This is a little bit simplistic, local hydrology could change, like rainfall patterns or the presence/absence of water courses and aquifers. Rainfall in an environment can't really be replaced by transported water. For some areas it could result in habitat loss or change and species loss or change. For agriculture yes it could possibly be piped in but this still requires building sufficient infrastructure to replace the loss of flows, and in the meantime some people may no longer be able to earn an income/work the land. This is not to say that I don't broadly support afforestation and reforestation efforts, but to say that it's just water moving from one place to another is missing the river for the trees.