Headwater streams control the non-perennial fraction of the global river network.

Quantifying the fraction of a river network that does not flow year-round is crucial, as the wetting and drying of channels governs important hydrological and biogeochemical services of watersheds. However, this remains challenging due to limited experimental data and the difficulty of accurately representing the total length of rivers draining a landscape. Here we present new global estimates of non-perennial stream fractions by extrapolating low-resolution global simulations and detailed field observations from experimental sites spanning diverse climatic settings. Our findings show that non-perennial streams are far more prevalent than previously recognized, both regionally and globally. When small headwater streams are comprehensively accounted for, the global fraction of non-perennial channels rises above 0.7 (up to 0.78), with regional estimates in relatively humid regions such as Italy and the eastern USA exceeding 0.5. The study reveals that, owing to the abundance of small upland streams, the effect of channel wetting and drying in headwaters persists even in much larger basins. The systematic prevalence of non-perennial streams across different watershed sizes calls for a paradigm shift in water science, emphasizing the importance of adequately considering channel network dynamics in the assessment of hydrological, ecological and societal services provided by rivers.