Running a sprinkler system using well water requires understanding the source’s capacity, which differs significantly from municipal water. The safe duration for running a sprinkler is a balance between the pump’s output rate and the well’s ability to refill itself. Over-pumping can lead to significant mechanical damage and a temporary loss of water supply. To successfully irrigate, you must align your watering schedule with the unique characteristics of your specific water source, ensuring the long-term integrity of your well system.
Understanding Well Yield and Recovery Rate
The foundation of safe well operation rests on defining the well’s capacity, measured by its yield and recovery rate. Well yield is the maximum volume of water the pump can physically deliver, typically measured in gallons per minute (GPM) during a controlled test. This rate is limited by the physical characteristics of the well and the pump size.
The static water level (SWL) is the natural, undisturbed water level inside the well when the pump is not running. When the pump turns on, the water level drops to the pumping water level, also known as the dynamic water level. The difference between these two measurements is called drawdown.
The recovery rate defines how quickly the water level returns to the SWL after pumping ceases. This rate, also measured in GPM, reflects the aquifer’s ability to recharge the well through surrounding soil and rock. If your sprinkler system’s demand exceeds the well’s recovery rate, the water level will continuously drop, making sustained watering impossible.
Protecting Your Pump from Drawdown
Exceeding the well’s recovery rate causes excessive drawdown, which poses a serious threat to the submersible pump motor. As water is removed faster than the well can refill, the pumping water level drops closer to the pump intake. If the water level drops too low, the pump begins to draw in air, a condition often called “pumping off.”
This action can cause the pump to lose its prime and engage in rapid on-and-off cycles, known as short cycling. Submersible pumps rely on the surrounding water for cooling and lubrication; running them dry causes a rapid temperature increase that can overheat and burn out the motor windings. To prevent this damage, many modern well systems utilize a low-water cut-off switch that automatically shuts the pump off when the water level reaches an unsafe depth.
The pressure tank helps mitigate the effects of minor drawdown by storing a usable volume of pressurized water. This reserve prevents the pump from starting every time a small amount of water is used, reducing the frequency of pump cycling and extending the motor’s lifespan. However, the tank’s stored volume is insufficient to sustain high-demand irrigation for long periods.
Calculating Safe Daily and Hourly Run Times
Determining a safe run time requires balancing the water demand of your irrigation system against the well’s known recovery capacity. First, you must calculate the flow rate demand of your sprinkler zones by summing the GPM ratings of all simultaneously running sprinkler heads. If the GPM demand of your irrigation zone is higher than your well’s recovery rate, you are operating in a deficit and the well level will drop.
For instance, if your well’s sustainable recovery rate is 4 GPM, but your sprinkler zone demands 8 GPM, you are drawing down the well at a net rate of 4 GPM. To find your safe run time, you must know the usable storage volume of water in your well casing above the pump intake. If you have 120 gallons of usable storage, you can run the sprinkler for approximately 30 minutes (120 gallons divided by 4 GPM net draw-down).
The most sustainable strategy is to run your sprinklers in short, cyclical bursts that allow the well to recover between sessions. For example, instead of running a deficit system for 30 minutes straight, you could run it for 10 minutes, turn it off for 60 minutes to let the well recover the volume used, and then repeat the cycle. This approach prevents continuous, deep drawdown and protects the pump from overheating.
Strategies for Maximizing Irrigation Efficiency
The most effective way to maximize your well’s irrigation potential is to align your watering schedule with minimal water loss due to evaporation. Watering during the coolest part of the day, typically between 4:00 AM and 6:00 AM, significantly reduces the amount of water lost before it can reach the root zone. Watering in the evening is less ideal because moisture remains on the foliage overnight, which can promote fungal diseases.
A technique known as “cycle soaking” is highly beneficial, especially in dense clay soils or on sloped areas. This method involves splitting the total required watering time into two or three shorter cycles, separated by a 30- to 60-minute “soak” period. The initial short cycle moistens the topsoil, and the break allows the water to penetrate deeply without causing runoff, which also provides a necessary recovery interval for the well.
Consider switching to low-flow delivery systems like drip irrigation or soaker hoses. Traditional spray heads may demand 2 to 4 GPM per zone, while a drip system can often operate at less than 1 GPM. By drastically reducing the system’s GPM demand, you can run the irrigation for hours without exceeding the well’s natural recovery rate, allowing for a much longer and more consistent watering duration.