An Environmental Isotopes‐ and <i>C</i> – <i>Q</i> Relationships‐Based Study of Hydro‐Geochemical Dynamics in Meso‐Scale Andean Headwater Basins in Arid North‐Central Chile

ABSTRACT This work combines use of environmental tracers and hydrochemistry for the assessment of hydrological processes at the basin scale. We present analyses of hydrological behaviours in three arid, headwater basins, that is, Derecho, Cochiguaz and Incaguaz (29°59′ S–30°29′ S and 70°15′ W–70°32′ W) in the Andes of north‐central Chile, combining isotope data and concentration–discharge ( C – Q ) relationships. During 2 years, monthly surface water samples were analysed for stable isotopes (δ 2 H, δ 18 O), radioactive isotopes ( 222 Rn and 3 H) and chemical composition (Ca, Mg, Na, K, Si, HCO 3 , Cl, SO 4 and Fe, both total and dissolved concentrations). Even though the basins present relatively similar physiographic features (e.g., orientation, elevation distribution, dominant geology), noticeable differences in hydrological processes were revealed. Derecho presented episodic contributions of groundwater to the surface channel (inferred from 222 Rn), not observed in Cochiguaz and Incaguaz. This was also associated with longer residence times, as inferred from stable isotopes. Based on 3 H, we inferred the occurrence of an important subterranean component of some decades, which explains the permanent existence of surface discharge despite an extreme 15‐year drought in the area. Apart from seasonal snowfall, no significant contribution of glaciers or other cryogenic forms to the surface streams was found. The chemical composition of the waters reflects quite clearly the imprint of predominant igneous rocks in the three basins. Specifically, the C – Q relationships showed a consistent behaviour for both geogenic and exogenous constituents, and most of this behaviour was classified as chemostatic. This is consistent with the identification of an important older water component in the streamflow of the basins under study, associated with an important storage volume in the basins, determining that water remains in the system for long time periods, that is, longer than is required to reach water–rock equilibrium. The combined isotopic and C – Q analysis provides a reliable understanding of the basins' behaviour beyond what each approach could have provided separately.