Jida Wang (University of Illinois Urbana-Champaign, United States)

With the increasing frequency of extreme climate events and rising water demand, the spatial and temporal distribution of surface water resources has become increasingly uneven. This variability highlights the essential role of reservoir systems in water allocation and hydrological regulation. Along many major rivers, cascade reservoirs, which are multiple reservoirs constructed and operated in sequence, play a key role in water supply, flood control, and hydropower generation. However, their operational modes and cumulative impacts on downstream rivers, particularly in transboundary river reaches, remain insufficiently observed and poorly quantified. Our study focuses on the cascade reservoir systems along the Yangtze and Mekong Rivers, aiming to reveal their operational behaviors and associated hydrological impacts. We utilize high-resolution, wide-swath remote sensing data from the Surface Water and Ocean Topography (SWOT) satellite mission, which provides repeated observations at least every 21 days. This high temporal resolution enables the detection of short-term changes in water surface elevation (WSE), thereby improving the accuracy of characterizing intra-annual dynamics of reservoirs and identifying their operational modes over time. Based on the WSE time series of reservoirs derived from SWOT observations, we apply wavelet analysis to examine the data in the frequency, time, and phase domains, aiming to reveal the correlations among reservoirs across seasonal and multiple frequency scales, as well as their operational modes. Building on this, we further convert WSE data into water storage estimates to quantify the regulation intensity and temporal characteristics of each cascade reservoir systems. To evaluate downstream hydrological responses, we analyze the impacts of cascade reservoir operations on mainstem discharge, with particular attention to how reservoir operations within the Chinese section of the Mekong River influence inflows to Southeast Asian countries, as well as how these upstream reservoirs affect discharge volumes in the Mekong Delta. This research advances our understanding of the operational modes of cascade reservoir systems, particularly in transboundary river regions, and provides scientific support for sustainable regional water management and international river governance.