Seasonal variations and long-term trends of groundwater over the Canadian landmass

Detailed knowledge of groundwater storage improves the understanding and management of water resources. Observations from Gravity Recovery and Climate Experiment (GRACE) satellites have provided data on global terrestrial-water-storage (TWS) changes since 2002. Combining GRACE-TWS and land-surface model (LSM) estimates of soil water, snow-water equivalent and surface-water storage provides a method to quantify groundwater storage (Wground). This study examines the Wground seasonal variations and trends for Canada’s landmass during the period 2003–2016 using GRACE-TWS and the Canadian LSM EALCO (Ecological Assimilation of Land and Climate Observations) model. The results show the study region has a maximum seasonal variation (ΔWground) of 118 mm (volume equivalent 700 km3), with the maximum/minimum Wground appearing in July/April. Eastern Canada has relatively large ΔWground values, up to 400 mm in Newfoundland. The Prairie region has the smallest value (<50 mm). The western and central regions show the maximum/minimum Wground mostly in spring/fall. In contrast, eastern Canada has the maximum/minimum Wground mostly in fall/spring. South Ontario and the Prairie area show the maximum/minimum Wground in summer/winter. Additionally, the Wground trends over the 14-year study period present large spatial variability, with increasing trends of up to 10 mm/year in eastern Canada and decreasing trends (similar magnitudes) in the west. The increasing trend largely offsets the decreasing trend in the study area, and the overall Wground for the region does not show a significant trend during 2003–2016. Comparison of Wground with groundwater well measurements present similar long-term trends but with a phase difference in seasonal variations.