Marked heterogeneity in hydrology among headwater catchments gives rise to main stem seasonal dynamics
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Headwater streams play a crucial role in the landscape as sources of water and solutes to downstream locations. These streams are diverse and influenced by local physiography (geology, geomorphology) and meteorological factors, such as the amount and form of precipitation inputs. We investigated water contributions and flow paths of four tributaries in Lookout Creek a 64 km2 catchment within the H.J. Andrews Experimental Forest, USA. We coupled composite precipitation and stream grab samples that were analyzed for water stable isotope ratios and major anions and cations. Seasonal patterns in isotope ratios and deuterium excess revealed that most water in the streams originates from fall and winter precipitation events. However, seasonal variability was substantial in tributary water contributions across the catchment. Tributaries strongly influenced by groundwater contributed between 7–90% of the flow downstream of a junction with the main stem with maximum contributions occurring in summer. On the other hand, precipitation-dominated tributaries had maximum contributions during high flows in the wet season . Groundwater-dominated streams had a more constant discharge per unit area over a year compared to streams influenced by episodic precipitation events. The spatial variation of summer unit discharge among 15 small tributaries draining through culverts reflected the heterogeneity in source water, ranging from 0.3 to 78 L/s/km². Our study also revealed substantial heterogeneity in chemical solute behavior, indicating variations in rock-water interactions through time and runoff generation processes. The concentration-discharge behavior of geogenic species (Ca, Mg, Na, K, and Si) showed strong dilution patterns for precipitation dominated systems, while chemostatic behavior was observed in groundwater dominated systems. End-member mixing analysis emphasized the higher degree of variability in streamflow contribution among tributaries compared with the main stem, indicating different contributions of groundwater vs. precipitation in these tributaries. This research contributes valuable insights into the spatial heterogeneity and temporal dynamics of flow and source water in headwater streams.