Hydrological characteristics and changes in the Nu-Salween River basin revealed with model-based reconstructed data

The Nu-Salween River (NSR), the longest free-flow river in Southeast Asia, plays an irreplaceable role in social development and ecological protection. The lower NSR region is particularly valuable as it is inhabited by approximately 6.7 million people. The basin has limited hydraulic conservancy infrastructure and insufficient ability to cope with climate change risks. Studying the hydrological characteristics and changes in the basin provides the scientific basis for rational protection and development of the basin. However, owing to the limitation of observation data, previous studies have focused on the local area and neglected the study of the lower reaches, which is not enough to reflect the spatial characteristics of the entire basin. In this study, the ECMWF 5th generation reanalysis data (ERA5) and Multi-Source Weighted-Ensemble Precipitation (MSWEP) were applied to develop a geomorphology-based hydrological model (GBHM) for reconstructing hydrological datasets (i.e. GBHM-ERA5 and GBHM-MSWEP). The reconstructed datasets covering the complete basin were verified against the gauge observation and compared with other commonly used streamflow products, including Global Flood Awareness System v2.1, GloFAS-Reanalysis dataset v3.0, and linear optimal runoff aggregate (LORA). The comparison results revealed that GBHM-ERA5 is significantly better than the other four datasets and provides a good reproduction of the hydrological characteristics and trends of the NSR. Detailed analysis of GBHM-ERA5 revealed that: (1) A multi-year mean surface runoff represented 39% of precipitation over the basin during 1980–2018, which had low surface runoff in the upstream, while areas around the Three Parallel Rivers Area and the estuary had abundant surface runoff. (2) The surface runoff and discharge coefficient of variations in spring were larger than those in other seasons, and the inter-annual variation in the downstream was smaller than that in the upstream and midstream regions. (3) More than 70% of the basin areas showed a decreasing trend in the surface runoff, except for parts of Nagqu, south of Shan State in Myanmar, and Thailand, where surface runoff has an increasing trend. (4) The downstream discharge has dropped significantly at a rate of approximately 680 million cubic metres per year, and the decline rate is greater than that of upstream and midstream, especially in summer. This study provides a data basis for subsequent studies in the NSR basin and further elucidates the impact of climate change on the basin, which is beneficial to river planning and promotes international cooperation on the water- and eco-security of the basin.