The Confederacy of Heaven by Margaret R. Taylor

Nasan Rattlingbones was supposed to die.  That’s what exile means in a post-apocalyptic Canada that hasn’t seen rain in almost two hundred years, but this disgraced clan warrior has other plans.  She will survive alone in the wasteland of junipers and broken, haunted cities – just as soon as she figures out how.  Enter Oscar, a strange bird who claims to be her spirit guide but can’t quite prove it.  She doesn’t believe a word when he says she's been chosen by the Stars to save the world.Oscar may not be what Nasan hoped for in a spirit guide, but he does find her a place to stay with the rich people who’ve built a walled city around their very own reservoir.  She tries to build a new life there while dogged by the memories of the botched water raid that killed half her clan.The angel-like Stars who rule the universe reconsider Earth’s drought-curse once every century.  The human race gets just one witness for the defense – and the throne at Calgary wants to send a man who will forfeit rain for universal passage to Heaven.  Nasan needs to help her newfound friends stop Calgary, but she just as desperately needs to get home and face down the chieftain who wrongly banished her.  Meanwhile she’s beginning to suspect that Oscar is in on some sort of heavenly conspiracy.  Whether he’s telling the truth or not, she may just have to step up to the challenge and save the world anyway.