Reviews and syntheses: Threshold shifts in riparian plant water sources along hydrological gradients

Abstract. The hydrological connection between riparian plants and the river underpins riparian ecosystem functioning and informs ecological water delivery and buffer-zone delineation, yet isotope evidence remains scattered across case studies. From a library of 131 stable hydrogen and oxygen isotope (δ²H, δ¹⁸O) water-source studies, we selected 25 studies with an explicit gradient design and re-analysed their data in a standardised form, 11 distance-calibrated transects (41 sites), three time series (158 records) and one water-uptake-depth sequence (43 records). Distance to channel, depth to water table and flow permanence were unified on a single "hydrological connectivity" axis to test for comparable thresholds in water-source switching. In arid and semi-arid regions plant water sources switch abruptly along the distance gradient, and the transition zone in which the stable-water fraction (deep soil water plus groundwater) crosses 50 % consistently falls at 50–400 m from the channel (shrubs 50–100 m and trees 100–400 m in hyper-arid regions, 50–200 m in semi-arid regions). In humid regions no such switch occurs; the river-water signal attenuates over metre-to-hectometre distances, with the attenuation scale increasing with aridity. Recharge mode determines the form of the gradient: channel-recharged systems produce a switch, precipitation-recharged systems attenuation. Life form shifts transition-zone position through rooting depth, and woody species show greater seasonal plasticity than herbs. Distance to channel is itself only a proxy for connectivity, since groundwater contribution increases with distance in a channel-recharge system but decreases with distance in a reservoir-recharge system. We propose defining the functional width of a riparian zone by the outer edge of the transition zone (400–1000 m for trees in arid inland rivers, 100–200 m in semi-humid rivers) and replacing a single target water level with a zoned combination of depth and fluctuation, together with a species-configuration list along the gradient. The synthesis provides water-source-based evidence for delineating restoration widths and scheduling ecological water delivery.