fishin4cars
True friends just call me Larkin
the 2" drain serves a very important part of the system, This is what allows for a quick flush of water, this is what removes the buildup in the bottom of the filter out and flushes the media in the filter. there is really virtually no pressure there for the most part. If you want to use the water for watering plants during a water change yes it can be done, I would put a 2" sleeve on the end of the pipe and not glue it. make up the fittings needed to reduce it down to a water hose connection that can just be slipped on and off as needed. it's a great idea but you don't want it glued so that you can remove it for back flushing. When back flushing a filter you want to completely dump one full filter volume for sure, maybe two depending on the size pond and how high the filter is. On my filter falls I do two complete flushes, 2500 gallon pond, filter holds about 25 gallons so two back flushes is only 50 gallons in my 2500 gallon pond, any additional water i would like to change i could change out the way you suggested. On the Trash master, it holds 33 gallons of water on a 300 gallon pond, one full back flush is a 10% water change, 30 seconds of back flush and I'm done for the week, the wettest my hands get is going to pick dying lily leaves and turning on the refill hose.