Land Use and Hydrologic Sensitivities of In-stream Water Quality in Complex Coastal-urban Watersheds

We employed sensitivity analysis to study the complex interactions of various water quality indicators with their drivers in the coastal-urban watersheds of southeast Florida, U.S.A. The total nitrogen (TN), total phosphorus (TP), chlorophyll-a (Chla), and dissolved oxygen (DO) were used to represent the stream water quality. Land use/cover and hydrologic variables along with the upstream (inlet) concentrations and distance from the coastal outlet were used to represent the sources and drivers of stream water quality at each monitoring station. Separate analyses were performed for the wet and dry seasons, acknowledging the variable climatic influence in different seasons. Power-law based nonlinear partial least square (PLS) regression models were developed with bootstrap resampling to achieve a robust estimation of parameters. The model showed good performance in both wet and dry seasons. The estimated model parameters were used to analytically derive relative sensitivity coefficients to determine the relative influence of different drivers on the stream water quality. Numerical sensitivity analyses were also performed with a range of perturbed model drivers to estimate the changes of stream water quality under different changing scenarios of land use/cover and hydrologic variables. Results showed that the major sources of in-stream pollutants were the agricultural land uses and the upstream sources. In both wet and dry seasons, TN showed relatively strong sensitivity to the upstream concentrations and distance from the coastal outlet; whereas TP and Chla showed relatively high sensitivity to the upstream concentrations and the watershed land uses. For DO, relatively strong sensitivity was found to the groundwater depth and watershed hydrology, respectively, in the wet and dry seasons. The sensitivity coefficients and mechanistic insights obtained from the study will guide the management of urban stream water quality to maintain healthy aquatic ecosystems.