Plain Language Summary
Filling a reservoir prevents that water from flowing back to the ocean, thus causing sea level to fall. But sea level does not change by the same amount everywhere: the mass of the water in the reservoir causes sea level to rise in locations near the reservoir, but fall in locations that are farther away. We use databases that include the locations and capacities of reservoirs to estimate how constructing reservoirs has changed sea level, and explore how these changes have varied in space and in time. We find that while constructing reservoirs since 1900 has caused sea level to fall on average, there are some locations that experience a dramatic sea level rise, as much as 40 mm, over a short period of time, usually only a few years. As more reservoirs are built, we expect this trend to continue. In fact, reservoirs that are currently being planned—if completed—will generate a sea level rise of a few millimeters in some low-lying coastal areas. This rise would be in addition to the rise in sea level from many other known factors.
1 Introduction
Global and regional changes in sea level are driven by a wide range of processes, including the redistribution of mass from melting glaciers and ice sheets, the continued adjustment of the solid Earth to ice mass changes during the last glacial period, steric expansion of ocean water, ocean circulation changes, and both natural and artificial changes in terrestrial water storage (Cazenave et al ., 2018; Kopp et al ., 2015). Proper characterization of each of these different components of the sea level budget is necessary for accurate projections of future changes in sea level. In this context, the role of terrestrial water storage remains underexplored and serves as the focus of the present study. We provide the most complete picture to date of the impact that artificially impounded water has had on sea level over the last century, resolved both spatially and temporally, and an estimate of future sea level changes due to projected reservoir construction.
Chao et al. (2008) use the International Commission on Large Dams World Register of Dams (WRD) (www.icold-cigb.org) database to construct an estimate of the global mean sea level (GMSL) change due to the construction of 29,484 reservoirs worldwide. They estimate the total volume of impounded water to be 10,800 km3, corresponding to a GMSL fall of ~0.55 mm yr-1 in the half century prior to their publication. Sea level changes associated with the impoundment of water on land will be geographically variable (Fiedler and Conrad, 2010) and each reservoir will have a unique sea level “fingerprint,” or gravitational, rotational, and deformational (GRD) response to mass redistribution (Gregory et al ., 2019). The redistribution of water from the ocean to the reservoir will (1) increase the gravitational attraction of the reservoir on the surrounding water and thus raise the local sea surface height, (2) induce a change in Earth’s moment of inertia, and (3) drive local crustal subsidence. Indeed, relative sea level (RSL) will rise within ~2000 km of a reservoir being filled, despite a drop in GMSL, and it will fall by increasing amounts at larger distances from the source of impoundment. The local signal, which can have a peak value an order of magnitude larger than the GMSL change associated with the impoundment, is primarily a result of processes (1) and (3). Calculating the global sea level pattern associated with water impoundment requires knowledge of both the size and location of a reservoir.
The WRD database adopted by Chao et al. (2008) includes the largest global tabulation of reservoirs and the most complete estimate of the total volume stored in those reservoirs. However, it does not provide locations for these reservoirs and thus cannot be used to generate maps of the associated sea level change. To compute such a map, Fiedler and Conrad (2010) adopt a dataset (Vörösmarty et al ., 1997) that includes the locations of 674 reservoirs currently built and scale their result upwards to match the GMSL cited by Chao et al . (2008). We extend their analysis in three ways. First, we make use of a much larger database of reservoirs. Second, we explore both the spatial and temporal patterns of RSL change associated with water impoundment. Finally, we project the signals into the future using a database of planned dam construction (Zarfl et al ., 2015).
2 Reservoir Databases
In the present study, we use two different databases of reservoir construction. For the time period 1900 – 2011, we use the Global Reservoir and Dam (GRanD) database (Lehner et al. , 2011), which aims to geospatially reference all reservoirs with a capacity of more than 0.1 km3. It contains 6,329 reservoirs that were completed after 1900 and reports their location, year of construction, and capacity (Fig. 1a). The total volume of these reservoirs is 5,979 km3, which is less than 8,300 km3, as reported in the WRD (Chao et al ., 2008). Time series of integrated water volume impoundment (and equivalent GMSL fall) for the GRanD database is shown in Fig. 2 (blue histogram). Although the GRanD database contains only ~72% of the volume reported in the WRD, we do not scale our total impounded water volume to match the WRD values because the primary purpose of this study is to estimate the spatial variability of sea level, and water impoundment not included in the GRanD database has an unknown geographical distribution.