Applying water accurately

Most of us who have been irrigating for decades are used to irrigating based on time: 12 minutes for fairways, nine minutes for greens and so many minutes on the tees, for example. We may irrigate all the greens for the same period of time or only use irrigation on the greens to water something in. Although this has worked well for many, the modern way to think about irrigating is by depth as opposed to time.

It’s like when we use to think about watering by controller zone as opposed to watering by area; greens, tees, fairway, rough. In truth, watering by time is actually watering by depth; you just need to know how to relate time to depth. This is done with precipitation rates.

Precipitation rate is the rate at which your sprinklers apply water in inches per hour. Precipitation rate is directly influenced by sprinkler flows and spacings. Change one, and the precipitation rate changes.

Spacing is pretty much set – the sprinklers are where they are and that doesn’t really change with time.  However, flow is influenced by the nozzles in the sprinkler as well as the pressure at which they operate.  Change a nozzle, and the precipitation rate changes. Have the pressure higher or lower than design due to worn pumps, elevation change or partially closed valves, and the precipitation rate changes. Depending on how good your hydraulics are, precipitation rates can actually change during an irrigation cycle depending on what program you are running.

Let’s look at a few examples. Say you have fairway sprinklers spaced on a 65-foot triangular spacing with a nozzle using 27.4 at 80 psi. The precipitation rate would be 0.72 inches per hour. If the pressure is actually only 60 psi, then the precipitation rate is 0.60 inches per hour as the flow is reduced to 22.8 gpm. If one of your crew replaces nozzles and the new nozzles are larger and use 35.2 gpm, then the precipitation rate increases to 0.93 inches per hour.
If only one nozzle is changed, then the precipitation rate is somewhere between the original 0.72 inches per hour and 0.93 inches per hour. Translating these into time, if you want to apply 0.20 inches per water at 80 percent efficiency, that is 21 minutes of run time at 0.72 inches per hour, 25 minutes at 0.60 inches per hour and 16 minutes at 0.93 inches per hour.

You could then easily also translate that into water use – minutes of run time multiplied by gallons per minute per sprinkler.

Precipitation rates are supposed to be imbedded in your central control database. By entering the nozzle in the database and entering a spacing, the software generates a theoretical precipitation rate that the computer then uses to schedule. If you specify a depth of water you want to apply, the software calculates a corresponding run time for that sprinkler for that depth. Unfortunately, the calculations are theoretical and the spacing configuration and distances that you may have out on the golf course do not always correspond to the options the database gives you. As such, you need to adjust the run times, but you can do that with the station adjust factors.

Ideally, you would have only three precipitation rates for your irrigation system, one each for greens, tees and fairways. It may be ideal, but it is also impossible. Most golf courses will have 18 different precipitation rates just on the greens. The more varied the precipitation rates you have, the harder it is to manage the turf and properly schedule irrigation. To start, you should try and minimize the number of different sprinkler/nozzle/spacing combinations in your irrigation system as it will only get worse over time as the system is maintained and nozzles are replaced or pressures vary. Keep in mind that as sprinklers settle and become unleveled, the precipitation rate will also change as the distance of throw will be affected.

The most accurate way to determine precipitation rates is to do a cup test (audit) of individual golf course features. By actually measuring the amount of water the sprinklers apply, you obtain the precipitation rate for that point in time and you can enter that into your database to make your scheduling more precise. It will also lower the gap between how much water your irrigation control software says you’re your using versus what your pump station flow meter says your pumping.
        
It is extremely difficult to decide how much water to apply to an area if you do not know at what rate the sprinklers apply water. By minimizing the number of different precipitation rates you have out on the golf course, the easier it will be to schedule your irrigation applications. Keep in mind that precipitation rates will change over time and instill in your maintenance staff the need to always replace nozzles with the nozzle that was originally there and to keep sprinklers level and at grade. These simple maintenance procedures will keep your irrigation system from having hundreds of different precipitation rates and as a result higher quality turf conditions by applying water accurately.
 
Brian Vinchesi, the 2009 EPA WaterSense Irrigation Partner of the Year, is president of Irrigation Consulting Inc., a golf course irrigation design and consulting firm headquartered in Pepperell, Mass., that designs irrigation systems throughout the world. He can be reached at bvinchesi@irrigationconsulting.com or 978-433-8972.