Retracked Jason-2 Altimetry over Small Water Bodies: Case Study of Bajhang River, Taiwan

Abstract

River surface heights (RSHs) are retrieved from Jason-2 (J2) 20-Hz radar altimeter waveforms over the narrow Bajhang River in Taiwan using waveform retracking and water detection techniques. Our technique is demonstrated over the narrow river, with changing widths of 100–200 m in the dry season and up to 1 km after heavy rains, which is significantly smaller in size compared to previous studies. Here, we used radar backscatter coefficients and accurately geolocated Google Earth Images validated by GPS campaigns for water detection and employed optimized waveform retrackers to retrieve 20-Hz accurate RSHs. The waveforms of the selected J2 ground tracks are carefully analyzed to remove land-contaminated data. Several waveform retracking algorithms, including offset center of gravity (OCOG), threshold, modified threshold, and ice retrackers are used to find the optimal algorithm by comparing the retracked RSHs with the nearby I-Chu stage gauge records and evaluating the computed improvement percentages (IMPs). The results indicate that the J2-derived RSHs over Bajhang River can be significantly improved by waveform retracking using the original and modified threshold retrackers with a 50% threshold level. The IMP reached 73.53% and the standard deviation of differences between the optimal retracked RSHs and stage gauge records is ∼31 cm. Our successful demonstration of the use of J2 to measure water level changes of the small river with seasonally varying widths implicates a potential new hydrologic application further exploiting satellite altimetry.