Monitoring transboundary water systems from space: SWOT opportunities for data-scarce and politically sensitive basins

Transboundary water systems under increasing water scarcity are often characterized by limited monitoring capacity and restricted data sharing, particularly in regions affected by political tensions or conflict. These limitations severely constrain the assessment of basin-wide hydrological conditions and informed water management. The Surface Water and Ocean Topography (SWOT) mission offers a step change in this context by enabling consistent, independent observations of inland surface waters across national boundaries.

SWOT provides near-global measurements of water surface elevation, river width, river slope, lake and reservoir area, and their temporal dynamics at sub-monthly sampling. Through these observations, SWOT enables the estimation of variables that have historically been poorly monitored from space, including river discharge, lake and reservoir inflow and outflow, and changes in surface water storage. This multi-variable capability is particularly valuable in transboundary basins where in-situ data are sparse, inaccessible, or politically sensitive, and where monitoring needs extend beyond rivers to lakes and reservoirs that regulate downstream flows.

As a demonstration, we highlight applications in the Amu Darya basin, one of the most critical yet least transparent river systems in Central Asia. The basin exemplifies a setting where upstream regulation, poorly monitored tributary inflows, and downstream irrigation withdrawals strongly shape the water balance, while reliable hydrological data remain limited. By combining SWOT-derived river width, water surface elevation, and surface water extent, we show how basin-scale water balance components can be inferred without reliance on shared in-situ observations. This includes diagnosing upstream regulation signals, quantifying inflows from ungauged tributaries, assessing downstream irrigation diversions, and resolving the dynamics of lake and reservoir inflow and outflow. In this way, SWOT effectively functions as an independent hydrological observing system, enabling physically consistent assessments of water availability and use across political boundaries and providing a robust evidence base for scientific analysis and dialogue in transboundary basins under increasing water scarcity.