Hydrometric data discretization into environmental flow components: a new, practical approach to explore concentration-discharge relationships

Abstract

The intricate dynamics of streamflow and water quality are often explored through the lens of concentration-discharge (C-Q) relationships. Interpreting C-Q relationships and using mathematical models to explain them can, however, be challenging due to hysteresis effects, low-frequency data, or noisy data. To address these challenges, this study introduces a noise-filtering approach by aggregating discrete data collected over several years into bins corresponding to distinct environmental flow components (EFCs). Covering a spectrum from extreme low flows to small and large floods, this method simplifies C-Q analysis by focusing on watershed hydrochemical responses to varying flow conditions. The objectives of the study were to explore the variability of stream water quality across a gradient of EFCs, categorize watersheds based on their median dominant export behaviour type (e.g. dilution, mobilization, chemostatic), and evaluate the predictability of export behaviour type from watershed characteristics. The study relied on 30 watersheds located in Québec, Canada, ranging in size from 11 to 2610 km2, with daily streamflow data and at least monthly water quality data spanning at least two years over the 1989-2020 period. EFC-specific summary statistics for dissolved organic carbon (DOC), total nitrogen (TN), and total phosphorus (TP) concentrations were computed and used to assess C-Q relationships and classify the general tendencies of watershed export behaviour. Results reveal nuances in watershed export behaviour depending on the specific water quality parameter and flow components assessed. Some differences in watershed hydrobiogeochemical behaviours could be explained by differences in watershed physiographic characteristics. The EFC-based approach to C-Q analysis provides watershed scientists and managers with a simple and comprehensive tool to utilize existing data, enabling a deeper understanding of watershed nutrient export dynamics.