Our team at Gardenworks Inc. gets asked this a lot especially during droughts and when water conservation is important. The easy answer is “run your system as long as it takes to apply the necessary amount of water without allowing any water to run off onto the sidewalk”. “There… wasn’t that easy?” No? Well, turns out you are not alone. Most people do not realize that there is actually a scientific method to determine sprinkler run times for lawns. This method isn’t easy or much fun but it is accurate. First, there is terminology that is needed to be known such as Precipitation Rate and Distribution Uniformity and the knowledge of how these are determined. Once those are known as well a few other things, formulas are used to achieve the final determination of how long the sprinklers should run – typically on a weekly basis. The final step is to program your irrigation schedule then monitor. Please follow along. Precipitation rate (PR) is - how quickly water is applied to the target area and is measured in inches per hour. The lower the PR, the longer sprinklers need to run (the goal is to get close to the infiltration rate of the soil) Distribution Uniformity (DU) – is how evenly water is broadcasted over the target area and is depicted as a decimal point. The closer DU gets to 1.0 the more efficient your irrigation system is and the less time sprinklers need to run to make up for inefficiencies. (Most DU’s on older system are less than 50% efficient) Both Precipitation rate (PR) and Distribution Uniformity (DU) are best determined by a field test consisting of capturing water in “catch cans” from the sprinklers for your lawn over a set amount of time and then measuring the amount of water in each can after the time has ended. The average amount of water in the lowest 25% of the catch cans is divided into the average amount of water in all of the catch cans to achieve the DUlq (Distribution Uniformity lower quarter). 24 catch cans are typically used during the test. The six cans with the lowest amount of water in them are used to determine DUlq. Once these two numbers are known the next term we will need to know is – Evapotranspiration rate (Eto). The Et rate is the amount of water lost from lawn and landscaped areas by transpiration (plants breathing) and evaporation. This is actually measured for much of California by your tax dollars and the California Irrigation Management Information System (CIMIS). There are yearly Et rates for all of California available on a website and they can be found in yearly, monthly, weekly and daily rates. The Et for Santa Rosa for the month of June is about 5.9” ( 1.2”per week). Et is basically the amount of water lost to the air by plants and soil. The higher the Et rate the more you need to water to add back to the plants what they have lost. Et is lowest in December and January and highest in July. Next comes the Plant Factor (Kl) – which is a declaration of how needy a plant is with regards to water. It is stated as decimal as well. A high water use plant such as a lawn has a plant factor of 1.0 -.8. Moderate water using plants are .7 - .5 and low water using plants are .4 or less. Now come the calculations: Step 1: Plant Water Requirement (PWR): Eto x Kl = PWR Step 2: Run Time Multiplier (RTM) - this equation adjusts the amount of time needed for watering to compensate for less than perfect Distribution Uniformity (DU): 1 divided by the sum of .4 + the sum of (.6 x DUlq) = RTM Step 3: Irrigation Water Requirement (IWR) – use the RTM above to determine total inches of water needed to apply the PWR from step 1 above: PWR x RTM = IWR Step 4: Irrigation Run Time (IRT) – number of minutes to run each week: IWR divided by the Precipitation rate (PR) x 60 = IRT Step 4: Daily Run Time (DRT): IRT divided by the number of days watering will occur = DRT Step 5: Cycles of watering per day (CPD) to avoid run off: DRT divided by time to run off = CPD Step 6: Station run time (SRT) is the daily run time (DRT) divided by the cycles per day = SRT So there you have it. The proven and taught approach to scientifically determining how long you should run your lawn sprinklers. Or… if you live around Santa Rosa, you can call TURF TIME at 543-3466 or go to www.srcity.org/turftime and they will tell you. Please feel free to contact us at Gardenworks Inc. if you would like more information about the scientific approach to calculating lawn sprinkler run times and water budgets.
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