Assessing the impacts of climate change on river discharge requires catchments that reliably reflect regional climatic signals, yet such signals are often masked by local controls, particularly in small and heterogeneous basins. This study determines the minimum basin area—termed the hydrological representativeness scale—above which streamflow primarily reflects regional climate variability rather than local factors. Using 41-year naturalized discharge records from 113 hydrometric stations across five major basins, an iterative Kruskal–Wallis test was applied to specific discharge values to identify the transition from locally dominated to hydroclimatically representative basin behavior. The results identify a transitional zone between 850 and 950 km², with an optimal threshold at approximately 900 km². Above this scale, streamflow becomes more regionally coherent and less variable, whereas smaller basins exhibit stronger local heterogeneity and greater hydrological noise. The robustness of this threshold was confirmed through multiple independent validations, including Dunn's post hoc analysis, inter-station correlation, temporal sub-period testing, runoff coefficient analysis, variability metrics (Cᵥ and Cₛ/Cᵥ), and sensitivity analysis. Although the numerical threshold is region specific, the proposed methodological framework is transferable to other regions, offering a practical tool for prioritizing representative monitoring basins and supporting water resources planning and related hydroclimatic assessments.