Left High and Dry: Deep Soil Water Depletion and Ecohydrological Resilience on China's Loess Plateau

Reforestation and afforestation on China's Loess Plateau have more than doubled vegetation cover since the late 1990s, yet mounting evidence shows that these gains often come at the cost of long‐term ecohydrological resilience. The region's thick loess vadose zone acts as a deep but finite reservoir of soil water, storing legacy rainfall that current vegetation continues to exploit. This desiccation process is not instantaneous, but unfolds over decadal timescales as deep soil moisture—recharged during wetter periods decades earlier—is gradually depleted. Four lines of evidence reveal the hydrological tradeoffs of large‐scale greening: (1) bomb‐peak tritium archives indicate that 1960s precipitation is still migrating through the subsurface 7–14 m belowground, confirming piston flow infiltration and decades‐long residence times; (2) roots of afforestation species routinely extend 5–15 m, drawing on this paleo moisture; (3) stand‐age trajectories show that deep soil water is predictably exhausted within 15–25 years of planting, forming chronic dried soil layers; and (4) drought‐driven collapse of hydrological niche separation among species increases competition and ecological risk. Together, these dynamics constrain the water balance and reveal an accumulating hydrological debt. Adaptive strategies—thinning, rotation, spatial mosaics, and rainwater harvesting—can slow desiccation and better align vegetation structure with water availability. The Loess Plateau offers globally relevant lessons for dryland restoration: greening must account for rooting depth, recharge rates, and the vertical architecture of water storage. Sustaining ecological function requires harmonizing surface vegetation with the subsurface memory of water. This article is categorized under: Engineering Water > Sustainable Engineering of Water Science of Water > Hydrological Processes Science of Water > Water and Environmental Change