| 14 Oct 2025
Investigating terrestrial water storage change in a western Canadian river basin with GRACE/GRACE-FO and fully-integrated groundwater–surface water modelling
Abstract. As hydrological trends shift in response to a warming climate, accurate characterization of hydrologic conditions and hydrologic change are imperative for water resources management, which is particularly important in the Canadian Prairies. In the study herein, a HydroGeoSphere (HGS) fully integrated groundwater–surface water (GW–SW) model is employed to evaluate trends and drivers of surface and subsurface water storage changes in the South Saskatchewan River Basin (SSRB). Terrestrial water storage anomalies (TWSA) derived from the Gravity Recovery and Climate Experiment (GRACE/GRACE-FO) are compared to HGS results; strong correlation is identified. The HGS solution facilitates decomposition of TWSA into constituent water storage components, namely surface water, soil moisture, and groundwater, and the GRACE/GRACE-FO solutions are used to validate the regional-scale TWSA and the interannual trends present in the SSRB TWSA time series. Meteorological and oceanic drivers and their impact on interannual hydrological trends in the SSRB are examined. Time-frequency analysis reveals a harmonic trend present in the SSRB TWSA with a period of 2.7–3.0 years, the inverse of which is present in the Oceanic Niño Index. The largest intra-annual water storage fluctuation is found in the soil profile, followed by snowpack, while groundwater experiences longer, multi-year cyclicity. Warmer oceanic conditions align with dry conditions in the SSRB and less snowpack, which leads to negative TWSA anomalies. Incorporating both high-resolution GW–SW models and regional-scale satellite gravimetry-derived estimates of TWSA facilitates a comprehensive analysis of hydrological dynamics in the Canadian Prairies and improved characterization of surface water and groundwater storage changes.
The authors investigated terrestrial water storage variability in the South Saskatchewan River Basin (SSRB) for the period 2002-2019 with use of GRACE/GRACE-FO data and the HydroGeoSphere (HGS) groundwater-surface water model. The results of the HGS model were used to decompose the terrestrial water storage anomalies (TWSA) derived from GRACE/GRACE-FO data into surface water, soil moisture, and groundwater. For each of the results, the time-frequency of the intra-annual water storage fluctuation was determined, and the impacts of meteorological and oceanic drivers were examined.
Dear authors,
It is an interesting study to read, and the description of the data is detailed and well-explained. In my opinion, the article could benefit from a more explicit or separate description of the methods and more elaboration on the limitations or challenges of the study. Below are my comments.
Specific comments:
Parts of the methodology were described in the Results and Discussion section. For example, in line 205, 206 and line 257-262. Having this placed in a separate methods section might make it easier to grasp what was done in which way and why. I recommend considering a separate methods section in addition to the description of the data.
I am missing the limitations and challenges in your chosen approach. For example, in line 105-108, you share about the advantage of CGS compared to mascon solutions. However, it was not mentioned how this (does not) show in your results, nor how it influences your outcome. It would be interesting to have these included. Could you elaborate more on these?
Section 2.1 describes how is dealt with the gaps in the GRACE/GRACE-FO data records in between the two missions for the JPL mascon data. How was this handled with the CGS method? This was not clear to me. Also, figure 3 shows a gap in the data, but figure 4 has a linear interpolation for the TWSA.
Figure 2(b): Please add a scale, north arrow and legend in the sub-figure.
Line 187-190: NAO is mentioned to be included in the analysis, but the only mention of it is in section 2.4. Is it included in the analysis? If so, where? If not, consider removing this part.
Line 202: The HGS TWSA values are compared to GRACE-derived values. In figure 3, it also shows a difference between the two GRACE-derived values (JPL and CGS). Could you specify which of the two it is compared to, or if it is to a combination of both, how this is done?
Figure 7 is interesting and nice. It would be clearer if the different subfigures had a title or something else clearly indicating which data they are showing (e.g., HGS TWSA, HGS surface water).
Line 287: Using word precipitation for the sum of rainfall + snowmelt might be slightly confusing for the reader. Was there a clear reason to define it as such? If not, perhaps the writers could consider using a different phrasing, such as “water input”.
Line 290 and Line 295 mention two different numbers: 90% confidence bounds and 95% confidence intervals. Which one is the correct one? Or if both are, could you clarify this in more detail to avoid confusion?