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.
解析: beginning 开始的一系列分词都是修饰主语 The hydrologic cycle,分别是 beginning /passing / passing / returning。值得注意的是, thence along and into the ground surface 是 thence passing along and entering into the ground surface 的省略形式,但是看上去意思却很清楚。类似这样的省略,是出题者合理地利用了省略来达到改编学术论文时压缩文章篇幅的目的 。