Drip irrigation is a water-efficient technique that delivers moisture directly to the root zone of trees, minimizing loss from wind and evaporation. Determining the correct run time is not a fixed number but a calculation based on the tree’s specific needs and the irrigation system’s mechanics. A successful watering schedule involves finding the ideal water quantity, translating that into a precise duration, and then establishing the appropriate frequency. This methodology ensures the tree receives deep, sustaining irrigation without wasting water or causing damaging runoff.
Calculating the Required Water Volume for Trees
The first step in setting up drip irrigation is determining the total volume of water the tree requires for a single deep watering event. This volume depends primarily on the tree’s size, estimated by the area underneath its canopy, known as the dripline. The dripline roughly corresponds to the location of the most active water-absorbing roots.
To calculate the relevant area, one measures the radius of the canopy spread, squares that number, and multiplies it by 3.14 (pi). For established trees, the goal is typically to saturate the soil to a depth of 12 to 18 inches, which encourages deeper, more resilient root growth. Shallow, frequent watering only wets the top few inches, leading to vulnerable, surface-level roots.
A general rule of thumb is that one inch of water coverage over one square foot of area equals approximately 0.62 gallons. Therefore, if the calculated canopy area is 100 square feet, and the goal is to apply the equivalent of one inch of water, the tree needs 62 gallons. The volume needed for a two-inch soaking would simply be double that amount, resulting in 124 gallons. This total volume is the foundation for determining the actual run time of the irrigation system.
Determining the Specific Run Time
Once the total required water volume is established, the next step is to translate that gallon amount into an exact run time for the drip system. The duration depends on the output rate of the emitters used, which is measured in Gallons Per Hour (GPH). Common emitters for trees range from 1 to 8 GPH, depending on soil type and tree size.
The total output of the entire system is calculated by multiplying the flow rate of a single emitter by the total number of emitters placed around the tree. For instance, if a tree has ten 2-GPH emitters, the system’s combined output is 20 gallons per hour. The formula to find the necessary run time is simply the Total Required Gallons divided by the Total Emitter GPH.
If the tree requires 60 gallons of water and the system delivers 20 gallons per hour, the resulting run time is three hours (60 gallons / 20 GPH). This calculation ensures the precise volume of water determined in the previous step is delivered to the root zone.
Scheduling Frequency Based on Soil and Season
After setting the run time, the final variable is determining how often this duration should be repeated, which is influenced by soil composition and seasonal weather conditions. Soil type dictates how quickly water drains and how much moisture it can retain. Sandy soils drain rapidly and retain very little water, meaning they require shorter, more frequent irrigation events.
In contrast, clay soils are composed of tiny, compact particles that absorb water slowly but hold onto it for much longer. Trees in clay soil should be watered less frequently, allowing the water to be applied slowly over a long period to prevent surface runoff.
Seasonal changes also necessitate adjustments to the frequency schedule. During the peak heat of summer, when evapotranspiration rates are highest, trees require more frequent watering to compensate for accelerated water loss from the leaves and soil. As temperatures drop in cooler months, the frequency should be reduced because the tree’s water demand decreases. Deep, infrequent watering is preferred for established trees, often occurring every one to three weeks during the growing season, depending on the heat and soil type.
Ensuring Effective Water Delivery and Monitoring
The theoretical run time must always be verified by physically checking the soil to ensure the water is reaching the target depth and area. Proper emitter placement is necessary for effective delivery, with emitters located within the active root zone, starting away from the trunk and extending toward the dripline. The drip system should wet at least 50% of the soil surface area beneath the canopy.
The most practical method for verifying the saturation depth is the “shovel test” or the use of a soil probe. About one or two hours after the irrigation cycle finishes, a probe or small shovel can be used to check the soil moisture. A probe will move easily through wet soil but will meet noticeable resistance when it hits dry soil, indicating the depth of water penetration.
If the moisture does not reach the desired 12 to 18-inch depth, the run time needs to be increased. If water is found well below the target depth or is creating surface runoff, the duration should be decreased. This monitoring step closes the loop on the calculation, ensuring the calculated duration achieves the goal of a deep, sustaining soak.