Contribution of Diffuse- and Point-Source Nutrient Loads to Emergent Scaling in Urbanized River Networks.

River network structures are known to be scale-invariant, characterized by Horton scalingratios, and power-law relations between geomorphological variables (e.g., Hack’s law). Rivernetworks also manifest scaling character for their functions, such as hydraulic geometry, orin-stream nutrient uptake processes. Human-related variables (e.g., human settlements) alsoreveal globally consistent scale-invariance over stream orders within a river basin. Based onscaling properties of river networks, recent work demonstrated scaling of dissolvedorganic carbon removal at the basin-scale (exported from diffuse sources). Ourrecent study showed that point-source nutrient loads discharged from wastewatertreatment plants (WWTPs) exhibited scaling properties in three large river basins inGermany. In the current study, we link these scaling ideas to investigate how spatiallyheterogeneous nutrient loads discharged from multiple urban point sources (i.e.,WWTPs) scale across urbanized basins. Available data from three German riverbasins for size distribution and locations of WWTPs are used to determine scalingof nutrient loads based on stream order hierarchy and spatial patterns based onGeomorphologic Width Function approach. Nutrient loads from spatially heterogeneouspoint sources are super-imposed in diffuse nutrient loads to produce spatially heterogeneouspatterns of nutrient concentrations. We examine the joint contribution of hydrologicdilution and in-stream processes to the emergent scaling across the entire basin ofwater-quality impairments (nutrient concentrations exceeding specified thresholds). [ABSTRACT FROM AUTHOR]