Evaluating the Emergent Controls of Stream Water Quality with Similitude and Dimensionless Numbers

Abstract

The emergent hydrologic and land-use controls of coastal-urban stream water quality were evaluated by using similitude and dimensional analysis, considering southeast Florida a prototype of growing coastal-urban environments. The goal was to test a fundamental hypothesis that the coastal-urban stream water quality processes represent emergent ecohydrological-biogeochemical similitudes (parametric reductions). The in-stream total nitrogen (TN), total phosphorus (TP), and biomass (Chl a) were normalized by their immediate upstream reach concentrations to formulate the dimensionless numbers of TN/TN0, TP/TP0, and Chl a/Chl a0. Stream dissolved oxygen (DO) was normalized by its saturated concentration (DOsat) to obtain the dimensionless DO/DOsat number—avoiding a misleading scaling by upstream concentrations in the presence of a DO sag phenomenon. The emergent controls of stream water quality were represented by a small set of dominant driver dimensionless numbers. For each water quality indicator, nine original variables (including predictors and response) were reduced to three to four important and mechanistically meaningful dimensionless numbers. The hydrologic control number (role of watershed hydrology versus the external Everglades) and salinity number (ratio of downstream to upstream salinity) exhibited the key controls on stream TN/TN0 across the wet and dry seasons. In contrast, the land-use number (ratio of agricultural plus vegetated lands to built lands), hydrologic control number, and salinity number dominated TP/TP0 and Chl a/Chl a0 incorporating the two seasons. However, DO/DOsat was controlled by the hyporheic exchange number (role of watershed groundwater versus surface hydrology) and land-use number in the wet and dry seasons, respectively. The formulated similitude and dimensionless numbers provided important insights and understanding that may help achieve healthy coastal-urban streams.