Changes in discharge and solute dynamics between hillslope and valley-bottom intermittent streams

Abstract. To gain understanding on how alluvial zones modify water and nutrient export from semiarid catchments, we compared monthly discharge as well as stream chloride, carbon, and nitrogen dynamics between a hillslope catchment and a valley-bottom catchment with a well-developed alluvium. Stream water and solute fluxes from the hillslope and valley-bottom catchments showed contrasting patterns between hydrological transitions and wet periods, especially for bio-reactive solutes. During transition periods, stream water export decreased >40% between the hillslope and the valley bottom coinciding with the prevalence of stream-to-aquifer fluxes at the alluvial zone. In contrast, stream water export increased by 20–70% between the hillslope and valley-bottom catchments during wet periods. During transition periods, stream solute export decreased by 34–97% between the hillslope and valley-bottom catchments for chloride, nitrate, and dissolved organic carbon. In annual terms, stream nitrate export from the valley-bottom catchment (0.32 ± 0.12 kg N ha⁻¹ yr⁻¹ [average ± standard deviation]) was 30–50% lower than from the hillslope catchment (0.56 ± 0.32 kg N ha⁻¹ yr⁻¹). The annual export of dissolved organic carbon was similar between the two catchments (1.8 ± 1 kg C ha⁻¹ yr⁻¹). Our results suggest that hydrological retention in the alluvial zone contributed to reduce stream water and solute export from the valley-bottom catchment during hydrological transition periods when hydrological connectivity between the hillslope and the valley bottom was low.