An approach for decomposing estimated river water-quality trends into different flow classes

Abstract:

A number of analysis approaches have been developed to quantify the overall trend in river water quality, but most approaches cannot quantify the trends under different flow conditions. We propose an approach called FN_2Q, which is an adaptation of the flow-normalization (FN) procedure of the well-established WRTDS (“Weighted Regressions on Time, Discharge, and Season”) method. The FN_2Q approach provides a daily time series of low-flow and high-flow flux estimates that represent lower and upper half of daily streamflow observations, respectively. These daily estimates can be summarized into any time period of interest (e.g., monthly, seasonal, or annual) for quantifying temporal trends. The method is illustrated with a record of daily discharge and 632 discrete measurements of total nitrogen concentration collected between 1985 and 2018 from the South Fork Shenandoah River at Front Royal, Virginia. This case study demonstrates the utility of the FN_2Q approach toward not only categorizing the changes in river water quality but also supporting understanding of the drivers of those changes, which can inform management strategies toward improving water quality. The open-source FN_2Q approach can be easily applied to river monitoring records elsewhere to quantify water-quality trends under different flow conditions to enhance understanding of the river dynamics.