Quick Answer
To get 1 inch of water with your sprinklers, you need to run them for enough time to put down 1 inch of water across the area you are watering. The exact run time depends on your sprinkler precipitation rate – how much water your sprinklers put out per hour. Typical spray head precipitation rates range from 1 to 1.5 inches per hour. So to get 1 inch of water, you would need to run spray head sprinklers for 1 to 1.5 hours. Rotary sprinklers and drip irrigation typically have lower precipitation rates, so would need to run longer. The key is to measure your system’s precipitation rate and use that to calculate the needed run time.
Measuring Your Sprinkler Precipitation Rate
The precipitation rate of your sprinkler system, often abbreviated PR, is simply a measure of how much water your sprinklers deliver per hour of operation. It is typically measured in inches per hour.
To determine your precipitation rate:
1. Place several straight-sided containers (like tuna cans) in different areas of the landscape being watered. The containers should be spaced out across the entire watered area.
2. Run each sprinkler zone to be tested for a set amount of time, such as 30 minutes. Make sure no runoff occurs.
3. Use a ruler to measure the depth of water caught in each container, in inches. Calculate the average depth across all containers.
4. Divide the average depth by the run time. For example, if the containers averaged 0.5 inches of water after 30 minutes of sprinkler run time, your precipitation rate would be 1 inch per hour (0.5 inches ÷ 0.5 hours = 1 inch/hour).
Repeat this test periodically, as precipitation rate can vary over time due to changes in water pressure, sprinkler wear, and adjustments to the system.
Calculating Run Time
Once you know your system’s precipitation rate, calculating the required sprinkler run time to achieve a given depth of water, like 1 inch, is simple:
Run Time (in hours) = Desired Depth (in inches) ÷ Precipitation Rate (in/hour)
For example, if your measured precipitation rate is 1.25 inches per hour, then to apply 1 inch of water you would need to run your sprinklers for:
1 inch ÷ 1.25 in/hour = 0.8 hours
So with a precipitation rate of 1.25 in/hour, you would need to run your sprinklers for 0.8 hours, or 48 minutes, to get 1 inch of watering.
Some more examples:
– Precipitation rate of 2 inches/hour → Run sprinklers 0.5 hours (30 minutes) to get 1 inch
– Precipitation rate of 0.75 inches/hour → Run sprinklers 1.33 hours (1 hour 20 minutes) to get 1 inch
The lower the precipitation rate, the longer the required run time for a given depth. Rotary sprinklers may only have PRs around 0.5 in/hour, so could need to run 2 hours to deliver 1 inch of water.
Considerations and Variables that Affect Run Time
When determining required sprinkler run times, keep these factors in mind:
– Sprinkler type – Spray heads, rotary sprinklers, and drip emitters all deliver water at different rates. Sprays tend to be highest, rotary heads lowest.
– Water pressure – Higher water pressure enables sprinklers to discharge at a higher precipitation rate. Lower pressure results in a lower PR.
– Spacing – Proper spacing between sprinkler heads results in uniform coverage and standard PR values. Sprinklers spaced too far apart leave dry spots and lower the system PR.
– Wear – Old, worn sprinkler nozzles and drip emitters discharge less water over time, reducing precipitation rate.
– Wind and evaporation – Wind can blow spray droplets away from the intended area, lowering PR. High heat and wind increases evaporation losses before the water can soak in.
– Soil type – Sandy and porous soils absorb water faster than heavy clay soils. PR may need to be reduced on sandy soils to avoid runoff.
– Slope – Sprinklers on a slope may runoff before the water has time to infiltrate the soil. Reducing PR prevents runoff on slopes.
– Plants – Landscapes with a mix of plants with different water needs make determining run times tricky. Set times based on the highest water users.
– Season – Plants need more water during hot, dry summer months and less in cooler spring and fall. Adjust run times seasonally.
– Environment – Local climate impacts evaporation rates and plant water needs, requiring seasonal adjustments to run times.
The best way to determine and adjust run times for your unique environment is to frequently measure precipitation rate in different seasons and for each zone of your irrigation system. This ensures you are applying an appropriate depth of water for your landscape’s needs.
