Vegetation plays an essential role in water partitioning, as it strongly influences evapotranspiration, infiltration and water retention. To analyse the influence of vegetation on water partitioning under innovative land management strategies, we used stable water isotopes as natural tracers to monitor precipitation, soil water and groundwater fluxes over the growing season of 2021 (March–October). We selected eight plot sites with four contrasting land covers and soil types in the drought‐sensitive Demnitzer Millcreek Catchment (DMC) in NE Germany. The land use types include forest, grassland, and arable with the latter being subdivided into conventional (e.g., crops) and innovative (e.g., agroforestry) sites. Two weather stations, a flux tower, and in situ soil moisture monitoring complemented our isotopic data with a hydroclimatic context. The year of 2021 had near‐normal precipitation totals compared to the prolonged drought of 2018–20. Soil water storage was highest at the agricultural sites, while lowest at the forest, though this reflected both the influence of soil properties (as forests dominated sand soils while crops loam soils) and the greater evapotranspiration from forests. We also estimated soil water ages and found the greatest isotopic variability and fastest turnover of water in the upper soils of arable sites. The forest soil water had the most limited variability in isotopic composition and tended to be older, revealing lower levels of groundwater recharge. Conventional and innovative cropping sites were similar to each other, likely due to the early tree development stage in agroforestry schemes under the latter. Our investigation revealed the forest sites are potentially most vulnerable to limited water availability in the DMC and land use changes in agricultural land lacked major differences in ecohydrological fluxes over the study year. The study further underlines the need for long‐term observations of recent adaptive land use changes and drought‐sensitive vegetation to improve our understanding and evolve drought resilient land management strategies considering time lags in impacts and non‐stationarity.

Abstract. Root water uptake is an important critical zone process, as plants can tap various water sources and transpire these back into the atmosphere. However, knowledge about the spatial and temporal dynamics of root water uptake and associated water sources at both high temporal resolution (e.g. daily) and over longer time periods (e.g. seasonal) is still limited. We used cavity ring-down spectroscopy (CRDS) for continuous in situ monitoring of stable water isotopes in soil and xylem water for two riparian willow (Salix alba) trees over the growing season (May to October) of 2020. This was complemented by isotopic sampling of local precipitation, groundwater and stream water in order to help constrain the potential sources of root water uptake. A local flux tower, together with sap flow monitoring, soil moisture measurements and dendrometry were also used to provide the hydroclimatic and ecohydrological contexts for in situ isotope monitoring. In addition, bulk samples of soil water and xylem water were collected to corroborate the continuous in situ data. The monitoring period was characterised by frequent inputs of precipitation, interspersed by warm dry periods which resulted in variable moisture storage in the upper 20 cm of the soil profile and dynamic isotope signatures. This variability was greatly damped in 40 cm and the isotopic composition of the sub-soil and groundwater was relatively stable. The isotopic composition and dynamics of xylem water was very similar to that of the upper soil and analysis using a Bayesian mixing model inferred that overall ~90 % of root water uptake was derived from the upper soil profile. Sap flow and dendrometry data indicated that soil water availability did not seriously limit transpiration during the study period, though there was a suggestion that deeper (> 40 cm) soil water might provide a higher proportion of root water uptake (~30 %) in a drier period in the late summer. The study demonstrates the utility of prolonged real time monitoring of natural stable isotope abundance in soil-vegetation systems, which has great potential for further understanding of ecohydrological partitioning under changing hydroclimatic conditions.

Abstract. The partitioning of water fluxes in the critical zone is of great interest due to the implications for understanding water cycling and quantifying water availability for various ecosystem services. We used the tracer-aided ecohydrological model EcH2O-iso to use stable water isotopes to help evaluate water, energy, and biomass dynamics at an intensively monitored study plot under two willow trees, a riparian species, in Berlin, Germany. Importantly, we assessed the value of in situ soil and plant water isotope data in helping to quantify xylem water sources and transit times, with coupled estimates of the temporal dynamics and ages of soil and root uptake water. The willows showed high water use through evapotranspiration, with limited percolation of summer precipitation to deeper soil layers due to the dominance of shallow root uptake (>80 % in the upper 10 cm, 70 %–78 % transpiration/evapotranspiration). Lower evapotranspiration under grass (52 %–55 % transpiration/evapotranspiration) resulted in higher soil moisture storage, greater soil evaporation, and more percolation of soil water. Biomass allocation was predominantly foliage growth (57 % in grass and 78 % in willow). Shallow soil water age under grass was estimated to be similar to under willows (15–17 d). Considering potential xylem transit times showed a substantial improvement in the model's capability to simulate xylem isotopic composition and water ages and demonstrates the potential value of using in situ data to aid ecohydrological modelling. Root water uptake was predominately derived from summer precipitation events (56 %) and had an average age of 35 d, with xylem transport times taking at least 6.2–8.1 d. By evaluating isotope mass balances along with water partitioning, energy budgets, and biomass allocation, the EcH2O-iso model proved a useful tool for assessing water cycling within the critical zone at high temporal resolution, particularly xylem water sources and transport, which are all necessary for short- and long-term assessment of water availability for plant growth.