The precipitation rate, measured in “inches of water per hour,” is the most direct way to quantify how much water a sprinkler system applies to a landscape. This rate expresses the speed at which water lands on a given area, similar to natural rainfall. Understanding the precipitation rate is crucial because the output varies significantly between different systems, making simple run time insufficient. Determining this rate allows homeowners to transition from arbitrary schedules to a precise, conservation-focused irrigation plan, ensuring plants receive the exact volume of water needed.
Why Precipitation Rate is Crucial for Efficiency
Knowing the precipitation rate is fundamental for achieving water efficiency and maintaining plant health. This measurement directly addresses overwatering, which causes significant water waste through surface runoff. When a sprinkler applies water faster than the soil can absorb it, the excess water pools or flows away, failing to benefit the root zone.
Conversely, inaccurate watering based on guesswork can lead to chronic underwatering, encouraging shallow root growth. Plants with shallow roots are susceptible to drought stress and heat damage. A calculated precipitation rate allows the homeowner to match the system’s output precisely to the soil’s absorption capacity and the plant’s hydration needs. This precision conserves water, reduces utility costs, and helps prevent issues like fungal diseases that flourish in overly saturated soil.
The Simple Catch Can Method for Measurement
The most accessible way to determine the precipitation rate is by conducting a simple catch-can test. This method uses common, straight-sided containers, such as tuna cans, to collect the sprayed water. Straight sides ensure accurate measurement of the collected depth.
To begin, place five to ten cans randomly throughout one irrigation zone, positioning them near and far from the sprinkler heads. This accounts for variations in water coverage uniformity. Run the sprinkler system for a precisely timed interval, typically 15 minutes.
After the time is up, turn off the system and use a ruler to measure the water depth collected in each can, recording the measurement to the nearest eighth of an inch. Calculate the average depth by summing all measurements and dividing the total by the number of cans used. This average depth represents the water applied during the test period.
To find the hourly rate, multiply the average depth collected by a conversion factor. Since 15 minutes is one-quarter of an hour, multiply the average depth by four to find the inches per hour. For example, if 0.25 inches was collected in 15 minutes, the rate is 1.0 inch per hour. This test should be repeated for every zone, as rates often differ.
Equipment Variables That Affect Water Delivery
The precipitation rate is not fixed and is significantly modified by the specific hardware and operating conditions.
Sprinkler Head Type
The type of sprinkler head installed is a primary factor. Spray heads disperse water in a continuous fan pattern and typically have a much higher precipitation rate than rotor heads, which distribute water by rotating a stream.
Water Pressure
Water pressure plays a large role, as sprinklers are designed to operate within a specific range to maintain their spray pattern. Low pressure shrinks the coverage area and creates larger droplets, leading to uneven distribution. High pressure atomizes the water into a fine mist, increasing loss due to wind drift and evaporation.
Nozzle Size and Spacing
The size and flow rate of the nozzles, measured in gallons per minute (GPM), directly determine the volume of water released. A larger nozzle increases the flow rate and results in a higher precipitation rate over the same area. The spacing and overlap of the sprinkler heads are also important. The system is designed for the spray from one head to reach the base of neighboring heads to ensure even coverage.
Calculating Optimal Watering Duration
Once the precipitation rate is measured, determining the correct watering duration is a straightforward calculation. Most established lawns require 1 to 1.5 inches of water per week during the active growing season, though this varies by grass type and climate. Dividing the desired water depth by the measured precipitation rate yields the total hours required.
For example, if a lawn needs 1 inch of water and the rate is 0.5 inches per hour, the total run time is two hours. Applying this duration in a single session is often impractical due to the soil’s infiltration rate. To prevent runoff and maximize absorption, the concept of “cycle and soak” must be used, especially for compacted or sloped areas.
The total watering time should be broken into multiple, shorter cycles separated by a 30 to 60-minute soak period. This pause allows the water from the first cycle to fully penetrate the soil without overwhelming the surface. A two-hour total run time might be programmed as four 30-minute cycles with a one-hour rest between each.