Catchment Concentraton-Discharge Archetypes Explained

Coupled hydrological and biogeochemical processes interact within catchments, producing hydrographs (Q(t)) and chemographs (C(t)), with the inter-dependence represented by an empirical function: C = aQ^b, where a and b are constants. Three archetypes of C-Q relationships have been observed in stream networks: (1) dilution; b<0; (2) accretion; b>0; and (3) constant C; b~0. Each relationship can exhibit either a relatively constant variance (homoscedastic) or decreasing variance with increasing Q (heteroscedastic). For the third type, the homoscedastic case has been referred to in the literature as chemostatic, while we describe the heteroscedastic case as chemo-convergence. We offer conceptual models for specific linkages between hydrologic and biogeochemical coupling to generate these observed relationships. We seek to understand how the spatial structure of solute sources coupled with hydrologic responses affect C-Q patterns, and investigate the following broad questions: (1) How does the coupling of flow-generating areas and biogeochemical source areas vary across a catchment under stochastic hydro-climatic forcing?, (2) What are the feasible hydrologic and biogeochemical responses that lead to the observed C-Q relationships?, and (3) What implications do these coupled dynamics have for implementation of best management practices for reducing exported solute loads? Our overarching hypothesis is that each of these C-Q patterns can be produced by explicitly linking landscape-scale hydrologic responses and spatial distributions of solute source properties within a landscape. To test this hypothesis, we developed a conceptual catchment model coupled to a dual-domain source-zone model to simulate solute export from each landscape unit. Outputs from the source-zone are then routed through the catchment to generate hydrographs and chemographs. This approach allows explicit links to be identified between specific hydrologic responses and spatial patterns of solute sources that generate these archetypical C-Q patterns. The model simulations reproduce the three major C-Q patterns observed in published data, offering valuable insight into catchment processes. The findings have important implications for effective catchment management and water quality improvement.