the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Jun 2026
Extending terrestrial water storage anomalies beyond the GRACE era using tree rings
Abstract. Satellite observations from NASA’s Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On have transformed monitoring of terrestrial water storage (TWS) anomalies, but their short record (2002–present) limits characterization of long-term variability and extremes. Here, we evaluate whether tree-ring width indices (RWI) can be used to extend TWS variability across western North America. Using 144 chronologies, we find predominantly positive correlations between RWI and GRACE-observed TWS anomalies, with mean annual correlations of r = 0.25 (IQR: 0.07–0.50) and the strongest relationships during summer (mean r = 0.27; 37% of sites significant at p < 0.1). Correlation strength is highest in moisture-limited environments and among Pinus species. In the Upper Colorado River Basin, a two-site composite model explains 69% of the variance in GRACE-observed summer TWS anomalies (r = 0.83) and enables reconstruction back to 1567 CE. The reconstruction exhibits a standard deviation of 29.9 mm and a peak-to-trough range of 167.6 mm, and agrees well with other hydroclimatic records, including reconstructed Palmer Drought Severity Index over 1567-2000 (r = 0.79) and climate-data-driven TWS estimates over 1901-2000 (r = 0.73). Using climate-data-driven TWS estimates as a longer reference record, moving-window analyses show that calibration stability improves rapidly from 10-year to approximately 25–30-year windows, after which additional years yield more gradual gains. Together, these results suggest that the now nearly 25-year GRACE and GRACE Follow-On record is approaching the duration needed for more stable tree-ring–TWS calibration, and that targeted extension of moisture-sensitive chronology networks through the satellite era may provide a robust pathway for reconstructing multi-century TWS variability in western North America and other similar regions.
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Status: open (until 21 Oct 2026)
- RC1: 'Comment on egusphere-2026-3086', Anonymous Referee #1, 21 Jul 2026 reply
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RC2: 'Review of “Extending terrestrial water storage anomalies beyond the GRACE era using tree rings”', Benjamin Creutzfeldt, 21 Sep 2026
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General assessment:
This manuscript presents an interesting approach to extending GRACE-based terrestrial water storage information beyond the satellite era using tree-ring chronologies. Conceptually, reconstructing TWS from tree rings is particularly interesting because it goes beyond their more conventional use as proxies for individual climate variables such as precipitation or temperature. TWS represents an integrated hydrological state variable reflecting several storage compartments, and tree growth may therefore contain information on integrated water availability rather than only on individual meteorological drivers.I particularly appreciate the interdisciplinary combination of satellite gravimetry, hydrology, dendrochronology, and paleoclimatology. The approach could provide valuable long-term context for recent droughts and water-storage changes and help place recent extremes in a historical perspective. The authors appropriately present the Upper Colorado River Basin reconstruction as a proof of concept.
The main limitation remains the very short calibration period between GRACE observations and available tree-ring chronologies and the some statistictical methods applied. I should note that I have left academic research, do not have access to all references cited in the manuscript, and do not consider myself sufficiently up to date with current statistical reconstruction methodology to assess all statistical aspects in detail. Given the importance of these statistical aspects, I recommend additional review by an expert in reconstruction methods and/or dendroclimatology.
Overall, I find the concept scientifically valuable, but following aspects should be clarified.
General comments:
1. Spatial scale, previous work, and stationarity
Sect. 1, L48–60; Sect. 2.4, L108–136; Sect. 3.3, L219–225Creutzfeldt et al. (2015) investigated relationships between tree rings and locally observed gravity-derived TWS. The present study substantially extends this concept towards satellite gravimetry and basin-scale reconstruction. However, the spatial-scale issue is not eliminated but changes in nature: basin-integrated GRACE TWS is ultimately reconstructed from two local tree-ring chronologies. The spatial representativeness of these two sites for the entire Upper Colorado River Basin should therefore be discussed more explicitly.
A related issue is stationarity. Modern GRACE TWS may contain anthropogenic signals, for example from reservoir operations or groundwater abstraction, whereas tree growth may respond mainly to climate-driven water availability. Applying a relationship calibrated against a potentially mixed natural–anthropogenic TWS signal to several centuries before the modern period assumes that the TWS–RWI relationship is sufficiently stationary. This assumption and its implications should be discussed.
