Lake and reservoir volume variations in South America from radar altimetry, ICESat laser altimetry, and GRACE time-variable gravity

Thanks to 25-years of radar altimetry measurements from TOPEX/POSEIDON, Jason 1, 2 and 3, ENVISAT and others, water level changes of major lakes and rivers can be measured regularly, remotely, with unprecedented precision, facilitating monitoring of continental water storage variations. In addition, the 18 laser altimeter measurement campaigns over 6 years by the Ice, Cloud and Land Elevation Satellite (ICESat) satellite complement these radar altimetry measurements and also validate them, expanding our monitoring capabilities. We extract derived water height estimates for various lakes in South America from radar and laser altimetry measurements. Using available imagery from the Moderate Resolution Imaging Spectroradiometer (MODIS), we estimate time-variable surface extent of these water bodies, and together with the altimetry, we derive corresponding estimates of relative volume variations for a number of lakes and reservoirs in South America. We see good agreement between our water extent and available products, with some differences when snow covered area are present. We find good agreement between the temporal variations of lake water heights obtained from ICE Sat, radar altimetry, and in situ measurements, when available. Lake-dependent biases still exist, likely due to differences in reference datum, choice of geoid model, and differences in data processing schemes. The is a good agreement in volume changes obtained from imagery and the various altimetry techniques. Furthermore, using the latest one-degree global iterated mascon solutions from NASA Goddard Space Flight Center, we successfully compare them to independent volume changes assessments for these water bodies, derived from mass variations from time-variable gravity measurements from 15 years of Gravity Recovery And Climate Experiment (GRACE) observations. The agreement in water volume changes from GRACE with those from altimetry and imagery is higher for lakes in arid and semi-arid regions than on mountainous areas. GRACE volume estimates remain limited to the spatial resolution and the quality of the hydrology models used to correct for soil-moisture and snow contributions. Overall, studies like the one presented here further validate the power of leveraging these data synergies to monitor continental water storage changes from space, taking advantage of what each of the techniques has to offer. We show that water level measurements from laser and radar altimeters have similar precision, of a few centimeters over inland water bodies, and prove that GRACE global mascons allow to retrieve small lake relative volume changes, even for small bodies, thanks to their higher sensitivity to small wavelength mass variations. (C) 2020 COSPAR. Published by Elsevier Ltd. All rights reserved.

Automated summary

By utilizing radar and laser altimetry techniques along with satellite observations, we successfully derived estimates of relative volume changes from various lakes and reservoirs in South America. Comparisons between different data sources showed good agreement, but also revealed some discrepancies, particularly in snow-covered areas. The latest global iterated mascon solutions demonstrated their accuracy and sensitivity, enabling us to more effectively monitor changes in surface water bodies.