Concentration-discharge patterns in a small urban headwater stream in a seasonally dry water-limited tropical environment

Abstract

The simplicity of the hydrochemical stationarity concept renders it attractive for partitioning solutes between geogenic and anthropogenic sources. The current study used a small urban headwater stream in a seasonally dry environment to address two research questions: (1) What concentration (C)-discharge (Q) patterns exist in small urban headwater streams?; and (2) Do the C-Q patterns persist across C-Q metrics and temporal scales? Four C-Q metrics were tested: concentration-discharge (C-Q), concentration-cumulative discharge (C-ΣQ), load (L)-discharge (L-Q) and normalized concentration-normalized discharge (NC-NQ). C-Q and NC-NQ revealed discharge-invariant behaviour for Ca, two linear relationships with threshold-like transitions from negative to positive slopes for Mg, K and Na, and positive linear relationships for Fe, Pb and PO43−. The threshold-like transitions with distinct breakpoints were more apparent in C-ΣQ patterns for all solutes. These patterns are consistent with three hypotheses: (1) negative linear to zero slope relationships indicate dilution followed by discharge-invariant behaviour (Ca); (2) negative to positive linear relationships (Mg, K and Na) point to dilution followed by solute enrichment or flushing; and (3) positive to negative linear relationships (Pb, Fe and PO43−) suggest initial solute mobilization followed by dilution. The three dominant behaviours were robust across weekly, fortnightly and monthly timescales. Significant linear L-Q relationships were observed for all solutes, suggesting that loads can be predicted from discharge. Our findings suggest that C-Q relationships are highly dynamic, and multiple processes control streamflow hydrochemistry at different times depending on antecedent discharge. The application of multiple C-Q metrics provided additional insights not apparent by using a single metric. The insights are critical to understanding of catchment hydrology and conceptual representation of hydrochemical processes in models.