Yesterday I topped up my small pond with a hosepipe, something I prefer not to do if I can avoid it. In warm dry weather the rate of evaporation from the surface of the pond and especially from the leaves of all the plants growing out of the pond, can be quite high. When the water level has dropped a few inches its best to hold out for rain rather than top up with fresh water straight away, but eventually there comes a point where plants such as water lillys are affected and also the total volume of free water for the animals in the small pond becomes restrictive. So out comes the garden hose with a good sprinkler attachment and I fill it up slowly, right to the brim. I also fill up the empty rain butt to half full, not expecting any rainfall in the next few days, to make it easier to fill watering cans and if the dry warm weather continues, I’ll tip that into the pond as well.
Ideally there would be such a large reservoir of rainwater collected from a big roof area into huge storage containers that tap water would never be necessary, but that isn’t practical with the setup I have here at the moment. Rainwater is generally preferable to tap water because the tap water contains a background level of nitrate which is not good for the animal and small insect life in the pond, also the PH of tap water is treated to be less corrosive to pipework whereas the pond life is happier, I think, with the slightly more acidic rainwater.
Not only that but also tap water has been expensively treated to make it suitable as drinking water for humans. Its really a waste to use it for other purposes unless absolutely necessary.
Another way to cut down on the need to top up any small pond is to cut back the vegetation somewhat. Water irises once they have flowered can be taken right back to near the surface level, although then you will miss out on the spectacular seed heads.