My parents have a cistern and a sump pump which would be useful to have some sort of water level monitoring.
In the past, we would check the cistern level with a pressure gauge calibrated in inches of water. There was a rubber squeeze bulb which which would pressurize a plastic line which fed into the cistern, and the resulting level would be shown on the gauge.
In order for the sump pump to be monitored accurately, the plastic hose from the pressure sensor must be slightly above where the water level when the pump turns off.
The Cistern hose is connected to a small air pump which is cycled occasionally.
Sensors on this board include:
I remember this from a Better or Worse comic: How much sump could a sump pump sump if a sump pump could pump sump?