lw,

Is this needed for pump sizing? You need to look at this from another angle. What is your water demand for each 3/4" lateral. What is the peak flow rate that you will need when your plants are mature? In other words, How much water will your plants require at their maximum need and what is that timing? Is this for a tree farm or nursery?

lehua,

This is for a pump sizing. This is actually for a vegetable farm, and some laterals will be used for sprinkler heads while others will be for soaker hose line (250 ft per lateral, which I'm getting efficient work on at about 20 PSI currently).

I'd like timing for the line to be 1 hour on, preferable watering at night or early morning, and the plants will not need more than a gallon per day (or even two during fruiting seasons)

I'm not sure why I need to look at it from a water demand angle. I can make the sprinkler heads and soaker hose all work efficiently at about 20 psi for my use, and with a consideration of constant water flow for that particular hour, it seems like I will only need to know what kind of pump will sustain that constant pressure on the mainline. However, I am new at this whole irrigation design thing, so I am looking into online courses on design and integration.

Just a few hours ago, I had an epiphany and realized how you were right to ask those questions, and how I was so wrong-headed in mine.

I found out today that about 4 GPM is pretty efficient for my soaker hoses, and my sprinklers, though 5 or 6 GPM would probably be better. How can I calculate my total GPM need on the mainline for 20 4GPM laterals?

lw,

Flow is additive so 20 lateral would require a 80 to 100 gpm single source if you use one zone. One usually requires a 20% added insurance flow to make sure you have adequate flow. That is 100 to 120 gpm. If you had two zones you would need 50 to 60 gpm and so on. You can research a 3/4" pipe flow at 20 psi on-line. The flow rate should be around 7 to 12 gpm so you are good there. If your system was using an undersized pipe size, your system would have very high water velocity with water hammer and noise. Your 2" pipe is should be okay at 20 psi but it might be close. You could use a higher pressure for the mainline and reduce the pressure at each lateral. In line pressure regulators are not expensive for 3/4" pipe. You could run the mainline at 40 psi and reduce to 20 psi at the laterals. So a centrifugal pump with 40 psi head with a flow rate of 140 gpm would be adequate or any flow rate lower is you go to more zones. Now it is just a cost isssue. GL JMHO Aloha.