The question of how long to run an irrigation system is complex, extending beyond a simple time setting on a controller. Assuming a uniform duration for all parts of a landscape often leads to overwatering in some areas and underwatering in others. Proper irrigation aims to deliver water deeply and infrequently, encouraging plant roots to grow downward. This makes the landscape more resilient during dry periods. The correct run time is a variable calculation based on three primary factors: the soil’s physical properties, the specific output rate of the sprinkler system, and the current environmental conditions.
How Soil Texture Impacts Watering Duration
The composition of your soil is the foundational factor determining how quickly water must be applied and how often. Soil texture dictates the infiltration rate, which is the speed at which water moves from the surface into the root zone. Ignoring this rate leads to either wasted water through runoff or deep percolation past the roots.
Sandy soil, characterized by large, coarse particles, has a high infiltration rate because the pore spaces are large and well-connected. Water drains through sandy soil very quickly, giving it a low water-holding capacity. Irrigation in this type of soil must be applied for shorter durations but more frequently to prevent the water from leaching past the root zone.
Conversely, clay soil consists of tiny, densely packed particles that create very small pore spaces. This structure gives clay a high water-holding capacity, meaning it retains moisture for longer periods, so it needs less frequent watering overall. However, its low infiltration rate means that water must be applied very slowly to avoid surface pooling and runoff. Rapid application results in wasted water running down sidewalks or slopes.
Converting Water Needs to System Run Time
Once the soil type is understood, the next step is to translate the plant’s water requirement into a specific time setting for the controller. This requires knowing the system’s precipitation rate, which is the amount of water applied over a specific area in an hour, typically measured in inches per hour. This rate is determined through a simple field test using catch cups or straight-sided containers, like tuna cans, placed across the irrigation zone.
After running the system for a measured period, the average depth of water collected reveals the system’s output. For example, if a system runs for 15 minutes and collects 0.25 inches of water, the precipitation rate is 1.0 inch per hour. The total run time is calculated by dividing the desired water depth (e.g., 1 inch per week) by the precipitation rate and multiplying by 60 minutes.
This calculation method differs significantly for micro-irrigation, such as drip emitters, which are rated in Gallons Per Hour (GPH) per plant, not inches per hour over an area. Since drip systems apply water directly to the plant’s root zone, the calculation is based on the total gallons needed by the plant divided by the emitter’s GPH output. This often results in much longer run times, sometimes lasting for hours, to deliver the necessary volume of water slowly and directly into the root area.
Using Short Cycles for Maximum Absorption
Even after calculating the correct total run time, applying all that water in one continuous cycle can be inefficient, particularly in areas with slow-absorbing soil or significant slopes. The “cycle and soak” method is a technique used to address this by breaking the total watering duration into smaller segments. Its purpose is to ensure that the water has sufficient time to penetrate the soil profile fully.
For instance, if the total calculated run time is 30 minutes, the controller should be programmed for three 10-minute cycles. Each short cycle is followed by a “soak” period, typically 30 to 60 minutes, allowing the water to soak into the soil without causing runoff. This rest period allows the initial water to infiltrate, preparing the ground for the next application.
This method maximizes the amount of water that reaches the deep root zone and avoids water pooling or running onto impervious surfaces. By cycling the water application, the necessary amount of water is delivered in a way that respects the soil’s ability to absorb it. The cycle and soak technique modifies how the water is applied, not the total amount of water applied.
Seasonal Adjustments to Irrigation Schedules
The final consideration for run time is that the water needs of a landscape are not static; they fluctuate constantly based on weather conditions. Plant water use is driven by evapotranspiration (ET), a measure that combines the water evaporated from the soil surface and the water transpired by the plants. This rate changes daily based on factors like air temperature, solar radiation, humidity, and wind.
To account for this variability without recalculating the run time every month, smart controllers use a seasonal adjust feature that scales the established base time. The core run time calculated for the hottest part of the year, typically set at 100%, serves as the maximum. In milder periods, such as spring or fall, the controller’s percentage is reduced to 50% or 75%, proportionally shortening the duration of each cycle.
This scaling prevents the system from overwatering when the weather is cool and water demands are lower. During periods of dormancy or extended rainfall, the system can be temporarily turned off or set to a minimal percentage. Monitoring the local weather and making these seasonal percentage adjustments is an effective way to maintain an efficient, responsive irrigation schedule throughout the year.