Small streams, large impacts: headwaters control the non-perennial fraction of the global river network

The systematic wetting and drying of river channels exert a fundamental control on hydrological connectivity and biogeochemical functioning of watersheds. Quantifying the proportion of river networks that cease to flow seasonally or episodically is therefore essential, yet remains highly uncertain due to sparse observations and persistent underrepresentation of small streams in large-scale analyses. In this contribution, we integrate global-scale hydrological simulations with detailed field-based evidence from experimental catchments spanning diverse climatic regions to derive revised estimates of non-perennial stream occurrence worldwide. Our findings show that non-perennial streams are far more prevalent than previously recognized, both regionally and globally. When headwater streams are comprehensively accounted for, non-perennial reaches account for more than 70% of the global river network length, with upper estimates approaching 78%. Even in comparatively humid regions, such as Italy and the eastern United States, non-perennial streams represent over half of the total network length. Our analysis further demonstrates that the dominance of small upland channels allows wetting–drying dynamics to propagate their influence well beyond headwaters, leaving relevant signatures that persist even at the scale of large basins. These results highlight the need to systematically incorporate channel intermittency into large-scale hydrological models and assessments, with important implications for water resources evaluation, ecosystem functioning, and river management under ongoing climatic and environmental change.