A reservoir is a semi-closed system: the water entering and leaving it are not in balance, and organic matter which settles in the reservoir causes an "aging" process. The older a reservoir is, the poorer the quality of its water.
水庫又是半封閉系統，進、出的水體並不平衡，有機物質沈澱水庫，「水老質衰」，水庫越老，水質也越差。