1,720,984 research outputs found
Hyporheic Flows in Stratified Beds
No full textSurface-subsurface exchange fluxes are receiving increasing interest because of their
importance in the fate of contaminants, nutrients, and other ecologically relevant
substances in a variety of aquatic systems. Solutions have previously been developed
for pore water flows induced by geometrical irregularities such as bed forms for the cases
of homogeneous sediment beds and idealized heterogeneous beds, but these solutions
have not accounted for the fact that streambed sediments are subject to sorting processes
that often produce well-defined subsurface structures. Sediments at the streambed
surface are often coarser than the underlying material because of size-selective sediment
transport, producing relatively thin armor layers. Episodic erosional and depositional
processes also create thick layers of different composition within the porous medium,
forming stratified beds. A series of experiments were conducted to observe conservative
solute transport in armored and stratified beds. An analytical solution was developed
for advective exchange with stratified beds and provides appropriate scaling of the
physical variables that control exchange flows. The results show that armor layers are too
thin to significantly alter the advective pumping process but provide significant solute
storage at short time scales. Stratified beds with layers of significant thickness favor
development of horizontal flow paths within the bed and change the rate of solute transfer
across the stream-subsurface interface compared to homogeneous beds
Using tracers and hydrological hysteresis analysis to assess process consistency in a catchment conceptual model application
Full text linkAssessment of process consistency in hydrological modelling is crucial to get reliable model responses under conditions beyond the range of prior data availability. This is even more important in the case of conceptual catchment models because the assessment of process consistency may drive the selection of the degree of parsimony, which is warranted in a certain model implementation.
This study aims to analyse process consistency description for a simple conceptual rainfall-runoff model, by using water isotopic data and by the analysis of hysteretic relations. The continuous hydrological model conceptualizes the catchment dividing it into hillslope and riparian zone. A third conceptual tank represents the groundwater storage. The precipitation is used as input to the hillslope and the riparian areas, that are linked dynamically through a simple linear equation.
The model was applied to a headwater forested catchment located in the Italian pre-Alps (598-721 m a.s.l.), where rainfall, discharge, soil moisture and shallow groundwater level were monitored continuously. Moreover, samples for isotopic analyses were collected monthly and during selected rainfall-runoff events from rain and stream water, soil water and shallow groundwater. We applied an index for quantifying hysteresis between streamflow (independent variable) and groundwater level (dependent variable) at the rainfall-runoff event timescale. The index provides information on the direction, the extent and the shape of the loops. A set of 114 rainfall-runoff events were available from 2012 to 2016, to apply the model and compute the hysteresis index. The comparison of observed and modelled hysteretic relations was used to calibrate the hydrological model.
This model consistency analysis allowed us to investigate the goodness of the model in capturing the complex hydrological dynamics, keeping the number of parameters to be conditioned at the minimum. In particular, hysteresis analysis allowed to identify model parametrizations, which permitted an adequate mimic of the system-internal processes. Preliminary results show that the combined tracer analysis and examination of the hysteretic patterns provided indications on the degree of internal consistency of the model representation, making the model application more robust when extended beyond the range of data availability for model conditioning
Assessment of transient storage exchange and advection-dispersion mechanisms from concentration signatures along breakthrough curves
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Runoff events classification based on streamflow-water table hysteresis
Full text linkA framework for rainfall-runoff events classification helps reduce information into a manageable number of classes, and it allows watersheds comparisons. Hydrological signatures serve as proxies for tracking the catchment behaviour and represent a powerful tool for characterising the catchment response to a storm event. Despite that, they have rarely been used for rainfall-runoff event typology identification.
In this study, we propose a general framework for the classification of rainfall-runoff events based on the analysis of the hysteretic relation between streamflow and depth to the water table, and its relation with the event characteristics. Particularly, this study aims to: i) analyse the temporal variability of hysteretic patterns between streamflow and depth to water table in a small headwater catchment, ii) relate a set of hydrological and meteorological characteristics to the hysteretic index at event scale, and iii) identify clusters of events with similar characteristics.
