A Simple Concentration-flow Approach for Water Quality Assessment in a Tropical Semiarid Basin in Brazil

Abstract Rivers in tropical semiarid basins are typically intermittent, experiencing prolonged periods of zero or very low discharge. This study evaluated the applicability of empirical concentration-flow (C-Q) relationships for assessing water quality in the Middle Jaguaribe river, the main tributary of the Castanhão reservoir, the largest water-supply reservoir in the Brazilian semiarid region. C-Q equations incorporating two terms representing point sources (PS) and non-point sources (NPS) were fitted to 8–12 measurements of electrical conductivity, turbidity, biochemical oxygen demand (BOD), thermotolerant coliforms (TTC), total phosphorus (TP), total inorganic nitrogen (TIN), and chlorophyll-a (Chla), all showing satisfactory predictive performance (R² = 0.37–0.94). While conductivity and TTC decreased and increased monotonically with river flow, respectively, the remaining parameters exhibited a characteristic two-regime behavior. At low flows, concentrations decreased with increasing discharge, reflecting dilution under PS dominance. Beyond a threshold flow rate, concentrations increased with discharge, indicating wash-off and enhanced transport from NPS. The estimated threshold flow rates, at which PS and NPS contributions became equal, were of the same order of magnitude as the watershed’s average sewage discharge, providing physical support for the proposed framework. Overall, NPS dominated pollutant transport during 89–100% of the study period, highlighting the importance of diffuse pollution in tropical semiarid watersheds. As an application of the results, the proposed C-Q relationships and long-term discharge data were used to estimate nutrient and pollutant loads to the Castanhão reservoir, supporting water quality management. Moreover, climate change projections using these C-Q relationships under the intermediate-emission scenario suggest that increases in river discharge may slightly reduce electrical conductivity while increasing turbidity, BOD, TTC, TP, TIN, and Chla, indicating a tendency toward greater nutrient enrichment, increased eutrophication risk, and deteriorating sanitary conditions, with potential risks to human health. Under the high-emission scenario, however, the direction of all these trends was reversed due to projected decreases in river discharge. The present study advances current knowledge by not only investigating the applicability of C-Q relationships to several additional water quality parameters, but also, owing to the availability of low-flow measurements, providing a more detailed assessment of the threshold conditions associated with the transition from PS to NPS dominance.