"Can" is used to indicate capability or probability.
"May" is used to indicate permissibility.
You can go out - it is a question of whether you may go out = It is possible to go out but have you permission to go out.
With your example:
If the drought goes on much longer, there can be water rationing before the end of the month. ['...there may/ might/ could be ..]
All of these indicate a degree of uncertainty which is not the case with "can". Sentence 2 says there is a high degree of probability that a solution will be found.
Student or Learner