TL|DR: same place where the trees that were previously here were getting their water. I don't think the article is about planting trees in Sahara or Gobi. It would be more useful in the replenishment/repair after a fire.

Silly example, but in the U.K. (almost everywhere on this island) it rains 4629362619 days per year! (or it feel like this much:)

I do understand that U.K. forests are not diminishing as they are on Amazon but I think (not sarcastically) they are called Rainforests.

I guess it depends on the reason for the deforestation. If it is just for timber, then a cheap solution to repopulate the area would be taken up by the lumberjacks themselves (so they can go back in 10-20-30 years and collect again and repeat the cycle). If the deforestation is simple to create more land for crops/living areas, then this is irrelevant.

Does this only have to be used in areas strapped for water? I mean they mentioned Oregon, which gets pretty good amounts of rain naturally.