The use of Satellite Altimetry for the Development, Calibration and Validation of Global River Models
Yamazaki, Dai1; Lee, Hyongki2; Alsdorf, Douglas3; Bates, Paul1
1University of Bristol, UNITED KINGDOM; 2University of Houston, UNITED STATES; 3The Ohio State University, UNITED STATES

Water level dynamics in continental-scale rivers is an important factor for surface water studies and flood hazard management. The nadir-looking altimeter was initially developed for the measurement of sea surface altitude, but currently it is applied to inland waters using a so-called "retracking algorithm". On the other side, simulations of water levels became possible due to recent development of global river models, which enables calibration and validation of global river models using water levels measured by satellite altimeters. A significant latest achievement in global-scale river modeling is simulation of absolute water surface elevations (height above a geoid) in addition to relative water levels. The direct comparison of absolute water surface elevations between satellite altimetry and a global river model brings plenty of new information which can be used to indirectly estimate local topographic parameters such as channel cross-section shapes or floodplain topographies. Further than the tuning of model parameters, the information by satellite altimetry is helpful to improve the representation of the physical processes of global river models, which cannot be detected only from in-situ river gauges. For example, through the analysis of model simulation with satellite altimeters, we have found that the simulated river hydrodynamics cannot be improved by the calibration of channel cross-section parameters. The combined observation by in-situ gauges and satellite altimetry suggests that there is a contradiction of the relationship between the simulated river discharge and water level in a global river model. The absolute water surface elevations measured by satellite altimetry imply the importance of representing floodplain flow in addition to river channel flow in global river modeling. We found that the representation of the floodplain flow improves the simulation of river hydrodynamics as suggested by the information from satellite altimetry. The comparison of water surface elevations between the model and the altimetry are currently restricted to the mainstem of major rivers, mainly because accurate measurement of water surface elevations by currently-existing nadir altimeters requires river channels wider than a few kilometers. This limitation will be overcome by the future NASA/CNES satellite mission SWOT (Surface Water and Ocean Topography), which brings a wide swath altimeter for measuring water surface elevations at spatial resolution ranging from 10 m to 60 m in the cross-track direction and as small as 2 m in the along-track direction. The future integration between global river models and SWOT global measurement has the potential to increase our understanding and predictive skill of the detailed process of surface water storage and transport in the world major rivers.