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Precipitation rate calculation

Discussion in 'Irrigation' started by F6Hawk, Jul 22, 2005.

  1. F6Hawk

    F6Hawk LawnSite Member
    Messages: 195

    Is there a way to convert gallons used over square footage to get an average inches watered?

    For example, if I used 1171 gallons, and watered 12,000 sq feet, how many inches of precip did I achieve?

    If I am thinking correctly, then 12,000 ÷ 1,171 ≈ 10.25 gals/sq ft. Thinking in familiar terms, a 10-gal aquarium fills about 1.33 cubic feet of volume, which means approx 12 inches of water in a 1-foot square. Now that don't make sense, cuz I know I am not putting that much water down per square foot (neglecting evap & blowing for now).

    According to the RB calculator ( http://www.rainbird.com/calculators/convert.htm ), using gals/sqft/day converted to in/hr, if I put in 10.25 for g/s/d, I get .6851 in/hr. Would I then have to divide this by 3 (if I ran a zone for 20 mins) to get the actual inches laid down? If so, then that is only .2 inches per 20 mins per zone. Man, U am going to have to some SERIOUS running of the zones to get enough water on my yard!

    My soil is sandy in most places (though there are a few spots heavy with clay), so if I understand correctly, I need to water longer & more often due to evapotransporation, right? I can tell after watering once that not enough water is getting down, and I wam just trying to figure out how much time I actually need to run the system to do it right, short of laying cans out in the yard (I don't have enough yet to do that, am eating my veggies every night!).

    Thanks for any insight you can provide.

  2. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Messages: 50,258

    1171 gal = 0.0431 acre-in
    Your lawn = 12000 sq ft = 0.275 acres
    So, 1171 gal = (0.0431 / 0.275) = 0.156 inches, when spread evenly over your lawn
  3. jerryrwm

    jerryrwm LawnSite Bronze Member
    Messages: 1,274

    You can calculate your precipitation rate by setting a series of catch cans across the yard 1 foot apart. Run the system for a measured time and then measure the water in the cans. Get an average provided there are no real high catches and no real low catches. This will give you a pretty accurate rate of precipitation. Then you can calculate your run times based on that.

    If you know the flow (theoretical) of the heads, and the spacing is consistent, you can calculate precipitation rate by the following formula:

    GPM (1 full circle head) x 96.25
    __________________________________ = Precip rate (In/Hr)

    Head spacing x Row spacing

    If you only have half circle heads, then the constant is 192.5

    Using that information, you can determine run times per station per watering day for a given application of water per week (in/wk).

    Armed with this information set your controller accordingly. But don't get sucked into the 1" per week trap. No one has been able to show serious research studies where 1" per week for turf grass is the optimum. There are so many variables that make this almost useless. Things like temperature, humidity, wind, cloud cover, sunlight hours, soil type, type of grass, fertilizer applications, etc, etc. are all factors that affect how much water you need to put down. Some weeks you may need to put down .6 inches, and then two weeks later 2.0" isn't enough. Water your grass when it needs it and water it with what it needs. The grass will tell you when it needs water, and it will also show you when it has had enough to sustain healthy growth. One excellent way to water the turf is by knowing what the E.T. rate for your area is, and then supplying that amount of water plus 5-10% more. The E.T. rate can be obtained from the National Weather Service office near your location. Using that information, water your turf accordingly, and water deeply. Twice a week will help promote a healthy turf with a deep root zone. Some will say that a daily watering provides a good looking lawn, and it does, but if you have several days of extremely hot weather, and you miss watering for several days, the turf is stressed because the roots are shallow and the soil around them dries quickly. Case in point - you go on vacation for 7 or 8 days and the breaker for the controller trips the day you leave. When you get back that lawn is going to be thirsty as hell and show it when you get home. If on the other hand you had watered deeply just before you left, and the breaker trips, you will have missed only one watering and the grass will not be as stressed and it is easy to recover.

    Just my thoughts.

    Jerry R
  4. HBFOXJr

    HBFOXJr LawnSite Bronze Member
    Messages: 1,712

    Here in NJ I water my own and push my clients to water by precip rate and ET. I water every day, 1/7th of weekly ET need. Don't knock it till ya try it. Been doing it for over 5 yr now after over 30 yr in the biz and advising otherwise.

    I do not wait till the lawn is dry or stressed. Insects and disease are not a problem. Test holes show roots way, way down with full moisture all the way.
  5. jabbo

    jabbo LawnSite Member
    Messages: 215

    After I install my system I was going to do the same thing. I have always heard that if you water infrequently and long that you'll get deep roots. And maybe thats so, but I have very sandy dirt and I think my centipede will benefit more keeping the top 2 or 3 inches wet. Anything more than that is going to leach right thru.
  6. HBFOXJr

    HBFOXJr LawnSite Bronze Member
    Messages: 1,712

    negative on that top 2-3 inches of soiul moisture. it is not what u want. the roots will be deep and the moisture will remain constant thru the soil profile/root zone, if you water daily as I described and BEFORE drying and stress occur. if anyone starts this practice on a dry, stressed lawn, I always get them to pump up the moisture level of the entire root zone first, by irrigating longer periods and multiple times per day for a few days.
  7. jabbo

    jabbo LawnSite Member
    Messages: 215

    Yeah, I didn't really mean the top 2 or 3 inches, I just meant in sandy dirt long watering can sometimes waste more water than if you watered for short periods of time more frequently.

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