Intrinsic Spatial Scales of River Stores and Fluxes and Their Relative Contributions to the Global Water Cycle

The Earth's rivers vary in size across several orders of magnitude. Yet, the relative significance of small upstream reaches compared to large downstream rivers in the global water cycle remains unclear, challenging the determination of adequate spatial resolution for observations. Here, we use monthly simulations of river stores and fluxes to investigate the intrinsic spatial scales of the global river water cycle. We frame these scale‐dependent river dynamics in terms of observational capabilities, assessing how the size of rivers that can be resolved influences our ability to capture key global hydrologic stores and fluxes. By filtering reaches by estimated river widths, we quantify the relative contribution of global river reaches by size and estimate that over 17% of global discharge to ocean and nearly 9% of the world's river storage lies within rivers smaller than 100 m—hence revealing both strengths and limitations of current observational capabilities. Plain Language Summary The world's rivers come in many different sizes, from narrow streams to wide rivers flowing to the ocean. In order to make future plans for how rivers of different sizes should be observed from space, monitored by river gages, or represented in models, we use global simulations of river flow and volume to analyze how small and large rivers contribute to Earth's water cycle. By estimating river widths at all global rivers, we assess if different strategies for observing rivers would accurately capture key river fluxes and stores worldwide. We find that the world's smallest rivers, those narrower than 100 m, contain 9% of global river storage and contribute 17% of global discharge to the ocean, highlighting the need to monitor and model small rivers. Key Points We assess how river spatial scale influences contributions to the global water cycle using long‐term river discharge simulations Rivers narrower than 100 m contribute over 17% of discharge to the global oceans and 9% of global river storage We highlight the importance of accounting for contributions from Earth's small rivers, which may be challenging to accurately observe