Irrigation zones in a home system are areas managed by a single valve, typically grouping plants with similar water needs, sun exposure, and sprinkler types. Efficient watering requires applying the exact volume of water needed by the plant and soil, delivered at a rate the ground can absorb. This science-based approach promotes deeper, healthier root growth and results in water savings. Precision requires understanding the unique variables that dictate how long a zone should run.
Essential Factors Determining Watering Needs
The required amount of water for any zone depends on the soil structure and the target depth of the plant roots. Soil texture, determined by the proportion of sand, silt, and clay particles, dictates both the infiltration rate and the water holding capacity. Clay soils absorb water slowly (0.11 to 0.31 inches per hour) but retain moisture for longer periods. Conversely, sandy soils allow water to infiltrate rapidly (0.74 to 1 inch per hour) but drain quickly and hold less available water.
Loamy soils offer a balance, with moderate infiltration rates and good water retention, making them flexible for irrigation scheduling. The soil’s capacity to hold water influences how frequently irrigation is needed; sandy soils require more frequent, shorter cycles compared to clay. Understanding the soil type is foundational because applying water faster than the infiltration rate leads directly to wasteful runoff and shallow saturation.
The target root depth determines the necessary depth of water penetration. For established turfgrass, the goal is typically to saturate the soil six to eight inches deep to promote deep root systems. Shallow watering encourages roots to stay near the surface, making the grass susceptible to stress. Watering deeply and infrequently trains roots to grow into the moist, lower soil layers, increasing the plant’s resilience.
Measuring Water Delivery (Precipitation Rate)
To match the plant’s needs with the system’s output, it is necessary to measure the Precipitation Rate (P-Rate), which is the speed at which the irrigation system delivers water, expressed in inches per hour. This rate varies significantly between different types of sprinkler heads; for example, spray heads apply water much faster than rotor heads. The manufacturer’s stated rate is often inaccurate due to variations in water pressure, line size, and system uniformity.
A simple method to determine the P-Rate is the catch-can test, which requires placing several straight-sided containers, such as tuna cans, randomly throughout the zone. The sprinklers should be run for a short, measured duration, typically 10 to 15 minutes. After the run, the depth of water collected in each can is measured in inches.
To calculate the P-Rate, first find the average depth collected by dividing the sum of all measurements by the number of cans. The formula converts this average depth into an hourly rate: P-Rate (inches/hour) = (Average Depth collected / Run Time in minutes) \(\times\) 60. For instance, if the average collected depth is 0.25 inches after a 15-minute run, the P-Rate is 1.0 inch per hour.
Calculating Total Zone Run Time
Once the P-Rate is known, it is combined with the desired water depth to calculate the total run time required. The goal is to determine the duration needed to deliver the target inches of water into the soil. For example, if established turf requires 1.0 inch of water and the zone’s P-Rate is 0.5 inches per hour, the total run time can be determined.
The calculation is: Total Minutes Required = (Desired Inches of Water / P-Rate in Inches per Hour) \(\times\) 60. Using the example, to deliver 1.0 inch of water with a P-Rate of 0.5 inches/hour, the total run time is 120 minutes. This duration represents the total time the sprinklers must operate to saturate the root zone to the desired depth.
This total run time is the required duration for a full irrigation cycle, which may span several days or a week, depending on local climate and evapotranspiration rates. The amount of water needed is determined by how much moisture the plant has lost since the last watering. Using the calculated run time ensures the system delivers a precise volume of water.
Implementing the Cycle and Soak Method
The total run time calculated from the P-Rate and desired depth cannot usually be applied in a single, continuous session. Many soils, especially those with high clay content or areas with slopes, have an infiltration rate much lower than the sprinkler system’s P-Rate. Applying the full duration at once causes water to run off the surface before it can soak in, leading to waste and poor root saturation.
The cycle and soak method addresses this issue by breaking the total calculated time into several shorter cycles, separated by a period of rest. For example, a 30-minute total time might be divided into three 10-minute cycles. The purpose of the soak period, typically 30 to 90 minutes, is to allow the water from the first cycle to fully infiltrate the soil profile.
The initial burst of water breaks the soil’s surface tension, and the subsequent soak time ensures that moisture moves downward. When the next cycle begins, the soil is primed to absorb water more efficiently, preventing surface pooling and runoff. This methodology ensures the water reaches the lower root zone, promoting deep root growth and drought tolerance.