Comment on egusphere-2022-1051

Abstract

All major river systems in the Contiguous United States (CONUS) are impacted by dams. Many regional and global studies have looked at reservoir resilience to extreme events and quantified static characteristics, yet analysis of historical reservoir operations has been limited by a lack of data. Here we use the first national dataset of historical reservoir operations in CONUS, ResOpsUS, to analyze reservoir storage trends and operations over the last 40 years. We characterized seasonal operating patterns and show clear regional trends. In the eastern US which is dominated by flood control storage we see that storage peaks in the winter months with sharper decreases in operational range in the summer. While in the more arid western US where storage is predominantly for irrigation, we find that storage peaks during the spring and summer with increases in the operational range during the summer months. The Lower Colorado region is an outlier because it is arid and dominate by irrigation, but its seasonal storage dynamics more closely mirrored that of flood control basins. Consistent with previous studies we show that reservoir storage has decreased over the past 40 years, although our national fraction filled decreases are 50 % less than those shown previously. We also find that declines are occurring faster in more arid regions. Operational ranges (i.e. the difference between monthly max and min storage) have been increasing over time in more arid regions and decreasing in more humid regions. We also quantified hydrologic drought using the standardized streamflow index (SSI) and compared time it took for reservoir storage (expressed as anomalies in fraction filled) and SSI to recover. As would be expected, we see longer drought periods and more prolonged negative reservoir storage anomalies in the more arid basins. That said, nearly all regions have we show that the reservoir storage takes longer to recover from drought that the streamflow.