Abstract
Abstract. While GRACE (Gravity Recovery and Climate Experiment) satellites are increasingly being used to monitor total water storage (TWS) changes globally, the impact of spatial distribution of water storage within a basin is generally ignored but may be substantial. In many basins, water is often stored in reservoirs or lakes, flooded areas, small aquifer systems, and other localized regions with areas typically below GRACE resolution (~200 000 km2). The objective of this study was to assess the impact of nonuniform water storage distribution on GRACE estimates of TWS changes as basin-wide averages, focusing on surface water reservoirs and using a priori information on reservoir storage from radar altimetry. Analysis included numerical experiments testing effects of location and areal extent of the localized mass (reservoirs) within a basin on basin-wide average water storage changes, and application to the lower Nile (Lake Nasser) and Tigris–Euphrates basins as examples. Numerical experiments show that by assuming uniform mass distribution, GRACE estimates may under- or overestimate basin-wide average water storage by up to a factor of ~2, depending on reservoir location and areal extent. Although reservoirs generally cover less than 1% of the basin area, and their spatial extent may be unresolved by GRACE, reservoir storage may dominate water storage changes in some basins. For example, reservoir storage accounts for ~95% of seasonal water storage changes in the lower Nile and 10% in the Tigris–Euphrates. Because reservoirs are used to mitigate droughts and buffer against climate extremes, their influence on interannual timescales can be large. For example, TWS decline during the 2007–2009 drought in the Tigris–Euphrates basin measured by GRACE was ~93 km3. Actual reservoir storage from satellite altimetry was limited to 27 km3, but their apparent impact on GRACE reached 45 km3, i.e., 50% of GRACE trend. Therefore, the actual impact of reservoirs would have been greatly underestimated (27 km3) if reservoir storage changes were assumed uniform in the basin. Consequently, estimated groundwater contribution from GRACE would have been largely overestimated in this region if the actual distribution of water was not explicitly taken into account. Effects of point masses on GRACE estimates are not easily accounted for via simple multiplicative scaling, but in many cases independent information may be available to improve estimates. Accurate estimation of the reservoir contribution is critical, especially when separating estimating groundwater storage changes from GRACE total water storage (TWS) changes. Because the influence of spatially concentrated water storage – and more generally water distribution – is significant, GRACE estimates will be improved by combining independent water mass spatial distribution information with GRACE observations, even when reservoir storage is not the dominant mechanism. In this regard, data from the upcoming Surface Water Ocean Topography (SWOT) satellite mission should be an especially important companion to GRACE-FO (Follow-On) observations.
Highlights
Large rivers cross the Middle East and bring life to these dry regions, where throughout human history reservoirs have served to ensure an adequate water supply by storing water in times of surplus and releasing it in times of scarcity (Altinbilek, 2002), providing a buffer to mitigate seasonal and interannual variability in precipitation
Apparent basin storage exceeds True basin storage (TBS) when the point mass is concentrated in the basin center and diminishes rapidly as the mass moves offcenter due to leakage to regions outside the basin
We have investigated how spatial concentration and distribution of water masses affect GRACE water storage estimates
Summary
Large rivers cross the Middle East and bring life to these dry regions, where throughout human history reservoirs have served to ensure an adequate water supply by storing water in times of surplus and releasing it in times of scarcity (Altinbilek, 2002), providing a buffer to mitigate seasonal and interannual variability in precipitation. Regulation to ensure water supply and control floods may become even more critical in the future with projected increasing intensity of the hydrologic cycle (Easterling et al, 2000). Water inflows to reservoirs are mostly controlled by precipitation and water convergence from contributing basins, while outflows are subject to laws and policies serving agricultural, industrial, and urban needs, in addition to physical processes of evaporation and seepage. Most reservoirs are above ground, but below-ground reservoirs in aquifers (groundwater banks) with ∼ 2–3 km of water storage have been developed in the Central Valley of California, and are in service elsewhere
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