The total run time for a sprinkler is a calculation derived from how much water your lawn needs and how quickly your specific sprinkler delivers it. Effective lawn watering is guided by one core principle: apply water deeply but infrequently. This practice encourages turf roots to grow downward, making the grass more resilient to heat and drought stress. To achieve precision, you must determine the target amount of water, measure the application rate of your equipment, and calculate the required run time.
Determining the Depth of Watering
The primary goal of any watering session is to saturate the root zone of the established turf. This depth is typically between six and eight inches for a healthy lawn, as the deepest roots are where the grass draws moisture during dry periods. Watering only the surface encourages roots to stay shallow, leaving the lawn vulnerable.
Most established turfgrass requires about one inch of water per week during the peak growing season to maintain health and color. This one-inch rule serves as the baseline for the target volume of water you need to apply. This volume is typically delivered in one or two deep watering sessions per week, depending on your soil type and local climate conditions.
Applying this volume ensures the water penetrates deep into the soil profile, which is necessary for a strong root system. Watering only when the soil starts to dry out trains the grass to seek moisture deep underground, preventing the development of shallow roots that can quickly dry out.
Measuring Your Sprinkler’s Application Rate
Before calculating the run time, you must determine your sprinkler’s application rate, which is the speed at which it delivers water, measured in inches per hour. Different sprinkler types, such as spray heads or rotating heads, have vastly different output rates, making this measurement specific to your equipment. You cannot rely on a manufacturer’s estimate because variables like water pressure and nozzle condition can alter the flow.
The simplest and most accurate way to find this rate is by performing a catch can test. Place five to nine identical, straight-sided containers, such as tuna cans, randomly throughout the area covered by a single sprinkler zone. The goal is to measure the average water depth collected across the zone.
Run the sprinkler for a fixed, short period, such as 15 minutes, which allows for a measurable amount of water. After the time is up, use a ruler to measure the depth of water in each can to the nearest eighth of an inch. Add up all the measurements and divide by the number of cans to find the average depth collected.
To convert this average depth into the application rate in inches per hour, multiply the average depth by four, since the test was run for 15 minutes. For instance, if you collected an average of one-quarter inch in 15 minutes, your sprinkler’s application rate is one inch per hour.
Calculating Your Ideal Watering Time
Once you know the target depth of water needed and the rate at which your sprinkler delivers it, calculating the total run time is straightforward. The formula is dividing the target water depth by the sprinkler’s application rate. This calculation provides the total number of hours required to deliver the correct volume of water.
For example, if the target depth is one inch of water and your catch can test revealed an application rate of one-half inch per hour, the total run time is two hours (120 minutes). Conversely, a sprinkler with an application rate of one inch per hour would only need to run for 60 minutes to apply the same one-inch depth.
This calculated time represents the total duration needed to apply the full target volume. You must perform this measurement for each separate sprinkler zone or type of sprinkler head you use, as different zones will have different application rates. Using the correct calculation prevents both underwatering and the wasting of water.
Adjusting Frequency Based on Environment
The total run time calculated is the amount of water needed, but environmental factors dictate how that time should be applied. Soil type is a major variable affecting water absorption and retention. Clay-heavy soils absorb water very slowly, while sandy soils absorb water quickly but retain it poorly.
For lawns with clay soil, applying the total calculated time all at once will result in significant runoff and waste. Instead, use the “cycle and soak” method, which breaks the total run time into two or three shorter intervals, separated by an hour-long rest period. This rest allows the water to fully soak into the slow-draining soil, achieving deep penetration without surface pooling.
Sandy soil can handle the full run time in a single session, but its poor retention means you may need to water more frequently, such as twice a week with half the total volume each time. Watering should take place in the early morning (4:00 AM to 8:00 AM) to minimize water loss from evaporation and reduce the risk of turf disease. Seasonal changes also require adjustments, reducing frequency during cooler periods or following natural rainfall.