Question
How do i figure the water demand for my Irrigation system. I
want to install a large enough Rainwater catch basin. I have
the specs. on Toro 570 with 40 pSI. Using 3 full head 7 1/2
and 3 1/4 I get 345 gpm. Can this be right? Is there a
formula?  

Answer
Hey Robert, you are on the right track. Lets say you look at the specs on the toro nozzles and it says a 12 foot full circle head uses 2.5 GPM and the 12 foot 90 degree (1/4) uses 1 GPM at 40 PSI. So you have 3 full heads and 3 1/4 heads. You calculate the GPM as 3 x 2.5 GPM = 7.5 GPM and 3 x 1 = 3 GPM, and add these together 7.5 plus 3 equals 10.5 GPM. This means that if you have a zone with 3-12 foot full nozzles (at 2.5GPM each) and 3-12 foot quarter nozzles (at 1GPM each), it would use 10.5 GPM (gallons per minute). Lets say you ran this zone for 10 minutes it would uses 105 gallons of water (10.5 GPM times 10 minutes is equal to 105 Gallons (10.5x10=105)).I hope this makes it a little easier to understand. It really depends on how long you run this zone that would determine how many gallons of water you would need. If you ran it for 20 minutes you would use 210 gallons of water (10.5 x20=210). Good luck and let me know if you need anymore help, thanks and have a great day.