Groundwater Responses to Deluge and Drought in the Fraser Valley, Pacific Northwest

Groundwater level variations represent signals of superimposed physical processes, with memory. Groundwater level records are used to understand how aquifer systems respond to natural and anthropogenic perturbations. Here we analyze groundwater levels across the South Coast of British Columbia (BC) in the Pacific Northwest with the objective of determining groundwater responses to atmospheric rivers (ARs) and drought. An AR catalog was derived and used to associate precipitation amounts to AR occurrence. Droughts were quantified using dry day metrics, in conjunction with the standardized precipitation index. Historically (1980-2023), from September to January, approximately 40% of total precipitation was contributed by ARs. From April to September, more than 50% of days received no precipitation, with typically 26 consecutive dry days. We used the autocorrelation structure of groundwater levels, commonly used to characterize aquifer memory, to identify two distinct clusters of observation well responses. Cluster 1 wells respond to recharge from local precipitation, primarily rainfall, and respond rapidly to both ARs during winter recharge and significant rainfall deficits during summer. Cluster 2 wells are driven by local precipitation but are influenced by the Fraser River's large summer freshet which briefly recharges the aquifers, thereby delaying drought propagation. The results suggest that groundwater memory encapsulates multiple hydrogeological factors, including boundary conditions, influencing the response outcome to extreme events.