2. Derivation and standardization of RWI
Sect. 2.1, L64–72
The RWI processing is generally understandable, but it is unclear whether the authors performed these steps themselves or used already standardized chronologies from Williams et al. (2020). Please clarify this and provide methodological details if additional preprocessing was performed.3. PDSI comparison
Sect. 2.4, L147–155; Sect. 4.1, L254–270
The Steiger et al. (2018) dataset should be described more precisely as a multiproxy data-assimilation PDSI reconstruction rather than simply as “tree-ring-based”. Please specify which product was used, if it was spatially aggregated or statistic aggregated. Since both reconstructions contain regional tree-ring information, their agreement should be interpreted primarily as evidence of hydroclimatic consistency rather than fully independent validation.4. Climate-data-driven TWS reconstruction
Sect. 2.4, L152–156; Sect. 3.3, L233–240
The comparison with GRACE-REC is valuable because it is independent of the tree-ring information, but the exact product and processing should be documented more clearly. Please identify the GRACE-REC version and forcing used, explain how it was spatially aggregated and how summer values were derived. It should also be made clear that GRACE-REC is calibrated against GRACE and primarily represents climate-driven TWS variability. Finally, please explain why the comparison is restricted to 1901–2000 .5. Hydrological memory/Additional hydrological observations
Sect. 4.1, L260–270
The interpretation that TWS responds and recovers more slowly than near-surface hydroclimatic conditions is plausible, but the manuscript does not directly demonstrate a soil-moisture–TWS lag because PDSI is not a direct soil-moisture observation. This interpretation could be strengthened by observational studies such as Creutzfeldt et al. (2012), which showed different response and recovery times among near-surface soil moisture, groundwater, and integrated gravity-derived TWS during drought. This supports the concept of hydrological memory in deeper storage compartments.
The interpretation of delayed TWS responses would benefit from independent hydrological observations. Streamflow could provide an integrated basin response from the catchment outlet, complementary to the spatially integrated GRACE perspective, while groundwater observations could directly test delayed storage responses and multi-year drought persistence. I suggest use or mentioning these datasets as important targets for future work.6. Other controls on tree growth
Sect. 4.2, L282–300
Tree-ring width is not controlled by water availability alone. Nutrient availability, competition, diseases, disturbances, and other site-specific factors may introduce variability unrelated to TWS. These factors should be acknowledged as an additional limitation when interpreting RWI–TWS relationships.7. Potential for data-sparse regions
Sect. 4.2–4.3 and Conclusion
The potential application to data-sparse or poorly monitored catchments deserves stronger emphasis. Tree-ring chronologies could provide long-term information on hydroclimatic and storage variability where groundwater, soil-moisture, or streamflow observations are sparse or absent. This appears to be an important broader application of the approach.9. Interpretation of early instrumental data quality
Sect. 4.1, L271–280
The statement that early discrepancies likely reflect limitations in the “quality” of instrumental climate data appears too general. It would be more appropriate to refer to spatial coverage, station density, temporal homogeneity, and regional representativeness.
More importantly, disagreement between reconstructed TWS series cannot by itself be used to assess the quality of the underlying instrumental observations. Differences may also result from reconstruction methods, forcing data, spatial aggregation, or the variables being compared. The attribution to observational data quality should therefore be avoided.Minor comments:
TWS terminology: Throughout
Please use terrestrial water storage (TWS), TWS anomalies, and TWS variability/changes consistently. The study generally reconstructs anomalies rather than absolute storage, but these terms are sometimes used interchangeably.Sect. 2.1, L74–82 – Exact time series:
The methods refer to GRACE data for 2002–2016 but subsequently describe the comparison period as 2002–2017. Please clarify the exact periods used.Sect. 2.4, L108–114 – UCRB:
Please provide the basin area and a brief geographic description of the Upper Colorado River Basin (e.g. USA).Sect. 3.1, L160–175 / Fig. 1:
Please state explicitly that Fig. 1 refers to correlations with GRACE-derived JPL RL06 TWS anomalies and specify the temporal aggregation.Sect. 3.3, L219–225 – Two-site composite:
UNAWEP and USCHUL are introduced rather abruptly. Please provide basic information on their location, species, individual correlations with basin-scale TWS, and why these two sites were selected. Because the reconstruction ultimately depends on these two chronologies, their basic metadata should be readily available in the main text or figure caption.Concluding assessment
The manuscript presents an innovative and interdisciplinary approach for extending TWS information beyond the satellite era. Particularly interesting is the attempt to reconstruct an integrated hydrological state variable rather than an individual climatic driver. The approach may also be valuable in regions with limited long-term hydrological observations.The manuscript would benefit from clearer terminology and dataset documentation, a more explicit discussion of spatial representativeness and stationarity, and a more cautious interpretation of the PDSI and GRACE-REC comparisons. The short calibration period and some statistical methods remain the central limitations and makes independent statistical assessment particularly important.
With these revisions, the study could provide a useful proof of concept and a stimulating contribution to long-term terrestrial water-storage reconstruction, where the primary value lies in demonstrating the potential of the approach rather than in providing an exact reconstruction of historical TWS.
AI disclosure: AI-assisted tools were used to support language editing, structuring, and refinement of the review. The scientific assessment, interpretation, and final responsibility for the comments remain with the reviewer.
