Interactive effects of aridity and catchment position on blue-green water partitioning across river networks

Rainfall received by river basins is partitioned into blue water (surface and groundwater) and green water (soil water used by plants), largely determining how much water is available for terrestrial productivity, aquatic ecosystems, and human water use1,2. Environmental properties, such as aridity, are known to influence blue-green partitioning through evapotranspiration losses to the atmosphere; yet the imprint of climate on the blue–green water cycling within local catchments across river networks remains unknown. Here, we combine stable‑water‑isotope observations with a mass‑balance model for all 20,852 sub-catchments of the Mississippi River Basin and show that climate coupling—defined as the degree to which blue–green water cycling tracks aridity—decreases predictably with river‑network position. Headwaters partitioning fluxes and are more tightly coupled to climate, whereas downstream catchments converge toward similar partitioning states and systematically decouple from aridity. Because the Mississippi River Basin captures ~80% of Earth’s major climate–landform combinations outside tropical and polar extremes, this network‑position scaling provides a transferable constraint on where climate change and water use are most likely to perturb terrestrial water cycling across river basin.