Question

What year was groundwater the highest

Answer 1

What year was groundwater the highest

Answer 2

The answer to your question depends on the area that you are referencing. I would refer to a list of floods and other similar events near the area that your are researching as floods occur when rivers go over their banks and the ground becomes saturated with water, meaning that it is full of water. That is groundwater, and that is how to receive your answer :)