Run Time Calculator
Use this calculator to determine run time needed to apply a given depth of water based on your measured precipitation rate:
| Precipitation rate (in/hr): | |
| Desired depth (in): | |
| Run time needed: |
Typical Precipitation Rates by Sprinkler Type
Here are some typical precipitation rate ranges for different types of sprinklers:
| Sprinkler Type | Precipitation Rate |
|---|---|
| Spray heads | 1 – 1.5 in/hr |
| Rotor sprinklers | 0.5 – 1 in/hr |
| Impact sprinklers | 0.5 – 1.5 in/hr |
| Stream rotor sprinklers | 1 – 2 in/hr |
| MP Rotator | 0.4 – 0.8 in/hr |
| Drip irrigation | 0.1 – 1 in/hr |
However, precipitation rates vary widely based on specific products, spacing, water pressure and other factors, so measuring PR for your unique system is highly recommended.
Typical Run Times to Apply 1 Inch of Water
Based on typical precipitation rates, here are estimates of how long various sprinkler types would need to run to apply 1 inch of irrigation:
| Sprinkler Type | Run Time for 1 Inch |
|---|---|
| Spray heads | 1 to 1.5 hours |
| Rotor sprinklers | 2 to 3 hours |
| Impact sprinklers | 1 to 2 hours |
| Stream rotor sprinklers | 0.5 to 1 hour |
| MP Rotator | 1.5 to 2.5 hours |
| Drip irrigation | 1 to 10 hours |
Again, these are general estimates only. Always measure your specific system’s PR and use that to calculate required run times.
Impact of Too Little or Too Much Water
Applying the correct amount of water is important for plant health and effective irrigation:
– Too little water – Underwatering stresses plants, leading to drought symptoms like wilting, browning, dry patches, and stunted growth. It makes plants more susceptible to pests and diseases.
– Too much water – Overwatering can starve plant roots of oxygen, encourage fungal diseases, leach nutrients from the soil, and lead to runoff and erosion. Some plants, like succulents, are highly sensitive to overwatering.
Getting your sprinkler run times right avoids these potential issues. Measuring precipitation rate and calculating needed runtimes ensures your plants get adequate moisture without the risks of under or overwatering.
Watering Guide by Sprinkler Type
Here is a summary of estimated run times by sprinkler type to apply 0.5, 1, and 1.5 inches of water, assuming typical precipitation rates:
| Sprinkler Type | Run Time for 0.5 inch | Run Time for 1 inch | Run Time for 1.5 inches |
|---|---|---|---|
| Spray heads | 0.5 hour | 1 hour | 1.5 hours |
| Rotor sprinklers | 1 hour | 2 hours | 3 hours |
| Impact sprinklers | 0.5 hour | 1 hour | 1.5 hours |
| Stream rotor sprinklers | 0.25 hours | 0.5 hours | 0.75 hours |
| MP Rotator | 0.75 hours | 1.5 hours | 2.25 hours |
| Drip irrigation | 0.5 to 5 hours | 1 to 10 hours | 1.5 to 15 hours |
Use these as general guidelines only. Your specific run times still depend on accurately measuring your own system’s precipitation rate.
Tips for Efficient Watering
Follow these tips in addition to using proper run times:
– Water early in the morning or evening to reduce evaporation losses. Avoid mid-day watering.
– Adjust run times seasonally as weather and plant needs change.
– Inspect sprinklers periodically for blocked or broken nozzles causing uneven watering.
– Let lawns and plants dry out between waterings to encourage deeper roots.
– Consider upgrading old spray nozzles to more efficient rotating sprinkler heads.
– Zone your irrigation system according to plant water needs. Separate zones for turf vs. shrubs.
– Water natives and drought-tolerant plants less frequently than other landscapes once established.
– Install a smart irrigation controller to automatically adjust watering based on weather.
– Improve soils with compost to increase water retention. Mulch beds to slow evaporation.
– Consider drip irrigation for bedded areas to apply water right at plant roots and reduce evaporation.
Conclusion
Determining how long to run your sprinklers is easy once you know your precipitation rate. Simply measure how much water your system puts out in an hour, then divide by the depth you want to apply. While general sprinkler run time estimates exist, measuring your unique system’s output is the only way to set accurate runtimes to achieve proper watering without overwatering. Combine run time calculations with efficient watering practices for a healthy, thriving landscape.
