I have 2 ponds that had this happen in the bottom. If water leaks in it will flow the opposite direction when pressure is applied. Odds are you will need some source of water input (stream , well ect...) to maintain water levels during the dry times. I know in mine the water would stop flowing in like that under drought conditions when drained. I know it was not uncommon to see the pond drop around a inch per day in the summer with no water coming in.