THE EFFECTS OF VARIED STREAM FLOW ON SURFACE AND HYPORHEIC TRANSIENT STORAGE EXCHANGE

To understand how stream networks function both hydraulically and biogeochemically, an evaluation of transient storage exchange dynamics throughout the network is required. Transient storage is expected to vary through space in response to stream hydraulic and morphologic gradients, and through time as stream hydraulics change with varied discharge. The efficiency at which streams retain nutrients changes along the hydrograph; thus it is reasonable to assume some of these differences are controlled by changing transient storage exchange dynamics, as transient storage has been shown to promote biogeochemical cycling. To investigate the control of varied discharge at one location on transient storage exchange we applied a 1-D model with two storage zones to conservative solute injections and measured stream velocity distributions from a 3^rd-order stream in Massachusetts. This allowed us to quantify both surface transient storage (STS) and hyporheic transient storage (HTS) dynamics separately at varied discharge (n=3), and therefore determine how these disparate exchange processes change with respect with discharge. Our results showed that several model parameters (main-channel area, longitudinal dispersion, HTS area normalized to the main-channel, and the STS exchange coefficient) and storage metrics (the influence of HTS exchange on median transport time and average STS exchange flux) were positively correlated with discharge. Other storage metrics were negatively correlated (the influence of STS exchange on median transport times and average STS residence time); and some parameters and metrics showed no evident trend or were consistent over this range of discharges (STS area normalized to the main-channel, the HTS exchange coefficient, average HTS residence time, and average STS exchange flux). These results indicate that both STS and HTS exchange dynamics are sensitive to changing stream hydraulics through time, which may have important implications to biogeochemical processes. The assessment of STS and HTS for this reach over several discharges suggests that at each location along the network a range of storage exchange characteristics is likely to exist, partly based on discharge.