How long to run your irrigation system is simple and complex at the same time. You should run your irrigation system for as long as it takes to fill the plant’s root zone. No more, no less. However, several variables impact how long it takes to fill the root zone and makes this more complex. Understanding those variables and how they affect watering times is vital for water conservation.
For mature plants, in almost all cases, you should give a plant the same amount of water every time you irrigate. To better understand this concept, knowing a few key terms is helpful.
Soil Saturation Point – Soil holds only water, no air. Plants can’t live in this condition.
Field Capacity – The amount of soil moisture or water content held in the soil after excess water has drained away.
Permanent Wilting Point – There is water in the soil, but the water is not available for plants, so the plants die.
Oven Dry – The opposite of saturation. Oven dry occurs when there is no water in the soil. Plants don’t live here either.
Available Water Holding Capacity or Plant Available Water – The difference between permanent wilting point and field capacity. This is the amount of water available to plants. This is the zone where plants thrive.
Managing water efficiently means watering when water in the soil reaches a trigger point for watering. A popular trigger point for most plants is 50% of the available water holding capacity. Precision irrigation means watering when the water in the soil hits the trigger and irrigating until 95% of field capacity is reached. Maintaining this balance means plants are never too full or too thirsty. Keeping this balance helps plants thrive.
How Fast Does Water Move Through Soil – Infiltration refers to water entry into a soil surface, and percolation is the movement through the soil. Infiltration is impacted by the vegetative cover, temperature, texture, structure, and moisture. Percolation varies by soil type. For example, sandy soils have greater percolation than clay soils. A simple soil percolation test helps identify how fast water moves through the soil. You can learn how to do one here.
Plant Root Depth – The depth of the roots is key to how long to water. The easiest way to learn plant root depths is from charts. Here is an excellent chart for vegetables and herbs. Root depths can be found easily on your growers’ website or extension service websites. This is important because watering past the root zone wastewater and nutrients. Also, remember 70% of the water taken up by plants is in the top half of the root zone.
Consider The Slope – If you water on flat soil, the water tends to move down. However, if there is a slope involved, water will move in the direction of the slope, impacting how much water reaches your root zone.
How Much Water Does Your Irrigation System Deliver – Irrigation emission devices apply water at a wide variety of rates. For example, a pop-up sprinkler, with a half-circle nozzle, at 30 psi and 15-foot throw applies 1.85 gallons per minute or 1.58 inches per hour. A one gallon per hour drip emitter delivers one gallon every hour or 1.6 in/hr per one square foot.
Precipitation Rates for Drip (ft²)
- 6 in/hr = 1.0 gph/ft²
- 44 in/hr = 0.9 gph/ft²
- 96 in/hr = 0.6 gph/ft²
- 64 in/hr = 0.4 gph/ft²
All the major manufacturers have their gallon per minute or gallons per hour and precipitation rates on their websites.
One Final Factor – Distribution Uniformity – Distribution Uniformity measures how uniformly water is applied to the area being watered. For example, distribution uniformity of 100 means all the irrigated area is getting the same amount of water. Typical pop-up spray head irrigation has a distribution uniformity of around 50. To get the desired amount of water, you need to divide the applied water by the distribution uniformity. So with pop-up sprinklers, you need to water twice as much (need one inch. 2 inches x .5 = 1 inch). Drip irrigation systems often have 90 distribution uniformity or higher.
Putting It All Together – These are two simple examples to help you understand the concepts above.
Example 1 – Imagine irrigating cool-season turfgrass in sandy loam soil. First, check the root depth of cool-season turf. You find 2-6 inches for most cool-season grasses. Therefore, we will use 4 inches.
This week 1 inch of water needs to be replaced in the soil due to evapotranspiration.
The pop-up sprinklers used apply 1.58 inches per hour.
Distribution uniformity is 50, so every hour, .79 inches are applied. Therefore, to apply one inch of water, you need to run your sprinklers for 76 minutes.
However, running the sprinklers one time for 76 minutes might push the waterway past the root zone of 4 inches. Loam soils take up between ¼ and 2 inches per hour. 1 inch is all that is needed, but we also know 70% of the water uptake will be in the first 2 inches of soil. Splitting the water into four separate cycles would be best. ¼ inch each application to fill the 4-inch root zone for a total of 1 inch. To achieve this, we run the sprinklers for ¼ of 76 minutes or 19 minutes to fill the root zone with water. Watering every other day.
Example 2 – Imagine a shrub with a 12-inch root zone watered with a .6 gallon per hour emitter. The shrub needs one inch of water this week.
You could water one time during the week for one hour and achieve your 1 inch per week. However, we want to keep the water in the upper half or top 6 inches, so three irrigations of 20 minutes would be better.
The Key – Apply the proper amount of water each time you irrigate. The only thing changing is the frequency. If it gets hotter and the turf or shrub needs more water because the root zone is depleting quickly the frequency increases. So, with more heat, turf would move to daily water and the shrub to 4 times a week, for example, for the same amount of time.
If evapotranspiration were less (due to cooler temps, for example), water would be applied for the same time, and the frequency would be less. For example, once every third day for turf and maybe twice for the shrub.
Final Thought – This is an exact science, but every situation is different due to environmental factors, so observation is critical. A simple observation exercise turns your irrigation on and timing how long it takes before the water stops easily penetrating the soil. This is where water applied is greater than the water absorbed and wastes water. Knowing the length of time this takes helps tremendously in determining how long to run your water.
Understanding the concepts above is an excellent start to better water management and making better water managers. Smart controllers using Jain Unity do all the work for you and take hourly calculations to determine the best frequencies to water. It would be practically impossible to make all the calculations needed on an hourly basis for any water management; however, knowing the concepts help you to manage the controller better. Please check back in the next few weeks to learn how to determine watering frequency best.