Flexible water uptake across soil depths buffers transpiration in winter cereals during dry spells

Croplands are among the land systems most vulnerable to shifts in precipitation regimes and prolonged droughts, particularly in temperate climates. Characterizing root water uptake patterns is therefore essential to understand how crops maintain function and sustain transpiration and productivity under drought stress. We investigated water uptake patterns of winter cereals (wheat, barley) across two contrasting growing seasons (2024, 2025) at a temperate cropland in Central Germany. Within the footprint of an eddy covariance (EC) tower, we sampled plant leaves, precipitation, soil water, and groundwater for stable water isotope analysis at natural abundance and estimated xylem water isotope composition using the Craig–Gordon model. Soil moisture was monitored at nine depths (5–100 cm) at three profiles. Using multi-layer soil moisture time series and a dual-isotope mixing model, we identified the contribution of different water sources to transpiration, and quantified water uptake depth patterns. In 2024, more frequent rainfall maintained wetter soil layers and higher evapotranspiration (ET), whereas 2025 was marked by longer dry spells and consistently lower ET, although average water-use efficiency remained similar with greater variability in 2025. Both crops dynamically shifted their water uptake depth in response to the interaction between dry-spell timing and duration, soil water availability and phenology, and shallow uptake increased rapidly after rainfall. However, dry spells did not necessarily deepen the median uptake depth, Z 50 , of winter barley in 2025, whereas in 2024 winter wheat systematically relied on deeper soil water sources as the topsoil dried, which coincided with higher leaf area index (LAI). Our results indicate that water uptake depth in rainfed cereals is a conditional, reversible and dynamic response to topsoil drying rather than a systematic deepening, and that combining EC, soil moisture and isotope methods reveals where crops take up water with whether water and carbon fluxes are maintained.