Waves of Sorrow was first intended to be on Joshua Tree but was never finished, but the band finalized it for the anniversary release of the album, and this is what Bono had to say about it (via u2.se and u2.com [news item from 9th of November 2007]):
Interviewed in New York, Bono explains that Wave of Sorrow 'was one of the songs from the Joshua Tree sessions that we never finished... a song that was trying to describe experiences that myself and Ali had when we were working in Ethiopia during the famine.'
Jonas Steverud (Maintainer of U2MoL) (4th of December 2007)
Bono was interviewed and commented on Wave of Sorrow, unknown by who, and made the comment that the song was one of the Joshua Tree sessions they never finished. It was a song that tried to describe his experiences when he and Ali went to Ethiopia to work with the refugees there. He comments on that Ethiopia is supposed to be the homeland of the Queen of Saba, that bring Salomon great and rich gifts, and now a parched country. How refugees walked through the night to the camp where Bono and Ali worked for help and relief for them and for their children. It is a bit unclear from the interview if the refugees gave their children to the refugee workers to take care of or if it was just a strong picture of they carrying the children in their arms, but the first stanzas in the song clearly refer to this. According to Bono, the problem with the rain coming was that it took all the top soil with it, all the fertile soil.
Thanks to Chris Taguchi (of Salvation in the Blues fame) for pointing out the interview.
Jonas Steverud (Maintainer of U2MoL) ( 2:th of November 2008)