Estimates indicate that the aquifer contains enough water to fill Lake Huron, but unfortunately, under the semiarid climatic conditions that presently exist in the region, rates of addition to the aquifer are minimal, amounting to about half a centimeter a year.
分析：长难句之所以长，大部分的句子是因为有连词的缘故，并列连词如and，转折连词如but，yet，while，whereas，当这些连词出现的时候，意味着这个句子不仅仅只有一个主干，在此句子中我们就会看到一个转折连词but，所以同学们能够清楚的是but前后各会有一个主谓宾结构，前面的句子主语是estimates， 谓语是indicate，后面跟了一个that引导的宾语从句，而宾语从句起到主谓宾的实体作用，所以不能删，所以第一个主干是Estimates indicate that the aquifer contains enough water，but连词后面的句子under….region,做了一个状语，可以删除, 而amounting to….a year做了一个伴随状语，也可以删除，所以主干是rates of addition to the aquifer are minimal, 所以这个句子提完主干的结果是Estimates indicate that the aquifer contains enough water，but rates of addition to the aquifer are minimal. 翻译是估计表明蓄水层有足够的水，但是增加的水率却很少。