You can use a pump timer to turn pumps on and off.

Pumps are small and inexpensive and so are timers......

Common solenoid valves have a minimum spec. of 10-lbs.
With a gravity feed system you're going to need at least 23 feet of head in order to get close to 10-lbs. at the valve inlet. Not sure if that's practical in your situation.

HI foxden,

I just had a thought. Have you considered a float valve?
Might be just what you need.

Yes hank you are right. I use float valves for the same thing. I get them from a farm supply place Central Tractor. They are used for stock watering tanks. About $9.00 bucks