Variability and Complexity of Environmental Drivers on Catchment Transit Times Across Space: A Systematic Review

Catchment transit time, the time water takes to travel from precipitation to streamflow, is a key diagnostic of how catchments store, route, and transform water and solutes. Its spatial variability reflects interactions among hydroclimatic forcings, landscape structure, flow‐path organization, and storage dynamics, yet understanding its environmental drivers remains fragmented across scales, regions, and methodological frameworks. This review examines how environmental drivers shape transit‐time spatial variability, focusing on two widely used metrics: mean transit time (MTT) and young water fraction (Fyw). We first clarify the mathematical and physical distinctions between these two metrics, showing that MTT is not well suited for cross‐study comparisons because it is highly sensitive to methodological choices, tracer types, and sampling design. MTT is therefore presented mainly as a historical summary, whereas Fyw is identified as a more robust and transferable metric for comparative synthesis. Integrating primarily Fyw‐based evidence, we identify a possible scale‐dependent shift in environmental controls of transit time variability: from soil and hillslope–riparian processes in small catchments to hydrological connectivity and network organization at intermediate scales, to broader groundwater–geologic influences in larger basins. However, this framework remains tentative because available evidence is uneven across spatial scales and climate regimes. Future progress requires broader geographic coverage, stronger reliance on robust metrics such as Fyw, clearer quantification of uncertainty in MTT estimates, and more process‐based frameworks linking environmental drivers, hydrological processes, and tracer‐derived response metrics. These are essential for improving transferable understanding and prediction of water and solute transport, especially in ungauged regions. This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water and Environmental Change Science of Water > Methods