How to stop siphoning?

I live on a hillside. I am using a cistern and irrigation jet pump to water a large lawn. My problem is that the lawn is below the water level of the cistern and even when the pump is not on it siphons past the foot valve. The water level difference is about 8 ft. BTW, When we did not water for a couple of days we lost prime in the pump. Could it be that the foot valve is leaking. It is the only thing that keeps prime for the jet pump. On the same note. Next year we will be adding a connection to the same cistern from a well that has a static water level above the cistern. how do I prevent the same siphoning problem for that scenario?
Thanks
Kent

You have a water leak on the outlet side of the pump, or water couldn't run out the cistern, through the pump and down the hill. Right?

You lost prime because all the water ran out the tank, through the pump and down the hill through whatever leak you have. You have to find and fix the leak(s).

The pump is connected to a sprinkler by way of a hose, so the leak is at the sprinkler. What I am looking for is something that stays closed at minimal pressure but opens when the pump comes on. The pump is on a timer. Something similar to the way a foot valve works. This setup is just temporary this year. Next year we will put a submersible in the cistern and pressurize the hydrants with it. That is why the most important question is how to prevent the well from siphoning next year since the water level of the well is above that of the cistern. I will have a float switch in the cistern to control the well pump.

The tank is draining through the pump and sprinkler, not siphoning.

You can not use a check valve unless it is a motorized tpye that will open before the pump comes on.

The only thing I know of is a normally closed solenoid valve that will turn on (open) as the pressure switch closes.

OR!!! shut off the hose to the sprinkler.

Or look into one of these (if I'm interpreting your problem correctly)

http://www.hunterindustries.com/Resources/PDFs/Instructions/Domestic/23561w.pdf

That is exactly what I am looking for.
Thank you Riverside67