Satellite-based global-ocean mass balance estimates of interannual variability and emerging trends in continental freshwater discharge

TLDR

Continental freshwater discharge is essential to Earth’s water cycle, yet a comprehensive global observing system is lacking, and its sustained growth would signal an intensifying hydrologic cycle. The study aims to develop a method that can estimate long‑term global freshwater discharge trends as sea‑level and temperature records extend. The authors estimate global freshwater discharge by applying a 13‑year (1994–2006) satellite‑based ocean mass balance using precipitation, evaporation, and sea‑level data. The estimates show an average global discharge of 36,055 km³ yr⁻¹ with significant ENSO‑driven interannual variability, and a 13‑year upward trend of 540 km³ yr⁻² largely due to a 768 km³ yr⁻² rise in ocean evaporation.

Abstract

Freshwater discharge from the continents is a key component of Earth’s water cycle that sustains human life and ecosystem health. Surprisingly, owing to a number of socioeconomic and political obstacles, a comprehensive global river discharge observing system does not yet exist. Here we use 13 years (1994–2006) of satellite precipitation, evaporation, and sea level data in an ocean mass balance to estimate freshwater discharge into the global ocean. Results indicate that global freshwater discharge averaged 36,055 km 3 /y for the study period while exhibiting significant interannual variability driven primarily by El Niño Southern Oscillation cycles. The method described here can ultimately be used to estimate long-term global discharge trends as the records of sea level rise and ocean temperature lengthen. For the relatively short 13-year period studied here, global discharge increased by 540 km 3 /y 2 , which was largely attributed to an increase of global-ocean evaporation (768 km 3 /y 2 ). Sustained growth of these flux rates into long-term trends would provide evidence for increasing intensity of the hydrologic cycle.