I can't speak for Peru specifically, but natural ecosystems play a huge role in absorbing and holding on to rain water, ensuring it doesn't all just flow back into the sea. This has been known for ages - I remember a friend who did environmental sciences telling me stories of cities in developing countries that cut all the nearby forests and as a side-effect also ruined their water supply.

So with that in mind I'm a bit surprised at the skepticism on display here.