Changes in Dominant Streamflow Drivers as Network‐Scale Flow Regime Shifts From Intermittent to Ephemeral Across a Multi‐Year Drought

ABSTRACT Dynamic surface water wetting and drying are characteristic of most headwater stream networks, influencing ecological function and downstream water quality. While topography and subsurface properties have been shown to influence spatial surface water persistence, it is unclear how sensitive non‐perennial stream network flow regimes are to persistent drought conditions. To address this unknown, we monitored surface water presence and absence for three consecutive drought years at 31 locations across a 0.25 km 2 non‐perennial headwater stream network in central coastal California. We coupled these observations with landscape characteristics and climatic conditions to examine both physical and climatic drivers of spatiotemporal flow activation and persistence. We observed non‐stationarity in flow regimes throughout the stream network; reaches that were characterised by seasonal flow in 1 year became ephemeral as the drought conditions progressed. We observed declining spatial variability in surface water persistence and declining correlations between persistence and landscape attributes with ongoing drought. From year one to year three, as drought conditions intensified, correlation coefficients between surface water persistence and topographic wetness index declined from r = 0.60 to r = 0.17, stream channel slope declined from r = 0.47 to r = 0.21, profile curvature declined from r = −0.51 to r = −0.21, and elevation declined from r = −0.33 to r = −0.06. A principal component analysis suggests that, unlike prior years, flow activation events after several years of drought were more closely associated with precipitation event characteristics than antecedent storage states. This work suggests semi‐arid landscapes such as central coastal California may see shifts in the flow regimes of aquatic systems as landscape aridity intensifies with climate change. As predictive indicators of flow may shift from physical to climatic factors, the timing, intensity and frequency of individual storm events may play increasingly larger roles in driving annual‐scale flow conditions.