A feature of that size would require a constant top up system, probably two 1" mains water supplied ball valves set at slightly different levels so as to coupe with the inevitable evaporation.
In the summer months, during hot weather both valves would quite likely be running continuously!
On similar size constructions that I have been involved with, we have had to install a substantial reservoir fitted with a large pump to coupe with start up & turn off times.
If you can imagine a static pond full of water, the daily evaporation would still be noticeable but when you turn on the water flow there will be a huge demand for water & the main pond will drop dramatically in order to supply the extra water required to operate the water falls.
The same applies but in reverse if the water flow is turned off as the excess water from the water falls and streams will have go somewhere.
So a large reservoir has be located below ground to deal with the changing water levels.
This in effect results in quite a lot of fresh water continually going into the feature ..... however substantial water changes may still be necessary depending on the design of the pond i.e. if it has auto back flush filters or high levels of fish.