Nitrogen loss rates in streams: Scale‐dependence and up‐scaling methodology

We show that the large spatial aggregation of model parameters in common catchment scale nitrogen budget modeling leads to artifacts that may, for instance, be a factor in explaining reported decreases of calibrated in‐stream nitrogen loss rates, λ s *, with increasing stream size. In general, the common assumption of a single representative solute travel time for an entire stream reach may lead to considerable underestimation of the actual underlying local biogeochemical loss rate λ s by λ s *, which increases with actual λ s value and/or increasing mean solute travel time and travel time variability in the stream. We propose an up‐scaling methodology to overcome such model artifacts, in form of closed‐form expressions of catchment‐scale, in‐stream nitrogen delivery factors for diffuse and point sources, as functions of local‐scale nitrogen loss rates, λ s .