Drought-induced shifts in water uptake in winter cereals: Insights from multi-scale measurements across two contrasting years

Croplands are among the systems most vulnerable to shifts in precipitation regimes and prolonged droughts particularly in temperate climates. Although irrigation may increase agricultural productivity, it can’t offer a sustainable long-term solution to compound droughts due to intensified pressure on freshwater resources. Thus, characterizing root water uptake patterns is essential to understand how crops maintain function while sustaining transpiration during drought. We investigated water uptake patterns of winter cereals (wheat, barley) across two contrasting growing seasons (2024, 2025). The research site is in central Germany, it exhibits a suboceanic/subcontinental climate and has a shallow groundwater level (ca. 1.5 m). In the footprint of an eddy covariance (EC) tower, we sampled plant leaves, soil water, precipitation, river water, and groundwater to trace stable water isotopes. We monitored leaf area index (LAI) and installed soil moisture sensors (5–100 cm). Using soil moisture time series and dual-isotope mixing models, we quantified variation in water uptake depth throughout the growing seasons (March-July). In 2024, soil layers were wetted by regular rains in April with only short rain-free periods occurring. On the contrary, frequent and longer dry spells occurred in 2025, totalling 18 days in April and 15 days in May. Moreover, in 2024, ET_soil ranged from 1.2 mm day⁻¹ to over 7 mm day⁻¹ at peak LAI, while ET_EC-tower for the same period exceeded 5 mm day⁻¹. In 2025, despite high transpiration demand, ET did not exceed 5 mm day⁻¹ consistently in both methods. Soil water isotope patterns showed expected fluctuations, with deeper layers being depleted in δ²H and δ¹⁸O. We used the Craig–Gordon equation to determine xylem water isotope signatures, followed by mixing models to quantify water sources for transpiration. Xylem and soil water isotope time series suggest that despite more frequent rain events, winter wheat continued to draw water from stable, deeper sources rather than relying on enriched shallow soil layers (5–15 cm). During summer 2024 (June–July), δ²H and δ¹⁸O values in the topsoil enriched through higher soil evaporation, yet water uptake shifted to deeper layers, which agrees with ET_soil variations. Precipitation events in late spring 2024 enabled winter wheat to access deeper soil water sources (≥50 cm) to sustain high transpiration demand. During the drier conditions, barley altered water uptake depths yet transpiration demand was mainly sustained from water sources within 10–40 cm, and contribution from deeper layers was limited. Both species showed similar responses to dry spells, yet the timing of the drought shaped root plasticity and access to stable water sources. Our results demonstrate that water uptake strategies and water use efficiency are tightly linked to the timing and intensity of drought in annual crops, even when deeper water sources remain stable.