A Global Assessment of the Spatial-Temporal Origin of Soil Water Taken up by Vegetation

Vegetation modulates Earth’s water, energy and carbon cycles and provides a key link between water stores in the deep soil and the atmosphere. How its functions may change in the future largely depends on how it copes with droughts. There is evidence that in places-times of drought, vegetation shifts water uptake to deeper soil and rock moisture and groundwater. We differentiate and assess plant use of four types of water source: precipitation (P) in current month, past P stored in deeper unsaturated soils/rocks, past P stored in locally recharged groundwater, and groundwater from P fallen on uplands via river-groundwater convergence toward lowlands. We examine global and seasonal patterns and drivers in plant uptake of the four sources using inverse modeling and isotope-based estimates. We find that globally and annually, 70% (std 24%) of plant transpiration relies on current month P, 18% (std 15%) on deep soil moisture, only 1% (std 3%) on locally recharged groundwater, and 10% (std 22%) on groundwater or river water from upland more distant sources; (2) regionally and seasonally, recent P is only 19% in semi-arid, 32% in Mediterranean, and 17% in winter-dry tropics in the driest months; (3) at landscape scales, deep soil moisture, taken up by deep roots in the deep vadose zone, is critical in uplands in dry months, but groundwater and river water from uplands is up to 47% in valleys where riparian forests and desert oases are found. Because the four sources originate from different places-times, move at different spatial-temporal scales, and respond with different sensitivity to climate and anthropogenic forces, understanding space-time origin of plant water source can inform ecosystem management and Earth System Models on the critical hydrologic pathways linking precipitation to vegetation.