I'm woken by the sound of rushing water. I get up and hang out of the window. A river rushing by? Indeed.

I have lived on dry river banks for many years, so it's a sound I know well. But now we're in a suburb of Windhoek. It's cool and damp and you might think you were in a Cape Town winter, as it never seems to stop raining. Yet this is March and Windhoek is the capital of the driest country in the Sahel.