References cited in this review
Creutzfeldt, B., Ferré, T., Troch, P., Merz, B., Wziontek, H., & Güntner, A. (2012). Total water storage dynamics in response to climate variability and extremes: Inference from long-term terrestrial gravity measurement. Journal of Geophysical Research: Atmospheres, 117, D08112. [https://doi.org/10.1029/2011JD016472]
Creutzfeldt, B., Heinrich, I., & Merz, B. (2015). Total water storage dynamics derived from tree-ring records and terrestrial gravity observations. Journal of Hydrology, 529, 640–649. [https://doi.org/10.1016/j.jhydrol.2015.04.006]
Humphrey, V., & Gudmundsson, L. (2019). GRACE-REC: a reconstruction of climate-driven water storage changes over the last century. Earth System Science Data, 11, 1153–1170. [https://doi.org/10.5194/essd-11-1153-2019]
Steiger, N. J., Smerdon, J. E., Cook, E. R., & Cook, B. I. (2018). A reconstruction of global hydroclimate and dynamical variables over the Common Era. Scientific Data, 5, 180086. [https://doi.org/10.1038/sdata.2018.86]
Williams, A. P., Cook, E. R., Smerdon, J. E., Cook, B. I., Abatzoglou, J. T., Bolles, K., et al. (2020a). Large contribution from anthropogenic warming to an emerging North American megadrought. Science, 368, 314–318. [https://doi.org/10.1126/science.aaz9600]
Citation: https://doi.org/10.5194/egusphere-2026-3086-RC2
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- 1
Lucas E. B. Hoeltgebaum
Grant L. Harley
Fresh water stored on and beneath land helps people and ecosystems survive dry periods, but GRACE satellite records begin only in 2002. We tested whether tree rings can extend water storage records further back in time. Across western North America, tree growth often tracked yearly storage changes, especially in dry regions. These findings provide a new way to place recent droughts and water shortages in a much longer historical context.
Fresh water stored on and beneath land helps people and ecosystems survive dry periods, but...
This manuscript contributes to understand of the statistical relationships between tree-ring and GRACE-gravity data. This is an interesting because of the potential for tree rings to serve as proxies of historic water storage volumes. This manuscript emphasizes statistical relationships and takes a light approach to addressing the meaning of those relationships. Although statistical understanding can be useful on its own, the links between tree growth and water storage are understood well enough—both generally and specifically in the study region—that they deserve to be integrated into the statistical modeling and interpretation. By omitting most of the literatures on those topics, robust interpretation of the results of this work is difficult, and the utility of the proxy developed using tree rings remains unclear.
The hydrological meaning of GRACE variations is highly relevant to the current work, but the manuscript does not address this issue except cursorily. The nature of the hydrological store being investigated is just as important as the nature of the trees being investigated, and there are large variations in water storage across the study region. There is a substantial literature on the hydrological meaning of GRACE data in the region of this work, and it would improve the manuscript substantially if it were added to inform the methods and discussion of the results.
The utility of tree-rings for reconstructing hydrological history depends on why those correlations exist: it matters whether GRACE-relevant storage directly affects tree growth, or whether each of them are independently responding to climatic drivers. In the first case, we can use tree-rings to infer historic regional water storage, but in the second case we can only use tree-rings to infer the drivers of storage but not storage itself, because historic storage variations in the past were not subject to the same management effects as they are today, during the calibration period for the reconstruction.
Detailed comments:
L46 the hypothesis here is that trees respond to changes in water reservoirs measured by GRACE. However, these are vague causal statements, and they are in conflict with section 4.2, which says that tree growth in energy-limited regions is not well correlated with water availability. What is the non-statistical hypothesis of the study?
L68 stiffness of the splines is a crucial control on how much low-frequency variability is retained. What was done here? At what timescales is variance large in the GRACE data and are those same timescales emphasized in the tree-ring data? Large-scale changes in water storage are of course slower than soil moisture variability.
L88 L194 presumes causation, whereas noncausal correlation is not excluded.
L183 Pinus spp. are not likely to be directly accessing groundwater because they are mainly in uplands in semi-arid climates. This suggests the tree-ring and GRACE time series are both responding to a common driver, not that groundwater is controlling tree growth.
L233, Sec 4.1 if the TWS and PDSI reconstructions were based on the same tree-ring data, then these comparisons would be much more convincing. Why not include all 80 series of Steiger instead of just two series, if the goal is to understand whether the same tree-ring data can generate contrasting reconstructions?
L311 what climate-date-driven TWS anomalies? This analysis isn’t fully described. Omitting it would not hurt the manuscript.
Quibbles:
L28 conjugation of “characterize”
Table 1 is a little rough. Alphabetization by common name is annoying, column headings “species” and “genus” (though genus is not needed as a column at all because it is already listed in the species column), only one species name should be listed, authority names should not be italic.