Friction Loss in 400' of 3/4" is 0.12 psi. In 1" the friction loss is 0.08 psi. If you have enough 1" line, I would use that and reduce it to your 100' of 1/2 poly pipe.

At 8' of depth, your pressure is 3.5 psi. Adding the 0.08 psi (400' of 1") + 0.16 (100' of 1/2") to the depth psi of 3.5 you will have a total of 3.795 psi. I do not know what the back pressure of the type of diffusers you are using so you may see even a little bit higher number on the pressure gauge as a result from the diffuser back pressure.

A Gast 0523 is designed to operate at a continuous pressure of 10 psi. You should have more than enough pressure for your system. Happy ponding!

Cary Martin
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