Characteristics of surface water persistence in a non-perennial stream network

Abstract:

Intermittent and ephemeral streams make up the majority of the river network globally. Despite their prominence, we have a poor understanding of what drives the wet up and dry down dynamics of these systems, which has critical ecological and biogeochemical implications for how these systems will respond to future global change. Here, we present our work investigating the topographic and lithologic controls on the duration, timing, and sources of surface water at 40 locations during flow events in a 0.25 km² non-perennial stream network in a Mediterranean climate of the Coastal Range in central California. To identify changes in water chemistry and sources, we sampled our surface water locations for stable water isotopes (δ18O, δ2H), major ions, and nutrients across a range of flow conditions. The length of flowing surface water was dynamic, with an active surface network drainage density ranging from 0 to 8.1 km/km². We observed variability in dry down rates across the stream network and surface water pools that persisted throughout the dry season. Across the stream network, electrical conductivity values ranged from > 700 𝜇s/cm during dry conditions to < 50 𝜇s/cm at peak flows, suggesting spatiotemporal variation in surface water source. In particular, surface water pools were hot spots of progressively higher conductivity as the rest of the network dried down, suggestive of deeper sources sustaining flow. Our terrain analysis shows a complex relationship between flow presence and watershed characteristics, including slope, curvature, and contributing area. This work helps advance our understanding of the watershed characteristics that control the persistence of surface water, providing insights for managing water resources in intermittent and ephemeral stream networks.