杨鹏难句中一句求解！！

夏溪球子

Only when a system possesses natural or artificial boundaries that associate the water within it with the hydrologic  cycle may the entire system properly be termed hydrogeologic.
这里面the water within it 的it 指代前面的boundaries ，boundaries是复数，为什么能用it 指代呢？？求解！！

夏溪球子

谢谢！！可是杨鹏难句里的解释是说it 指boundaries 指边界里的水....

JIERONG1019

It指代的是system。
本句是only开头的倒装句，the entire system may be termed hydrogeologic only when a system possesses natural or artificial boundaries that associate the water within it with the hydrologic cycle .
that定语从句修饰boundaries，所以谓语用associate，定语从句里面使用了一个idiom，associate A with B. A是water，B是the hydrologic cycle. The water within it中的it值应该是系统，系统里才有水吧，如果指代boundaries的话，边界里面有水貌似逻辑不通。