difficult sentence 34

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

请问 it是指boundaries吗?可是单复数不一致???是不是书上有误??

列出原文，供参考：

Only when a system possesses natural or artificial boundaries that associate the water within it the hydrologic cycle may the entire system properly be termed hydrogeologic

只有当一个系统拥有自然的或人工形成的边界来把边界中的水与水文循环联系起来的时候，才有可能把整个的系统恰当的成为与水文地质学有关。

难句类型：复杂修饰、倒装

解释：only放在句首时，主句的情态动词may倒装，这是我们高中就已经学过的东西。然而句首的only与倒装的may距离如此之远，这在以前却没有见过，所以读上去让人不习惯。另外一个难点在于，前面的由when所引导的时间状语从句中，宾语boundaries后面跟了一个定语从句，that associate the water within it with the hydrologic cycle,而且从句中的associate with又被water within it所分开，读者还要分神去考虑it所指的是什么，所以读到后面倒装的may的时候，难免不知所云。

意群训练：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.