Determining how long a sprinkler system should run requires a calculation based on several specific variables. Efficient watering delivers the precise amount of moisture to the plant root zone without waste, such as runoff or excessive evaporation. Calculating the total runtime requires understanding the landscape’s water needs, the soil’s absorption capacity, and the output rate of the installed sprinkler heads. This approach ensures water conservation while promoting deep, healthy root growth.
Understanding the Cycle and Soak Method
Running a sprinkler continuously for a long duration often results in significant water waste, especially on sloped areas or in soils with high clay content. To counter this inefficiency, the “cycle and soak” method divides the total required watering time into multiple, shorter segments. This technique is designed to prevent the application rate from exceeding the soil’s infiltration rate, which is the speed at which the ground can absorb water.
For example, instead of watering a zone for 15 minutes straight, the timer is programmed for three 5-minute sessions. These shorter cycles are separated by a “soak” period, typically 30 to 60 minutes long, which allows the water from the previous cycle to penetrate deeper into the soil structure. The first short cycle breaks the surface tension of the soil, preparing the soil for subsequent applications.
Determining Water Requirements Based on Soil Type
Before calculating the minutes a system should run, the total quantity of water required must be established, which is heavily influenced by the soil composition. Most established lawns require approximately 1 to 1.5 inches of water per week, either from irrigation or natural rainfall, to remain healthy. However, the soil type dictates how this weekly amount should be delivered in terms of both frequency and duration.
Sandy soil, characterized by large particles and pore spaces, absorbs water very quickly, sometimes at a rate exceeding 2 inches per hour. It holds a low amount of available water for plants, making them prone to drought conditions within a few days. This type of soil needs less water per session but requires more frequent watering to replenish the quickly drained moisture.
Clay soil, conversely, has small particles and tight pore spaces, which makes its infiltration rate very low, often less than 0.25 inches per hour. Applying water too quickly to clay soil causes pooling and runoff, yet when fully saturated, it holds a large amount of water. Clay soil should be watered slowly and deeply, allowing a longer soak time, which means the required 1 to 1.5 inches can be applied less frequently, sometimes only once a week.
Loam soil represents an ideal mixture of sand, silt, and clay, offering a balanced water absorption rate, typically between 0.25 and 2 inches per hour, and an excellent water-holding capacity.
Calculating Zone Runtime by Sprinkler Head Type
Translating the landscape’s water requirement into an operational runtime in minutes depends on the sprinkler head’s precipitation rate. This rate is the speed at which the sprinkler delivers water, measured in inches per hour. Spray heads, which are designed for smaller areas, have a high precipitation rate, meaning they deliver water quickly in a fixed pattern. These heads may apply 1.5 to 2 inches per hour, requiring very short run times to avoid runoff.
Rotor heads, which are used for larger areas, rotate slowly and emit a single or multiple streams of water over a greater distance. Rotor heads have a significantly lower precipitation rate, typically ranging from 0.5 to 1 inch per hour. It is not recommended to mix different head types on the same irrigation zone because of their wildly different precipitation rates.
The specific runtime for a zone can be estimated by using the simple calculation: (Required Inches of Water / Sprinkler Head’s Precipitation Rate per Hour) multiplied by 60 minutes. For instance, if a zone needs 0.5 inches of water and the rotor heads have a rate of 0.75 inches per hour, the resulting total run time is 40 minutes. This total duration is then broken down using the cycle and soak method to ensure the soil absorbs the moisture without waste.
Adjusting Run Times for Seasonal Changes
The calculated runtime establishes a baseline for irrigation, but it is not a static schedule that remains constant throughout the year. Plant water needs change with seasonal temperature fluctuations and weather patterns. In hot, dry summer months, the rate of evapotranspiration increases, meaning plants lose moisture more quickly, necessitating an increase in the frequency or duration of watering.
Conversely, during the cooler spring and fall months, the rate of water loss decreases, allowing for a significant reduction in the runtime. Many modern irrigation controllers feature a “seasonal adjust” or “percent adjust” function that allows the user to modulate the base runtime across all zones simultaneously. Instead of manually recalculating each zone, a user can simply set the system to run at a percentage of the peak summer schedule, such as 50% in the spring or 25% in the late fall.