A New Approach for Identifying Water Quality Hot Spots in Watersheds

A new approach for analyzing “concentration–discharge” (C-Q) relationships that goes beyond traditional analyses to help identify stream segments important for water quality.

The Science

Concentration-discharge (C-Q) relationships are often used to describe how water moves through streams and the chemicals that are transported with it. These C-Q relationships are often developed and examined at individual locations within a watershed. A new, easy-to-use approach—“Differential C-Q”—develops relationships that accounts for and captures changes in the water chemistry over stream segments, rather than just individual locations. This new approach helps to better identify specific sources of harmful solutes (chemicals dissolved in the water), including hillslope areas contributing solutes, and critical stream segments that can adversely impact river water quality.

The Impact

The differential C- Q analysis is a valuable tool for assessing differences across stream reaches, rather than just at individual locations. It can be used to compare how harmful chemicals accumulate or are moved within and across different stretches of the stream network, and to monitor these processes in the face of environmental disturbances or changes. Watershed and land managers can use this approach to design strategies more effectively, identify locations to monitor water quality, or as an aid in designing pollution prevention and intervention strategies.


Concentration-discharge relationships reflect sources, storage, reactions, and transport of solutes in watersheds. These relationships are typically examined at individual sampling stations across the watershed, which do not provide sufficient information about accumulation or mobilization of harmful chemicals, pesticides, or other solutes. This study presents a new differential C-Q approach that can capture the increase, decrease, and/or fractional solute turnover over each stream segment. Water quality data on different chemical constituents, collected at upstream and downstream sites in the East River watershed in Colorado, was used to evaluate and compare this differential approach with traditionally used approaches. Traditional C-Q analysis across three stations in the watershed showed consistent, similar patterns between nitrate concentrations and stream discharge, while the differential C-Q approach showed different patterns across the sites. The new method showed that during high flows, the upstream reach showed gains in nitrate, while the downstream reach showed losses, indicating different processes and likely different sources of water (e.g., groundwater and snowmelt) between the upstream and downstream sites. In contrast to nitrate, other chemicals—phosphorus, organic carbon, molybdenum, and several other solutes—showed gains in the downstream reach, which likely due to contributions from lateral flows from the areas surrounding the streams in the low-relief, meandering terrain. The new C-Q approach identified different processes and patterns across the East River watershed that were not evident in the site-specific traditional approaches and clearly indicated when and where small increases in nutrients like phosphorus and nitrate can be particularly concerning given the potential for algal growth and eutrophication. Overall, the differential C-Q approach holds potential for aiding water quality managers in identifying critical stream reaches that assimilate harmful chemicals.

Principal Investigator(s)

Bhavna Arora
Lawrence Berkeley National Laboratory

Related Links


This work was supported by the Office of Biological and Environmental Research within the U.S. Department of Energy Office of Science.


Arora, B., Burrus, M., Newcomer, M. et al. “Differential CQ analysis: A new approach to inferring lateral transport and hydrologic transients within multiple reaches of a mountainous headwater catchment.” Frontiers in Water 2, 24 (2020). DOI:10.3389/frwa.2020.00024