Abstract
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.
Highlights
Large riparian forests and well-developed alluvial zones are two of the main contrasting landscape features between hillslope and valley-bottom areas in mountainous regions
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
When analyzing each hydrological period separately, we found that the Aridity Index (AI) was low during the transition period and it exhibited typical values of semiarid conditions (Table 1)
Summary
Large riparian forests and well-developed alluvial zones are two of the main contrasting landscape features between hillslope and valley-bottom areas in mountainous regions. There is a large flux of dissolved organic carbon (DOC) from riparian soils to stream ecosystems Riparian zones can act as important sinks of essential nutrients such as nitrate, substantially reducing nitrate export from catchments (Peterjohn and Correll, 1984; Hill, 1996; Vidon et al, 2004a). A well-developed alluvium can store a large volume of water, integrating the temporal variation of new and old solute inputs. In this sense, the alluvial aquifer acts as a wellmixed groundwater reservoir and can exhibit a chemical signature distinct from hillslope groundwater (Hooper et al, 1998). When the stream and riparian zone are surrounded by an alluvium with a large fraction of coarse material (hereafter, the alluvial riparian zone), high hydraulic conductivity
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