Diel fluctuations in summer streamflow depend on stream channel sediment storage and valley-floor vegetation in the forested western Cascades of Oregon, USA

During summer lowflow conditions, the zone of influence of evapotranspiration (ET) on streamflow is expected to be restricted to a small portion of a watershed, but the nature of the effective contributing area during lowflows is debated. We tested alternative hypotheses about the effective contributing area of ET-induced diel fluctuations in stream discharge based on high-resolution streamflow records since 2000 from eight small, forested watersheds at the HJ Andrews Experimental Forest in the Western Cascades of Oregon. Using field observations, satellite data, and a mathematical model, we tested the hypothesis that valley floor sediment storage influences the magnitude of diel fluctuations by controlling the area of potential connectivity between shallow groundwater (hyporheic flow) and surface flow. Correlations between minimum stream discharge and maximum air temperature were highest during early summer and wet years in WS1, which has a wide valley floor and high sediment storage. Field surveys and capacitance rod data in WS1 (young forest, alluvial reaches) and WS2 (old growth forest, bedrock reaches) revealed that diel fluctuations in local water tables were synchronized throughout the watersheds, and fluctuations were larger in vegetated alluvial reaches (1 to 2 cm) than in bedrock reaches, which lacked trees (<0.5 cm). Stream channels and tree heights delineated on LiDAR data were combined with published allometric relationships and sapflow data to estimate total transpiration from trees growing only in alluvial reaches; this could account for daily ET estimated from whole-watershed discharge records. Analytical solutions of a simple mathematical model describing the outflow from stream banks as a function of sapflow and hydraulic conductivity indicated that daily maximum and minimum sapflow could produce the fluctuations observed in water table heights in alluvial reaches of the stream channel.