Plant water use, especially in semi-arid systems, is critical to growth, competition, fire regimes, tree stress and biosphere-atmosphere energy budgets. Yet due to the difficulties of working belowground we lack good estimates of the timing and location and extent of root activity by different plant species. Here I describe a depth-specific tracer technique that describes the timing, location and extent of water use by plants in sites from around the world. Results show that 1) trees typically rely on surprising shallow soils (10-30 cm) and 2) trees and other woody plants are able to rapidly change rooting patterns within growing seasons to ‘follow the water’. I discuss how these rooting patterns are likely to affect forest distributions in response to anticipated climate change.