These are phrasal verbs so will help you understand one distinction between "about" and "over" (as already well explained by other tutors). You would need to study the use of prepositions and other sentences. A good start is to look up the different meanings of "about" and "over" in a learner's dictionary. http://www.collinsdictionary.com/dictionary/english-cobuild-learners/over?showCookiePolicy=true http://www.collinsdictionary.com/dictionary/english-cobuild-learners/about?showCookiePolicy=true

He is talking about the issue of water scarcity. we are talking over the issue of water scarcity

I think, talk over is more serious.

To 'talk over' something means to discuss it : 'We talked it over' It usually means a discussion between two or a few people about something of concern. The sentence you have written does not sound natural. The first one is better : 'He is talking about the issue of water scarcity.' This could refer to someone giving a presentation.