Plants rely not only on recent rainfall but also on water stored from earlier storms-known as carryover water-for growth. Yet we've lacked a global measure of how sensitive vegetation is to this water source, leaving a gap in our ability to predict drought impacts and ecosystem responses. Using satellite data that track changes in land water storage, we show that vegetation responds just as strongly to carryover water as to recent rainfall, despite its smaller contribution to total plant water uptake. Latest Earth system models, however, overstate this sensitivity, leading to distorted drought projections. We find that including a simple soil structure adjustment reduces this error and offers a practical path to improve model performance and ecological forecasts.