How Long Do I Run My Sprinkler?

Remember the term used for turf and landscape irrigation is Deeply and Infrequently.  Please read through the When to Water section to learn how frequently you should irrigate.  In this section we will discuss how much time it take to apply water 'deeply.'  Watch this quick video to learn about determining how deeply or how long to run the sprinkler then continue reading below.

 

To determine how long to run our sprinkler each time we irrigate, we need to determine what is called the Precipitation Rate.  The precipitation rate is simply the rate at which the irrigation system applies water.  In the United States, it is usually described in inches of water per hour (in/hr). In other parts of the world it may be described as mm/hr or cm/hr.

There are several ways to calculate the irrigation system's precipitation rate by using values for the water pressure and tables from sprinkler manufacturers.  But the simplest way is to use a rain gauge. Watch this video on using a rain gauge to measure the precipitation rate. Basically, you place a rain gauge some short distance from your sprinklers and run the sprinklers for a set amount of time, like 15 minutes.  After the 15 min are up, measure the water in the gauge and do the math to determine how many inches per hour that zone is applying.  In this example, let us suppose we collected 0.2 inches of water in 15 min.  Our precipitation rate would be 60/15 = 4         4*0.2 = 0.8in/hr    Our system applies 0.8inches of water per hour.

From our discussions of ET we talked about wanting to replenish the soil with moisture when we lost 0.75-1.0 inches of water from ET.  And that we should run the system long enough to replenish that 0.75-1.0 inches. In this example, let us assume we lost 1.0 inch of water to ET and we want to replenish that moisture. We should need to run our sprinklers for 1in divided by 0.8in/hr = 1/0.8 = 1.25 hours or 1 hour and 15 min. Below is a table giving an approximate precipitation rate based upon sprinkler type.  Your precipitation rate can vary greatly.  You should only use this table for this discussion. Measure your actual precipitation rate.

 

Soil Factors Relating to Precipitation Rate and Infiltration Rate

Precipitation rate (PR) is the rate at which a sprinkler applies water.  Infiltration rate (IR) is the rate at which a soil can absorb the water.  When the PR is greater than the IR, water will pond on the soil surface and can run off the site into the street, ditches, waterways, or streams.  Water that is running off the site, is wasteful and does not improve the health of your lawn or landscape. The system must be designed and operated to eliminate runoff. Below is a table that depicts the maximum precipitation rate that can be applied to various soil types.

Like it states above, these are averages and approximations.  The infiltration rate of the soil in your lawn will be different. But you can see the problem.  The soil used in many urban landscapes is frequently heavy clay or clay loam.  So the maximum precipitation rate should be approximately 0.2 in/hr, but many sprinkler heads apply 0.5 to greater than 1.0 in/hr.  It is highly likely that if we try to run these sprinklers on heavy clay soils, runoff will occur.

The solution is called Soak and Cycle, which means run the sprinklers long enough to saturate the soil but not cause runoff (Soak). Then wait an hour or several hours and run the sprinklers again (Cycle).  Repeat the process as many times as it takes to apply the full amount of water in one night/early morning.  This irrigation event, soaking and cycling should take place in one night, not over many nights.  That is, soak and cycle so that 1 inch is applied overnight.  Do not soak and cycle by applying 0.25 inches every night for 4 nights. Apply the full inch in one evening, stopping and pausing to allow the water to sink deeper into the soil.

Read on to learn more about Matching Precipitation Rates among sprinkler heads.