Merged similar topic.

Regards

I can comprehend the idea and the context of the sentence. In fact, I actually chose A and G
Nevertheless, I cannot understand "tighter succession" since I am non-native English.
Can you explain the concept of E to me, thanks ?

The correct words for the blanks are (i) worrying, (ii) tighter succession, and (iii) far more likely.

Here is a breakdown of why this combination works best:

Understanding the Context
The sentence sets up a contrast. The thing that is worrying the engineers is not a simple increase in total rainfall, but something else about the storms themselves. This suggests a shift in focus from the quantity of rain to the pattern of the rain.

Blank (i): (A) worrying
The phrase "is not so much" indicates that the real concern is something other than the predicted increase in precipitation. The blank needs a word that expresses this concern. Worrying fits perfectly, as the engineers are clearly troubled by the situation. "Comforting" and "unimportant to" are the opposite of the sentiment expressed.

Blank (ii): (E) tighter succession
This blank describes the pattern in which the storms will arrive. The consequence of this pattern is increased flooding, as described in the third blank. Therefore, the pattern must be one that leads to more flooding. Tighter succession means the storms will occur closer together, which would saturate the ground and increase the risk of flooding. "More predictable patterns" and "greater isolation" would likely lead to less, not more, flooding.

Blank (iii): (G) far more likely
The final blank describes the effect on flooding. The likelihood of flooding in low-lying areas is increasing due to the new storm pattern. Far more likely directly states this increased probability. "Somewhat infrequent" and "all but impossible" are the opposite of what the context demands, as the earlier part of the sentence establishes a cause for increased flooding.

The Complete Sentence
"What is currently worrying civil engineers is not so much a predicted increase in annual precipitation as the likelihood that many storms will come in tighter succession, thereby making flooding in lower lying riparian regions far more likely."