Differential C-Q Analysis: A New Approach to Inferring Lateral Transport and Hydrologic Transients within Multiple Reaches of a Mountainous Headwater Catchment

Abstract:

Concentration-discharge (C-Q) relationships are often used to describe how water moves through streams and the chemicals that are transported with it. However, these relationships are limited to investigating solute transport dynamics at individual sampling stations, such that they create an incomplete understanding of the solute behavior upstream or downstream of the sampling station. In this study, we present a slightly modified approach, the differential C-Q approach, that can capture the increase, decrease, or stationary response of solute concentrations in a river segment spanning multiple sampling locations. By following a different spatial scheme and organizing the river into multiple sections, the differential C-Q approach has been specifically designed to capture spatio-temporal variability in solute behavior at the reach scale. To demonstrate the advantages of this approach, we use water quality data collected over four water years (2015-18) in a mountainous headwater catchment of the East River, Colorado and compare traditional and differential C-Q relationships in predicting solute behavior across three sampling stations distributed throughout the river. While traditional analyses yield similar-shaped curves and solute hysteresis across stations, results from the differential C-Q analysis demonstrate significant differences in solute behavior within upstream versus downstream reaches of the East River watershed. In particular, the downstream reach is marked by significant gains in both groundwater and solute concentrations as opposed to the dilution and the declining trends observed in the high-relief, steep terrain upstream reach. Shale mineralogy was determined to have a major influence on in-stream concentrations pertaining to Ca, C, Mg, Mo, NO₃ and SO₄. Overall, differential C-Q analysis enables a better understanding of the lateral storage and interactions within catchments than traditional analyses, and holds potential for aiding water quality managers in the identification of critical stream reaches that assimilate harmful chemicals.