Modeling Complex Concentration-Discharge Relationships with Generalized Additive Models

Abstract

Concentration-discharge relationships in water chemical time series can provide important insights into sources, mobilization, and delivery of solutes and particulates into stream networks. The observed relationships are often complex, including nonlinear and hysteretic patterns reflecting seasonal, climatic, and land management changes in biogeochemical release and hydrological transport of solutes and particulates to streams. Using standard single concentration-discharge (c-q) slopes can obscure this wealth of information. In this study, we suggest a new approach using generalized additive models for evaluation of complex c-q patterns in low-frequency water quality data (only monthly or biweekly observations). We used these models to estimate c-q slopes together with their uncertainty, to provide evidence of changes in c-q behaviors and their controls. We estimated c-q slopes for a selection of Swedish streams and evaluated their nonlinear, seasonal, and temporal structure as indicators of changing hydrological or biochemical drivers.