大家好，请问一下：
http://focustaiwan.tw/news/afav/201609270011.aspx
Taipei, Sept. 27 (CNA) The Taipei Water Department urged people Tuesday
afternoon to store water as early as possible because the turbidity of the
city's water sources has risen under the influence of a powerful typhoon that
will make it difficult to purify.
... that will make it difficult to purify.
这里的关系代名词that是代替什么？
以句意来说，我觉得是代替前面一整句the turbidity of the city's water sources
has risen，但是that不是都只代替一个“名词”而已吗？
若代替a powerful typhoon，句意又不合。
请教一下，谢谢。