Spatio-temporal controls on diurnal streamflow fluctuations

Diurnal fluctuations in stream flow are widely observed across the world, yet the spatio-temporal sources of these fluctuations are poorly understood and conceptualized. Evapotranspiration (ET) is often assumed to be the main driver of diurnal fluctuations during the growing season. We compared eddy covariance derived evapotranspiration dynamics to groundwater and stream flow diurnal fluctuations across 5 growing seasons in the Tenderfoot Creek Experimental Forest, MT. We analyzed the timing of peak daily ET to the minima of hillslope, riparian and stream diurnal water levels (diurnal time differences; DTDs). DTDs were consistently between 5 and 8 hours during the growing season and there was a subsequent decoupling with more variability (up to 24 hours) during non-growing season periods. In order to better understand the mechanisms and transfer of ET signals that lead to diurnal fluctuations in stream flow, we analyzed the DTDs of shallow groundwater wells across different landscape positions (with varying degrees of saturation and hydrologic connectivity). The riparian zone DTDs were similar to those observed for streamflow. However, across hillslope well landscape positions, the DTDs were progressively different from the streamflow and riparian DTDs as a function of hydrologic connectivity to the stream network. Our analyses suggest that stream diurnal signatures result from landscape propagation of ET signals from hydrologically connected landscape positions that vary through time.