I have a fish pond which may have a slight leak. Or not - maybe the water is just evaporating. Searching it seems there is not simple way to calculate how fast water will evaporate. So suppose I put a glass beside my pond and measure how far the level of water in the glass has fallen in - say - 24 hours. Say this is D mm. If it is only the surface area of the water that matters then presumably the pond should have fallen by D mm as well. Is this method likely to be successful? @xts. Floating glass is a good idea - the po9nd is 3500 liters so its temperatuerew would vary much less than a glass, but floating glass should solve that. Will have to be tethered as there is a waterfall.