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.