PeterH:
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?
Allow me to re-describe my problem.
I have an uneven property where there are two ways for the water to flow, one is through a smallish drain towards the municipality and another to the pond on the premises. Usually the water output through the municipality pipe is low and when we have heavy rain, it becomes useless, which is why to alleviate flooding we prepare the pond to accept rainwater by pumping some water out. However during normal periods the pond is used as a water source for irrigation which is why the low pond levels should not be maintained all the time such as with a simple float switch + pump combination.
The way it works now is that when we see dark clouds we will start pumping. If there is heavy rain and the municipality's system is inundated and the pond is full then all pumping should stop until the water level has gone down again.
This is a relatively common problem in this part of the world and many do choose to maintain a permanently low water level but this wastes a lot of water.
Ideally the rain prediction should come from a barometer or a weather feed and the water levels indicating whether to pump or not should come from a series of sensors.