OK this ol boy is gonna tell those that spout about the rate of turnover in terms of the pond (ie 1.5 times).
There is no merit in the action going on in the Porous media shower in relation to pond turnover...the merit is occurring IN the dang pond...ya got that? That is why the amount of water moved is given in relation to POND turnover.....Hopefully you can figure this out with me.
Ya see. if a pond turns over faster then even more dense (ie specific gravity) particles will be carried with the water movement and too the PMS.
thereby keeping the pond cleaner by getting a greater percentage of the denser particulate to the PMS and to the PMS Faster (therby giving the denser particualte less time to dissolve into the water and become something negative and harder to deal with.
Ya see as a kid I hung around the grad students at Auburn (when it was fun) and learned about just how much (actually "little") water movement was needed to keep negatively bouyant particles afloat in the water columns... specifically how fast a current was needed to keep Striper eggs afloat long enough for them to hatch out. Unlike Koi, a Striper's eggs pretty much die if they rest on a surface. They need to lightly bob along within the water column to remain viable...but that was 30 years ago so that theory might be passe...but the science of how little current is needed to keep slightly negative (alot of koi pooh?) moving towards the PMS's quickly hasn't changed. The pooh is held aloft and doesn't fall apart before it gets to the PMS, it and all of its terrible by-products (which could also fertilize multiple algaes?) get sucked up and then broken down to much less toxic (and less usable fertilizers) components.
So there it is. My Opinion on why a "pond turnover" rate and not the gph through the Porous media shower, is the reference given by those that make and advocate PMS's.
Some parts are wrong, but I THINK the main reason why a "POND TURNOVER RATE" is central to the efficacy of PMS's is reasonable, and probable.