I didn't quite follow why you said "stop pumping if the property starts to flood up" but apart from that it seems to me that what you want is a bilge pump with the float switch used to set the level you want to hold the water at.
If the area outside the pond is already flooded, what that means is that any water that is pumped out of the pond will ultimately just flow back into the pond, which is why it's better to just stop and wait until the water drains out a little before we start pumping again.
I'm thinking of the issues regarding a physical float switch. Will it work correctly in a body of water as big as a pond?
Now I'm a little unclear what you're trying to achieve. If you're simply pumping the pond out to the surrounding land, couldn't you simply install an overflow pipe?
I think I follow you now. I suggest using a float switch in the pond set to turn the pump off at the 'low' level, and a float switch at the municipal output set to turn the pump off at the 'high' level, and a third override switch to turn the pump off when heavy rain is not expected. You could control this override switch manually. If you can tolerate a wired solution then you can get this working without any electronicsw at all - just the three switches wired in series with the pump.
If you come up with a weather prediction system you're willing to trust, I expect it would run on the PC. You could replace the manual override switch with a relay (power switch tail, X10 outlet etc) controlled from the PC via an Arduino. If the weather prediction system seems to work, you might choose to turn the whole system off manually during seasons where you think heavy rain is unlikely and want to avoid dumping your precious irrigation water due to a forecasting error.
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