It's winter here and over these month we normally receive ~1m of rain. Our pool has been slowly increasing tds over the years and last summer was nudging over 1500 tds. The local water supply is a dam so top up water tds over summer isnot an issue. The pool has a drain hole in the skimmer box at a level that will give us about 100mm free board, the drain is about 30mm in diameter and we use a rubber stopper to open or close it. The stopper is usually left out over winter and pool level is kept fine.
I had thought that maybe this situation would lead to increase in tds as the water escaping was not average tds, but lower tds, save mixing when the pool cleaner operated. So I took a piece of garden hose that was long enough to sit one end on the bottom of the pool and used some tape around the hose at the other end to make it a snug fit in the drain hole.
We had 50mm rain in last 24hrs, the discharge form the drain is at 1400 tds and water at the top of pool is 960 tds. Looks good so far for preferential dilution.