Two-thirds of the ecosystems on Earth are limited by sub-optimal water supply during at least part of their growing season. However, it is not clear whether the land-surface water balance is getting wetter or drier over recent decades. Furthermore, GCM projections of future climate cite changes in air temperature and precipitation, but do NOT compute the net change in water balance, the most important variable for ecosystems. This seminar will attempt to evaluate whether the western US is going to look more like southern Arizona, or western Oregon a hundred years from now.