The hydrological connectivity between channels and floodplains is modulated by vegetation and system geometry and plays a crucial role in determining the water transport timescale (WTT) in river deltas. It is well known that river deltas dynamically respond to many drivers including sea level rise, sediment composition, river discharge, and vegetation conditions, but less understood is how WTT changes throughout a delta's morphological evolution. In this study, a reduced-complexity model called pyDeltaRCM is used to examine the effects of sea level rise, vegetation, and sediment composition on system scale and local WTT throughout the evolution of a river delta. The model operates on stochastic parcel-based cellular routing systems for water, sediment, and phenomenological rules for sediment deposition and erosion. Due to the stochastic nature of pyDeltaRCM, ensemble results with or without vegetation were obtained for each set of boundary conditions. Absolute transport times are predicted to decrease with sea level rise, and WTT is anticipated to decrease as sand fraction rises. The impact of vegetation is influenced by the degree of hydrological connectivity with the delta floodplain. The findings of this study will have significant implications for enhancing our comprehension of the evolutionary patterns of hydrological transport in nonstationary coastal environments. Keywords: River delta, Water transport times, pyDeltaRCM, Vegetation, Sea level rise.

Coastal river deltas are highly significant both ecologically and socioeconomically, and are increasingly vulnerable to natural and anthropogenic factors. Vegetation, sediment supply and composition, sea level rise, human development, and subsidence are among the many factors that can modulate hydro-morphology in river deltas. The temporal and spatial dynamics of hydrological transport within a river delta are crucial indications of ecological function. The system's interplay with system morphology, boundary forcings, and internal conditions such as vegetation modulates the amount of time it takes for water to pass through the deltaic system, which is known as the water residence time. This study examined water residence times during morphological changes of river deltas using pyDeltaRCM, a reduced-complexity hydro-morphodynamic model that applied phenomenological criteria to precisely describe sediment accretion and erosion, water and sediment transport are simulated by stochastic parcel-based cellular routing algorithms. Vegetation, sea level rise, and input sediment composition were varied to quantify their effects of system morphology, transport patterns, and ultimately the water residence time distribution. Due to the stochasticity of the pyDeltaRCM modeling scheme, delta morphologies, and flow fields were produced across five realizations for each set of boundary conditions. Delta shorelines were delineated, and water residence time distribution was measured using the Lagrangian particle tracking approach. Results indicate that both input sand fractions and sea level rise exert significant control on system-scale water residence time, but vegetation has a secondary, limited, and inconsistent impact. The outcomes of this study enhance our understanding of system-scale hydrological transport dynamics in nonstationary coastal environments in the face of climate change.Keywords: River delta, Vegetation, Water residence times, pyDeltaRCM