomly within a sprinkler zone and running the system for a fixed time, such as 15 minutes. After the test, the water depth in each can is measured, averaged, and then extrapolated to an hourly rate. This test accounts for pressure variations and inconsistent coverage within the zone.
Drip irrigation systems are measured differently, focusing on the flow rate of individual emitters. Drip output is usually quantified in Gallons Per Hour (GPH), with common emitters flowing at 0.5, 1.0, or 2.0 GPH. To determine the total output for a specific plant, one must multiply the GPH of the emitter by the total number of emitters dedicated to that plant. For example, a shrub with three 1.0 GPH emitters has a total output of 3.0 gallons per hour.
Calculating the Proper Runtime Duration
With the required water depth and the system’s output rate known, the calculation for the proper runtime duration is straightforward. The fundamental relationship is Total Runtime equals the Desired Water Depth divided by the System’s Precipitation Rate. This calculation provides the total number of minutes or hours the system must run to satisfy the plant’s need for a given interval.
Consider a sprinkler zone with an average Precipitation Rate of 0.5 inches per hour (in/hr) where the lawn requires 1 inch of water for the current weekly cycle. Dividing the required 1 inch by the 0.5 in/hr rate yields a total run time of 2 hours, or 120 minutes, for that zone. If the goal is deep, infrequent watering, this 120-minute total might be applied once per week.
For drip systems, the calculation targets the volume of water needed by the plant rather than the depth. If a small tree requires 10 gallons of water per week, and the system delivers 3.0 gallons per hour (GPH), the total runtime is 10 gallons divided by 3.0 GPH, resulting in a runtime of approximately 3.33 hours, or 200 minutes. This total time is then scheduled based on the desired watering frequency.
This calculated runtime is the total duration needed to deliver the required water volume. This total time may need to be split into multiple shorter cycles to prevent runoff, especially on certain soil types or sloped areas.
Adjusting Runtime for Soil Type and Slope
The total runtime calculated must be modified based on the soil’s ability to absorb water. Soil composition plays a substantial role in determining how this total time is applied, even though it does not change the total volume of water required. Applying water faster than the soil can absorb it will lead to wasteful runoff and poor water penetration.
Soil Type Considerations
Sandy soil has a high infiltration rate, meaning it absorbs water quickly, often exceeding 1 inch per hour. While sandy soil can handle the entire calculated runtime in one continuous cycle, it retains less water. This means the total runtime may need to be applied more frequently, perhaps every two to three days.
Conversely, clay soil has a low infiltration rate, sometimes absorbing water slower than 0.25 inches per hour. If the calculated total runtime is applied continuously to clay soil, the water will quickly pool and run off, even on flat ground. For these conditions, the technique is “Cycle and Soak” programming.
Cycle and Soak Technique
Cycle and Soak involves splitting the total runtime into several shorter segments, separated by a recovery period of 30 to 60 minutes. For instance, a 30-minute total runtime might be split into three 10-minute cycles. This allows the water to soak in deeply between applications.
Seasonal Adjustments
The overall runtime needs seasonal adjustment. Runtimes typically increase in duration during the peak heat of summer and decrease significantly during cooler spring and fall months.