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Present and future climatic trends are expected to markedly alter water fluxes and stores in the hydrologic cycle. In addition, water demand continues to grow due to increased human use and a growing population. Sustainably managing water resources requires a thorough understanding of water storage and flow in natural, agricultural, and urban ecosystems. Measurements of stable isotopes of water (hydrogen and oxygen) in the water cycle (atmosphere, soils, plants, surface water, and groundwater) can provide information on the transport pathways, sourcing, dynamics, ages, and storage pools of water that is difficult to obtain with other techniques. However, the potential of these techniques for practical questions has not been fully exploited yet. Here, we outline the benefits and limitations of potential applications of stable isotope methods useful to water managers, farmers, and other stakeholders. We also describe several case studies demonstrating how stable isotopes of water can support water management decision-making. Finally, we propose a workflow that guides users through a sequence of decisions required to apply stable isotope methods to examples of water management issues. We call for ongoing dialogue and a stronger connection between water management stakeholders and water stable isotope practitioners to identify the most pressing issues and develop best-practice guidelines to apply these techniques.

Tracing and quantifying water fluxes in the hydrological cycle is crucial for understanding the current state of ecohydrological systems and their vulnerability to environmental change. Especially the interface between ecosystems and the atmosphere that is strongly mediated by plants is important to meaningfully describe ecohydrological system functioning. Many of the dynamic interactions generated by water fluxes between soil, plant and the atmosphere are not well understood, which is partly due to a lack of interdisciplinary research. This opinion paper reflects the outcome of a discussion among hydrologists, plant ecophysiologists and soil scientists on open questions and new opportunities for collaborative research on the topic "water fluxes in the soil-plant-atmosphere continuum" especially focusing on environmental and artificial tracers. We emphasize the need for a multi-scale experimental approach, where a hypothesis is tested at multiple spatial scales and under diverse environmental conditions to better describe the small-scale processes (i.e., causes) that lead to large-scale patterns of ecosystem functioning (i.e., consequences). Novel in-situ, high-frequency measurement techniques offer the opportunity to sample data at a high spatial and temporal resolution needed to understand the underlying processes. We advocate for a combination of long-term natural abundance measurements and event-based approaches. Multiple environmental and artificial tracers, such as stable isotopes, and a suite of experimental and analytical approaches should be combined to complement information gained by different methods. Virtual experiments using process-based models should be used to inform sampling campaigns and field experiments, e.g., to improve experimental designs and to simulate experimental outcomes. On the other hand, experimental data are a pre-requisite to improve our currently incomplete models. Interdisciplinary collaboration will help to overcome research gaps that overlap across different earth system science fields and help to generate a more holistic view of water fluxes between soil, plant and atmosphere in diverse ecosystems.

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The isotopic composition of xylem water (δX) is of considerable interest for plant source water studies. In‐situ monitored isotopic composition of transpired water (δT) could provide a nondestructive proxy for δX‐values. Using flow‐through leaf chambers, we monitored 2‐hourly δT‐dynamics in two tropical plant species, one canopy‐forming tree and one understory herbaceous species. In an enclosed rainforest (Biosphere 2), we observed δT‐dynamics in response to an experimental severe drought, followed by a 2H deep‐water pulse applied belowground before starting regular rain. We also sampled branches to obtain δX‐values from cryogenic vacuum extraction (CVE). Daily flux‐weighted δ18OT‐values were a good proxy for δ18OX‐values under well‐watered and drought conditions that matched the rainforest's water source. Transpiration‐derived δ18OX‐values were mostly lower than CVE‐derived values. Transpiration‐derived δ2HX‐values were relatively high compared to source water and consistently higher than CVE‐derived values during drought. Tracing the 2H deep‐water pulse in real‐time showed distinct water uptake and transport responses: a fast and strong contribution of deep water to canopy tree transpiration contrasting with a slow and limited contribution to understory species transpiration. Thus, the in‐situ transpiration method is a promising tool to capture rapid dynamics in plant water uptake and use by both woody and nonwoody species. Summary Statement: Tracing the isotopic composition of transpiration (δT) in real‐time showed rapid dynamics in plant water uptake and use by woody and nonwoody species. Daily δ18OT‐values weighted by the transpiration flux are a good proxy for δ18O of xylem water. [ABSTRACT FROM AUTHOR]