I have used water recirculated running down the front of perspex, into a catchment tank on stage & being pumped back again. There is no splash what so ever, but how big does it need to be?
This is the same principle as the ornamental garden waterfalls & the stage lighting is bounced off, high lighting the water flow.
Also I have used a rotating cylinder upstage of a cut cloth with scrim inserted, giving an impression of moving water over a water fall, similar to a moire paten.
Also with splash from an over head sprinkler rain effect, from upstage of s scrim, is effective in containing it.
But this probably not what you want.

The actual tank or pond catchment can be self contained, even on a truck. Therefore as it is a recirculating pump, there is no need of a drain.
With the perspex or plastic mirror, if you glue on rocks or what ever to break up & undulate the surface area, this would make it a bit more realistic.

I have yet to make a waterfall, but I have seen a small garden feature where the catch basin had alluminum screen (the same for sceen door and windows) across the top. This did help control some splash. Like Gaffa mentioned, keep it self contained. Build your catchment as part of the fall and face it to match.

Bewdy Spectrum!
That?s something I have never thought about!
It would have been lucrative a few years ago, when you had to spend a penny, to use the loo in theatres.
But that was only in the female toilets, the blokes got to soplash the porcelian for free of course.
I wonder if we shouldn?t bring it back & put a $2 coin vending machines on all the dunny doors.
Then ensure subtle waterfall music is played in the foyer/house  & with the interval music.
The coin mechanism is easy to make that weighs the coin, rejects slugs & has a refund as well!
Let?s face it if major Railway Stations & even some hotels get away with it here, why not amateur theatres?
It reminds a bit of a theatre we played in at John O?Groates in Scotland. Who would purposely stoke up the central heating in the middle of winter, during the show, to sell more ice cream & cool drinks,.
Now this could be lucrative & something to ponder!

It would've been funnier if, every time someone flushed the toilet, the face would spit more water farther.