Need assistance designing irrigation system

[sterns]

Hello all, I'm putting the finishing touches on a gravity fed irrigation system and I would like some informed input on my design.

The system starts at a creek about 500 feet uphill of my garden. The creek usually dries up during the driest parts of the year, which means I need a reservoir in the system.

I have a 250 gallon tank about 200 feet downhill from the creek. The elevation at the tank is about 8 ft below the creek.

I have two 3/4" lines from the creek to the tank. These lines are siphon lines because there is a small rise at the bank of the creek.

The tank has a 2" female pipe thread fitting at the bottom of the sidewall. There is currently a bushing which reduces to 3/4" which has a hose bib screwed into it.

I currently have a 3 way splitter attached to the hose bib. Each of the 3 outputs has a shutoff valve.

Attached to ports 1 and 2 are the two siphon lines. On port 3, I have a 1" pipe with a garden hose fitting.

Currently, the excess water from the siphon lines just flows out the top of the tank.

The 1" pipe runs the remaining 300 feet from the tank to the garden. I have not yet measured the water pressure in the garden, but I believe there is more than 20' of fall from the tank to the garden.

I would like to maximize the flow rate at the garden.

I believe the flow rate is restricted by the fittings at the reservoir. I estimate the inside diameter of the hose bib and 3 way adapter to be 1/2" maximum.

I am considering replacing the hose bib with a 1.5" header pipe which T's to a 1" fitting for my output and two additional 3/4" T's for the siphon lines. While I'm at it, I'll likely add a stand pipe and overflow that allows me to route the overflow.

Am I likely to see an appreciable improvement by doing this?

Bonus: Any simple low cost way to automatically shut off both the supply lines when the tank is full?

 

[etudiant]

I don't know whether it would be durable enough or whether frost is a consideration, but a standard toilet shutoff valve should be able to cheaply shut off the supply lines.
Separately, 250 gallons seems very little for irrigating a garden in a climate where the creeks run dry.
Also, based on your description, the irrigation flow is limited by the 3/4 inch bushing and because the tank is open, the maximum flow rate is set by the water level in the tank, which determines the pressure at the bushing outflow. The 1 inch pipe should be able to handle that volume nicely, but as the flow is choked by the bushing, don't expect a geyser.

 

[russ_watters]

Nice project. I think you'll see considerable improvement by increasing the size of the header and fittings. Etudiant is right though; you may be able to get the flow high at low pressure, but you won't get much pressure. Hopefully that will be acceptable.

Have you considered burying the pipe to eliminate the need to siphon? How about filling from the top of the tank? These changes will help fill the tank faster.