Abstract
Radar altimetry is now commonly used to provide long-term monitoring of inland water levels in complement to or for replacing disappearing in situ networks of gauge stations. Recent improvements in tracking and acquisition modes improved the quality the water retrievals. The newly implemented Open Loop mode is likely to increase the number of monitored water bodies owing to the use of an a priori elevation, especially in hilly and mountainous areas. The novelty of this study is to provide a comprehensive evaluation of the performances of the past and current radar altimetry missions according to their acquisition (Low Resolution Mode or Synthetic Aperture Radar) and tracking (close or open loop) modes, and acquisition frequency (Ku or Ka) in a mountainous area where tracking losses of the signal are likely to occur, as well as of the recently launched ICESat-2 and GEDI lidar missions. To do so, we evaluate the quality of water level retrievals from most radar altimetry missions launched after 1995 over eight lakes in Switzerland, using the recently developed ALtimetry Time Series software, to compare the performances of the new tracking and acquisition modes and also the impact of the frequency used. The combination of the Open Loop tracking mode with the Synthetic Aperture Radar acquisition mode on SENTINEL-3A and B missions outperforms the classical Low Resolution Mode of the other missions with a lake observability greater than 95%, an almost constant bias of (−0.17 ± 0.04) m, a RMSE generally lower than 0.07 m and a R most of the times higher than 0.85 when compared to in situ gauge records. To increase the number of lakes that can be monitored and the temporal sampling of the water level retrievals, data acquired by lidar altimetry missions were also considered. Very accurate results were also obtained with ICESat-2 data with RMSE lower than 0.06 and R higher than 0.95 when compared to in situ water levels. An almost constant bias (0.42 ± 0.03) m was also observed. More contrasted results were obtained using GEDI. As these data were available on a shorter time period, more analyses are necessary to determine their potential for retrieving water levels.
Highlights
Altimetry time-series of water levels were generated over the following Swiss lakes: Geneva, Neufchâtel, Thunersee, Vierwaldsee, Zurichsee, Walensee, and Zugersee using data from ERS-2, ENVISAT, JASON-1/2/3, SARAL, and SENTINEL-3A and B
As the number of Global Ecosystem Dynamics Investigation (GEDI) overpasses is below 10 except over Lakes Geneva and Neuchâtel, root-mean-square error (RMSE) and R were only determined over these two lakes, but the global biases were estimated over every lake and every beam
These values exhibit a wide range of values, from −0.37 to 0.62 m depending on the beam and the lake considered (Figure 4b)
Summary
Lakes and reservoirs are considered as sentinels, integrators, and regulators of climate change owing to rapid responses of their physical, chemical, and biological properties to Remote Sens. Variations in lake water levels are directly reflecting the impact of climate change and anthropogenic actions such as of strong rain event, a long-lasting drought, or excessive pumping in the lake or its underneath groundwater for irrigation or human consumption purposes. Decline in in situ water stage gauge numbers has been reported worldwide, including for lakes [7,8]. In this context, radar altimetry has demonstrated strong capabilities in the monitoring of inland water bodies, and especially of lakes [9,10]. Advances in sensor properties, such as the acquisitions at Ka-band on SARAL in 2013, which provided measurements with a smaller footprint and a higher bandwidth [11]; the generalization of the Synthetic Aperture Radar (SAR) mode [12] on the recent radar altimetry missions (i.e., Cryosat-2, Sentinel-3, and Sentinel-6/Jason-Continuity Service (CS)) to reduce the size of the altimeter footprint in the along-track direction; the Open-Loop (OL) or Digital Elevation
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