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RUNOFF GENERATION IN A FORESTED PRE-ALPINE CATCHMENT: HYPOTHESIS TESTING BY MEANS OF ISOTOPIC AND GEOCHEMICAL TRACERS
Full text linkUnderstanding of the complex runoff generation mechanisms is important for water management and to predict the effects of land use and climate change on streamflow. In particular, the mechanisms through which precipitation becomes streamflow and the spatio-temporal variability of the runoff components is still unclear. The integrated use of isotopic and geochemical tracers, combined with hydrometric data, has proven to be a useful method to investigate hydrological processes in small catchments. In this study, we used stable isotopes of water, major ions and electrical conductivity as tracers together with hydrometric data to i) compare the response of the water table and soil moisture to the streamflow response during rainfall-runoff events, ii) analyze the spatial and temporal variability in tracer signature (stable isotopes of water, EC and major ions) of different end members contributing to stream runoff, iii) assess how the main event characteristics (total rainfall amount, intensity and antecedent wetness conditions) affect the event water contribution to stream runoff, and iv) investigate how the use of different tracers affects the results of the two-component hydrograph separation.
The hydrometric and tracer data was analyzed for 15 rainfall-runoff events between September 2015 and October 2016 during different wetness conditions in a small catchment in the Italian pre-Alps. Continuous measurements for streamflow, precipitation, air temperature, shallow groundwater levels at six locations in different parts of the catchment (riparian, bottom of the hillslope, mid hillslope and upper hillslope), soil moisture at four locations along a riparian-hillslope transect were available for the study period. Samples for isotopic and geochemical analysis were taken from precipitation, stream, shallow groundwater, soil water at 5 locations (riparian at 10 cm and 20 cm depth; mid hillslope at 10 cm and 30 cm depth; upper hillslope at 30 cm depth). Electrical conductivity was measured directly in the field using a portable meter. Isotopic composition and ion concentrations were determined in the laboratory of Legnaro (Dip. TESAF, University of Padova) by laser absorption spectroscopy and ion-chromatography, respectively.
Mixing plot analysis shows that precipitation, near-surface riparian soil water and shallow groundwater are the main contributors to runoff. Two-component hydrograph separation showed that new water contributions increase with increasing rainfall amount and intensity and decrease with increasing antecedent wetness conditions. During events with high rainfall intensity with dry antecedent conditions, the new water fraction was high, especially in the initial phase of the event and near the streamflow peak, suggesting that streamflow is mainly generated by direct channel precipitation and saturated overland flow in the riparian zone. With increasing wetness conditions, pre-event water dominated runoff, suggesting the development of subsurface stormflow and an increasing contribution from groundwater pools located in different parts of the catchment. In the middle and at the end of large events in wet periods, the new water fraction in streamflow increased due to the expansion of the saturated area near the stream and flow through shallow soil layers. Differences in the event water fractions computed using isotopes, EC or major ions, suggest an ionic enrichment of rainfall due to the accumulation of ions in the ephemeral portion of the stream and the soil surface
Use of major anions and cations as tracers to analyse runoff generation processes in a mountain catchment
No full textUnderstanding the spatial and temporal variability of water sources in a humid forested catchment
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Response time and water origin in a steep nested catchment in the Italian Dolomites
Full text linkIn this study, we investigate the surface flow time of rise in response to rainfall and snowmelt events at different spatial scales and the main sources originating channel runoff and spring water in a steep nested headwater catchment (Rio Vauz, Italian Dolomites), characterized by a marked elevation gradient. We monitored precipitation at different elevations and measured water stage/streamflow at the outlet of two rocky subcatchments of the same size, representative of the upper part of the catchment dominated by outcropping bedrock, at the outlet of a soil-mantled and vegetated subcatchment of similar size but different morphology, and at the outlet of the main catchment. Hydrometric data are coupled with stable isotopes and electrical conductivity sampled from different water sources during five years, and used as tracers in end-member mixing analysis, application of two component mixing models and analysis of the slope of the dual-isotope regression line. Results reveal that times of rise are slightly shorter for the two rocky subcatchments, particularly for snowmelt and mixed rainfall/snowmelt events, compared to the soil-mantled catchment and the entire Rio Vauz Catchment. The highly-variable tracer signature of the different water sources reflects the geomorphological and geological complexity of the study area. The principal end-members for channel runoff and spring water are identified in rainfall and snowmelt, which are the dominant water sources in the rocky upper part of the study catchment, and soil water and shallow groundwater, which play a relevant role in originating baseflow and spring water in the soil-mantled and vegetated lower part of the catchment. Particularly, snowmelt contributes up to 64 ± 8% to spring water in the concave upper parts of the catchment and up to 62 ± 11% to channel runoff in the lower part of the catchment. These results offer new experimental evidences on how Dolomitic catchments capture and store rain water and meltwater, releasing it through a complex network of surface and subsurface flow pathways, and allow for the construction of a preliminary conceptual model on water transmission in snowmelt-dominated catchments featuring marked elevation gradients
What controls groundwater dynamics and hillslope-stream connectivity in an alpine headwater catchment?
No full textVariabilità spazio-temporale della precipitazione sottochioma a scala di parcella in un bacino idrografico delle Prealpi Vicentine
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Controls on water table response and hillslope-stream connectivity during wet and dry periods in an Alpine headwater catchment
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