Separation of River Network Scale Nitrogen Removal Between Surface and Hyporheic Transient Storage Compartments

Reach scale experiments have shown that the characteristics and distribution of surface transient storage (STS) and hyporheic transient storage (HTS) may be important controls on nitrogen (N) export to coastal waters. We investigated the relative impact that STS and HTS have on N removal at the river network scale using a daily time step river network N removal model applied to the Ipswich River (a 5th order basin) in northeastern Massachusetts, USA. Spatially distributed runoff and discharge were predicted using a daily time step Water Balance and Routing Model. Nitrogen inputs to streams were calculated using simulated runoff and N concentrations based on land use type. Field investigations in 1st through 5th order reaches of the Ipswich River provided the scaling rules for hydraulic characteristics of STS and HTS throughout the network. The size of the STS and HTS relative to the size of the channel cross section (As:A) had positive relationships with stream size whereas the coefficients of exchange between the transient storage compartments and the main channel remained generally constant. On average, the cross-sectional area of the HTS was 4x that of STS while the exchange coefficient of the STS was 18x greater than that of the HTS. Nitrogen removal was simulated in three channel compartments (STS, HTS and the main channel) for every river grid cell using hydraulic characteristics, simulated river discharge and a time specific removal rate (k). For our initial model runs we assumed that k was identical in all compartments to assess how gradients in STS and HTS hydraulic parameters as a function of stream size influence network scale N removal and its distribution across stream order. Model results indicate that N removed in the HTS potentially dominates N removal, both at the reach and river network scales, and that the relative importance of HTS increases in larger streams. This suggests that the longer residence time of water in the HTS compared to STS outweighs the effect of smaller exchange rates between the main channel and the HTS compared to STS in determining the fate of N. However, a better understanding of the rates of various N cycle processes in both the STS and HTS is needed to identify the fate of N in entire river systems.