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

摘要

To gain understanding on how alluvial zones modify water and nutrient exportfrom semiarid catchments, we compared monthly discharge as well as streamchloride, carbon, and nitrogen dynamics between a hillslope catchment and avalley-bottom catchment with a well-developed alluvium. Stream water andsolute fluxes from the hillslope and valley-bottom catchments showedcontrasting patterns between hydrological transitions and wet periods,especially for bio-reactive solutes. During transition periods, stream waterexport decreased >40% between the hillslope and the valley bottomcoinciding with the prevalence of stream-to-aquifer fluxes at the alluvialzone. In contrast, stream water export increased by 20–70% between thehillslope and valley-bottom catchments during wet periods. During transitionperiods, stream solute export decreased by 34–97% between the hillslopeand valley-bottom catchments for chloride, nitrate, and dissolved organiccarbon. In annual terms, stream nitrate export from the valley-bottomcatchment (0.32 ± 0.12 kg N ha?1 yr?1 [average ± standarddeviation]) was 30–50% lower than from the hillslope catchment (0.56 ± 0.32 kg N ha?1 yr−1). The annual export of dissolved organiccarbon was similar between the two catchments (1.8 ± 1 kg C ha?1 yr?1). Our results suggest that hydrological retention in the alluvialzone contributed to reduce stream water and solute export from thevalley-bottom catchment during hydrological transition periods whenhydrological connectivity between the hillslope and the valley bottom waslow.