Earth Observation Based Functional Characterization of Riparian Interfaces across Climatic Zones

Riparian zones are dynamic eco-hydrological interfaces regulating groundwater-surface water (GW-SW) exchange by controlling vertical and lateral fluxes of water, heat, sediments, and biogeochemical constituents along river corridors. Most Earth Observation (EO) studies, however, treat them as static land-cover units rather than functional exchange domains, limiting insights into subsurface connectivity and hydrological processes. To address this gap, this study develops a process oriented EO framework to delineate functional riparian zones based on hydrological connectivity and climatic sensitivity. Multi-decadal EO datasets were integrated with hydro-climatic indicators of spectral vegetation indices, inundation frequency, surface moisture proxies, and land surface temperature metrics to distinguish permanent and seasonal riparian interfaces and were interpreted as proxies for GW-SW exchange intensity, residence time, and flow directionality. The framework was applied across climatically heterogeneous river corridors spanning multiple Köppen-Geiger climate classes across India, representing distinct precipitation-temperature regimes. The results indicate that Permanent riparian interfaces occupy only 18-27% of the geomorphic floodplain area but account for >55% of persistent surface-subsurface connectivity. Contrastingly, seasonal riparian zones expand by up to 2.6 times during monsoon or high-precipitation periods. This further highlights the climate-driven activation of transient GW-SW pathways. Humid climatic regions exhibit stable vegetation persistence and low thermal variability and are indicative of sustained gaining conditions and shallow groundwater tables. Semi-arid reaches show high seasonal variability, episodic losing conditions, and rapid contraction of active interfaces. Climatic transition zones display the highest temporal instability from bidirectional GW-SW fluxes governed by threshold-controlled switching between hydrological states. Moreover, trend and non-parametric breakpoint analysis of extreme climate indices indicate regime shifts in 32-41% of seasonal riparian interfaces across the varying climatic zones across India after the early 2000s. Further, rainfall dominated basins show the strongest response due to weak hydrological memory and event-driven processes. Thus, Riparian zones emerge as transient control volumes regulating GW-SW coupling under changing climatic forcing. This approach advances riparian analysis from spatial mapping to functional characterisation and supports scalable, process-based riparian management focused on buffering capacity, resilience, and subsurface connectivity.