Addendum: Human alteration of global surface water storage variability

Check for updatesdue to the corrected lake areas.However, given that most of the error associated with area calculation occurs at high latitudes, where lakes are not as variable and where there are comparatively few reservoirs, the effect of this error on the percent storage variability occurring in reservoirs is minimal, especially at the basin-scale.In total, of the forty largest river basins for which we report percent storage change in reservoirs in Figure 4 and Extended Data Table 1, the mean difference between the original and updated values resulting from our new area calculation is ± 0.5%.Uncertainty bounds.While neither our water level retrievals from ICESat-2 nor our uncertainty analysis in comparison with USGS gauge data were affected by this correction, the corrected area calculations did affect our uncertainty estimates.This is because the uncertainty bounds reported in the original article were binned by lake area and then propagated throughout the results to produce overall uncertainty estimates.We have recalculated the uncertainty bounds binned by our corrected lake areas, leading to slight changes in uncertainty bounds that average 0.02 to 0.04 m (i.e., from 0.13-0.33 to 0.11-0.35)across all water bodies.Proportion of lakes observed by area.In the original article, we report the proportion of lakes that ICESat-2 observes by lake area.After performing this calculation accounting for our new area values, our results demonstrate that ICESat-2 actually observes a greater percentage of large lakes than we originally reported, increasing from 52% to 72% for lakes larger than 1 km 2 .Comparison with California Department of Water Resources (CDWR).Due to the area corrections, the area of California reservoirs shrank by 33% on average (owing to their location between 33° N and 40° N).As a result, we now find that our analysis using ICESat-2 slightly underestimates storage change relative to CDWR storage values (mean absolute error = 0.027 km 3 and mean absolute bias = -0.026km 3 , vs. 0.019 km 3 and -0.01 km 3 in the original article; see Extended Data Figure 6).Despite these updates, this corrected comparison to CDWR does not impact any of our key findings or conclusions.We have always known that our analysis underestimates lake storage variability due to ICESat-2's incomplete sampling as well as our use of a static, conservative water mask (see Methods section "Uncertainty in seasonal variability and bias analysis" for discussion of this sampling underestimation).Importantly, as this is a consistent underestimation, it does not influence our overall conclusions about the proportion of storage variability occurring in reservoirs since the underestimation due to both the temporal sampling and the use of a conservative lake mask is consistent across both artificial reservoirs and natural water bodies.The recalculation affects data reported in the Abstract, Results, Discussion, Methods, and Figures 3,4, Extended Data Figures 1,6 and Extended Data Tables 1234, which are all now updated in the HTML and