Groundwater Responses to Deluge and Drought in the Fraser Valley,
Pacific Northwest
Abstract
Extreme weather events are reshaping hydrological cycles across the
globe, yet our understanding of the groundwater response to these
extremes remains limited. 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 catalogue was derived and associated to local
rainfall defining extreme precipitation. Droughts were quantified using
dry day metrics, in conjunction with the standardized precipitation
index (SPI). From September to January, approximately 40% of total
precipitation is contributed by ARs. From April to September, more than
50% of days receive no precipitation, with typically 26 consecutive dry
days. We used the autocorrelation structure of groundwater levels to
quantify aquifer memory characteristics and identified two distinct
clusters. 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 also driven by local precipitation, and are additionally
influenced by the Fraser River’s large summer freshet, briefly providing
a secondary recharge mechanism to South Coast aquifers. Accordingly,
groundwater recessions are offset to later in the summer, contingent on
the Fraser River, mediating drought. The results suggest that
groundwater memory encapsulates multiple hydrogeological factors,
including boundary conditions, influencing the response outcome to
extreme events.