Analyzing global river discharge changes using remote sensing-based and in situ data

Abstract

Understanding how river discharge changes across space and time is fundamental for hydrologic science and water management. Given the reduction in global in situ observations, remote sensing and reanalysis products have become key data supplements. In 2024, Feng and Gleason released the Global River Discharge Reanalysis (GRDR) dataset, concluding that there is now "more flow upstream and less flow downstream" in global rivers. Motivated by their research, this study evaluates the quality of the GRDR dataset and establishes a "Remote Sensing-based and In situ" (RSI) dataset -combining remote sensing estimates with gauge observations - to analyze spatiotemporal trends in global river discharge from 1984 to 2018. First, an independent validation of GRDR was conducted using a more extensive global network of ground observations. The assessment reveals that GRDR’s overall performance is lower than reported in the original study, particularly for stream orders 1, 6, 7, and ≥ 8. For stream orders ≥ 8, the median Nash-Sutcliffe Efficiency (NSE) in this study is approximately 1.4 lower than that in the original report. Comparisons of GRDR with remote sensing (RS) derived data reveal that RS data outperforms GRDR in stream orders 1, 7, and ≥8. GRDR exhibits a tendency to severely underestimate discharge in stream orders 1 and 2, with a significant fraction of its relative bias falling below −50%. Based on the RSI dataset, long-term trends, seasonality, and the Longitudinal Hydrographic Distribution Index (LHDI) were further analyzed. Results show an increasing trend in discharge for river orders 1 to 5 and 7 (with a significant increase of ∼0.19%/year in order 4 rivers), whereas discharge in orders 6 and ≥8 is decreasing. Seasonally, for all stream order groups, discharge typically increases from March to May, whereas it decreases from December to February. Furthermore, the LHDI confirms that the global discharge centroid is shifting upstream. These conclusions are generally consistent with those derived by Feng and Gleason using the GRDR dataset. In summary, while the limitations of the GRDR dataset are highlighted through independent validation, the findings of this study ultimately support the conclusion that global river systems are experiencing increased upstream flow and decreased downstream flow.