Two pumps feeding 1 irrigation system?

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Drick

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I have a 375 foot deep 6 inch diameter well producing 2 gpm. The pump is a 1/2 HP rated at 5 GPM. The static water level is 6 feet below the top of the well. Its not great, but it provides all the water for my 4 bedroom house plus irrigation for 20,000 square feet of grass. This all works ok until the temperature climbs into the 90's which is rare in New Hampshire, but when it does happen I cannot draw enough water from the well fast enough to keep the lawn nice and green.

Fortunately my neighbor has a true artesian well with water flowing out at the rate of 4 GPM. I allowed him to dump this excess water onto my property via a 3/4 inch pvc pipe as I have a large area surveyed as wetlands and as such is wet much of the time anyway. I would like to use this "excess" water by adding a 500 gallon retention tank and pump and tying this into my irrigation system to supplement my well.

My question is this; Can I tie in both pumps to the same 1.25 inch pipe that feeds all my sprinkler zones? My thought is this - when my well starts to draw down and the pressure drops the pressure in the 1.25 inch sprinkler pipe will also drop. When this occurs the retention tank pump will see the drop and will be able to start and make up the difference.

Thoughts anyone? Any risk of cross contamination between the supplies? There will be check valves where both pumps tie into the irrigation system so I'm thinking no. Should I use a 10 GPM pump with the retention tank as the sprinkles system is designed close to that number and my deep well can deliver close to that (measured 9 GPM) when the well is full. As an alternative setup I could split the irrigation system in two and run each half on one pump which would help some, but I really wanted to use the retention tank to increase the water delivered to sprinkler heads/zones exposed to full sun so I could run these heads longer. And as luck would have it there is no easy way to group the sun exposed zones together if the system was split in half.

A couple more notes. This is a rather large spread out system. the 1.25 inch main irrigation pipe is 400 feet long and each pump would literally be a opposite ends of the 1.25 inch main irrigation line. The zone valves (12 total) are located in 4 locations along this pipe. Ideally each zone wants 7 GPM.

-rick
 
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