How do evapotranspiration-driven discharge fluctuations alter reach-scale ecosystem function?

Evapotranspiration on hillslopes can cause discharge in headwater streams to fluctuate on a diurnal basis. At some locations and times, the net result can result in spatial intermittency of stream discharge - specifically the diurnal presence and absence of surface discharge along a fixed study reach. We expect ET-driven (de)activations of stream flow, and more generally the diurnal variation in hydraulic gradients linking the stream and subsurface, to alter exchange fluxes and transit times along a stream reach. Variations should be both continuous as discharge rises and falls in response to ET and exhibit threshold behavior as the stream activates and deactivates. Associated with these changes, a number of system states fluctuate on a diurnal basis including changes in exchange flux, reach scale transit times, locations oscillating between saturated and unsaturated, changes in the relative flux along flowpaths that do not change in time, and changes in the geometry of some flowpaths as they activate and deactivate. Given the changes in both biogeochemical forcing (e.g., saturated to unsaturated conditions) and physical transport (e.g., changing timescales and relative fluxes along flowpaths, activation of new flowpaths) we expect diurnal variation will be observable in reach-scale ecosystem function, including respiration and nutrient spiraling.

Here, we present results of a series of four injections of both conservative and reactive tracers along a study reach with diurnal fluctuations in discharge, including two locations where flow is completely subsurface along the stream. We interpret physical transport by analyzing conservative solute tracer breakthrough curves using both time-series analyses and fitting a time-variable StorAge Selection (SAS) function, and ecosystem function using Resazurin-to-resorufin transformation rates (both modeled and based on field data).