Abstract
Riparian ecosystems in drylands face increasing risks from intensifying droughts, which lower water tables, reduce soil moisture and suppress streamflow—threatening vegetation and risking ecosystem collapse. Although riparian vegetation relies on subsurface water, the relative importance of groundwater versus rainfall‐infiltrated soil moisture during drought remains unclear. As climate change prolongs drought severity, understanding how plants shift between water sources is key to predicting ecosystem resilience and guiding sustainable groundwater management. We conducted a stable isotope study along the Santa Clara River in southern California (2018–2020) during recovery from a severe (2012–2019) drought. We sampled δ 18 O p in plant xylem water from four native riparian woody species ( Salix exigua , S. laevigata , Populus trichocarpa , P. fremontii ) and the non‐native grass Arundo donax . Shallow soil moisture and groundwater were sampled to characterize endmember δ 18 O signatures. Isotope mixing models were developed to track shifts in water source contributions for each species over three growing seasons. Riparian plants showed opportunistic water use, relying on shallow soil moisture during wet periods and shifting to groundwater during droughts. Native taxa including Populus and Salix species increased groundwater use by up to 60% during drought, reflecting hydraulic flexibility and drought tolerance. In contrast, the invasive A. donax depended on shallow soil moisture for 64–86% of its water under all conditions. These findings underscore the importance of quantifying species‐ and site‐specific groundwater use. Incorporating such ecological insights into groundwater sustainability planning will be critical for protecting riparian vegetation and maintaining ecosystem function in a changing climate.