Silly Question, But What's MA?

It's that surrounding countryside that the water runs off and ends up in your taps.

You have to be a little careful with that idea.  What matters is the countryside surrounding the location where your water comes from. Generally, that will be fairly local, but occasionally it won't be true. As an example, Birmingham gets it's water from Central Wales. So it's the Welsh countryside that matters.

Sorry, but my pedantry gene kicked in.

I thought it was Colin was the teacher?   

TBH, I did realise it'd be the surrounding countryside that gathers the water.  Not much chance of gathering decent rainwater from our High Street.