GloLakes: a database of global lake water storage dynamics from 1984 to present derived using laser and radar altimetry and optical remote sensing

Measurements of the spatiotemporal dynamics of lake and reservoir water storage are fundamental in the assessment of the influence of climate variability and anthropogenic activities on water quantity and quality, as well as wetland ecology and the estimating greenhouse gas emissions from lakes. Previous studies estimated relative water volume changes for lakes where both satellite-derived extent and radar altimetry data are available. This approach is limited to only 10 few hundreds of lakes worldwide. In this study, the number of measured lakes was increased by a factor 400 using high-resolution Landsat and Sentinel-2 optical remote sensing and ICESat-2 laser altimetry in addition to radar altimetry from the Topex/Poseidon, Jason-1, -2 and -3, and Sentinel-3 instruments. Time series of relative (i.e., storage change) or absolute (i.e., total stored volume) storage for more than 170,000 lakes globally with a surface area of at least 1 km 2 (representing 99% of the total volume of all water stored in lakes and reservoirs globally) were retrieved. Within these, we were able to 15 develop an automated workflow for near real-time global lake monitoring of more than 27,000 lakes. The historical and near real-time lake storage dynamics data for 1984 to current are publicly available through https://doi.org/10.25914/K8ZF-6G46 (Hou et al., 2022). the digital elevation models and can vary in different landscapes depending on relief and any change from above- to below-water topography. Khazaei et al. (2022) used 310 HydroLAKES-derived surface area, elevation, volume, shoreline length, and watershed area to estimate maximum lake depth, and applied a distance method (Hollister and Milstead, 2010) to develop a lake bathymetry map by converting Euclidean distance to shoreline into depth based on the estimated maximum depth. Doing so they produced the GLOBathy dataset, which provides the height-area-volume ( h-A-V ) relationship for each lake delineated by HydroLAKES. We used these h-A-V relationships as an alternative to the geo-statistical method, and estimated absolute lake storage dynamics from 315 1984 onwards and validated the resulting alternative product against in situ lake storage records. The SMAPE error resulting from the GLOBahty method is 70.5%, which is greater than that from the geo-statistical method. This led our choice to use the geo-statistical method in favour of the GLOBathy dataset in this study. ICESat-2) at global scale. This product comprises both relative and absolute volume estimates for all HydoLAKES delineated lakes larger than 1 km 2 . To achieve more frequent water extent measurements, a simple image gap-filling algorithm was implemented in each historical Landsat-derived surface water map. This process effectively solves missing 365 data issues caused by, e.g., cloud, cloud shadow, swath edges and the Landsat-7 SLC failure. The monthly Landsat-derived water extents in this study show strong correlations with GRSAD v2 for 5,323 reservoirs, with an average R of 0.88. We demonstrated that the geo-statistical HydroLAKES bathymetry estimation approach produced slightly better results than the GLOBathy method. The geo-statistical method was applied to estimate absolute lake volume dynamics from 1984 onwards by combining Landsat and Sentinel-2 imagery. Validation results showed an average R of 0.78 and a SMAPE of 52.5% 370 between estimated and reported volumes for 238 lakes in USA and Australia. In situ bathymetric measurements would be ideal to more precisely estimate total lake volume, but are not available for the majority of the millions of lakes globally.