The South Saskatchewan River Basin (SSRB), spanning from the Rocky Mountains to the Great Plains, has hydrologic complexities which make it prone to both droughts and floods. Hydrologic dynamics in the basin are driven by snowpack, surface water flows, soil moisture, and groundwater, with disproportionate regional sensitivities to different drivers. Model-based analysis of SSRB water balance necessitates consideration of surface water and groundwater, and interactions between the two components. Validating such models is difficult because large areas within the SSRB are effectively ungauged. Recognizing this challenge, the study herein utilizes a HydroGeoSphere (HGS) fully integrated groundwater – surface water model validated against in-situ data and Gravity Recovery and Climate Experiment (GRACE/GRACE-FO) time-variable gravity field data to evaluate interannual water storage trends across the SSRB. Results indicate alpine subbasins have high amplitude trends related to snowpack, while eastern subbasins have lower amplitude trends more closely related to soil moisture and groundwater. Snowmelt and rainfall in the alpine regions have a greater impact on surface water storage than in the prairie regions. Comparison against large-scale oceanic pressure and temperature anomalies reveals negative correlation between the Pacific Decadal Oscillation and alpine snowpack, however the relationship is not significant for snowpack in the prairie regions. The results of this study demonstrate how groundwater – surface water modelling combined with GRACE/GRACE-FO can support sustainable water management in the SSRB.