A novel Multiple Persistent Peaks (MPP) retracker to improve global inland water level monitoring from satellite radar altimetry

Abstract

Satellite altimetry has become an indispensable technique for large-scale monitoring of inland water levels, which is fundamental to understanding the hydrologic dynamics of surface water and informed water resource management in the scenario of changing climate. Yet the capacity of inland altimetry can be further improved regarding the accuracy and the spatial-temporal coverage of water level estimates. This study presents a new Multiple Persistent Peaks (MPP) retracker to improve the retrieval of inland water levels from satellite radar altimetry. Focusing on the long-term Jason-2/3 observations, we demonstrate how the MPP retracker works in the case of complex and multipeaked waveforms commonly encountered in inland radar altimetry. The MPP retracker determines surface elevations of the target water body by grouping and multi-objective analysis of multiple along-track height profiles corresponding to persistent peaks in the aligned averaged waveform. We evaluated the results globally at 41 gauge stations and 6 crossovers with various climate, terrain, and hydrologic settings. The root-mean-square-error (RMSE) of the altimetry-derived water levels with in situ measurements is between 0.19 m and 0.78 m (median value of 0.38 m) and within the range of 0.25–0.49 m at crossovers. The validations show that the new automated method outperforms classical retrackers in terms of accuracy and robustness and is applicable to both pulse-limited and SAR altimetry. Our approach successfully retrieved good quality water level time series at river reaches with highly heterogeneous landscapes where most retrackers exhibit errors of several meters. Based on the MPP retracker, we provided an efficient tool for sophisticated outliers filtering and correction in the final time series, which is desirable for expert evaluation and improving the quality of altimetry-derived water level products. The new automated retracker and the tool we provided is a critical step toward improved monitoring of inland water levels from satellite altimetry on a global and long-term scale.