The study area is a small forested catchment located in the Italian Pre-Alps, where hydro-meteorological data have been recorded since August 2012. A set of 112 rainfall-runoff events, occurred between 2012 and 2016, was investigated. A simple hysteresis index was applied to each event. The hysteresis index was used to characterize the direction (clockwise or anti-clockwise), the size and the shape of the hysteretic loops.
Results show that the hysteresis analysis was particularly useful for the identification of three main clusters of rainfall-runoff events. A first cluster was characterised by a clockwise loop, i.e., there was a faster streamflow response compared to the depth to the water table. The events in this cluster were short, with dry antecedent conditions, small streamflow peaks, event runoff depths and runoff coefficients. The second cluster of events was characterised by an anti-clockwise loop, i.e., there was a faster response of the depth to the water table compared to the streamflow. The events in this cluster were long, with wet antecedent conditions, large streamflow peaks, event runoff depths and runoff coefficients. A third cluster had characteristics similar to the first cluster, i.e. clockwise hysteretic loop and similar storm characteristics, but on average displayed a narrower hysteretic loop. The statistics showed a significant difference (p<0.05) among the clusters.
This analysis allowed us to successfully identify three clusters of rainfall-runoff events with specific characteristics and distinct hydrological behaviour. Concluding, the analysis of the hysteresis between streamflow and depth to the water table can be considered a useful tool for classifying rainfall-runoff events
A contaminant transport model for wetlands accounting for distinct residence time bimodality
Full text linkVegetation plays a major role in controlling the fate of contaminants in natural and constructed wetlands. Estimating the efficiency of contaminant removal of a wetland requires separate knowledge of the residence time statistics in the main flow channels, where the flow velocity is relatively higher, and in the more densely vegetated zones, where the velocity is smaller and most of the biochemical transformations occur. A conceptual wetland characterized by a main flow channel (MFC) and lateral vegetated zones (LVZs) is modeled here using a two-dimensional depth-averaged hydrodynamic and advection–dispersion model. The effect of vegetation is described as a flow resistance represented in the hydrodynamic model as a function of the stem density. Simulations are performed for a given flow discharge and for increasing values of the ratio between the vegetation density in the LVZs and in the MFC. Residence time distributions (RTDs) of a nonreactive tracer are derived from numerical simulations of the solute breakthrough curves (BTCs) resulting from a continuous concentration input. Results show that increasing vegetation densities produce an increasingly pronounced bimodality of the RTDs. At longer times, the RTDs decrease exponentially, with different timescales depending on the stem density ratio and other system parameters. The overall residence time distribution can be decomposed into a first component associated with the relatively fast transport in the MFC, and a second component associated with the slower transport in the LVZs. The weight of each temporal component is related to the exchange flux at the MFC-LVZ interface. A one-dimensional transport model is proposed that is capable to reproduce the RTDs predicted by the depth-averaged model, and the relationship between model and system parameters is investigated using a combination of direct and inverse modeling approaches
Assessing climate change impact on complementarity between solar and hydro power in areas affected by glacier shrinkage
Full text linkEffect of a meandering channel on wetland performance
No full textVegetation plays an important role in controlling mixing and contaminant removal in wetlands. Recent studies have shown that the hydraulic performance of a wetland can be significantly affected by the presence of a main flow channel (MFC) where vegetation density is much lower than the average vegetation density in the wetland. The existence of a main flow channel induces short-circuiting, which reduces hydraulic and treatment efficiency. A numerical study was carried out to analyze the effect of channel sinuosity and vegetation density on the hydraulic performance of a channelized wetland. A rectangular wetland characterized by a meandering channel and later vegetated zones (LVZs) was considered, and numerical simulations were carried out using a 2-D depth-average hydrodynamic and solute transport model. The hydraulic performance was analyzed as a function of the average vegetation density, channel sinuosity and the ratio of vegetation densities in the LVZs and the MFC. Results show that increasing sinuosity of the main flow channel can promote mixing and improve hydraulic efficiency. Different performance metrics also indicate negligible impact of the average vegetation density on the hydraulic performance, especially when the width of the MFC is relatively large
Reducing hydrological modelling uncertainty by using MODIS snow cover data and a topography-based distribution function snowmelt model
Full text linkThis work introduces a general multi-objective parameter estimation framework to exploit MODIS-based snow cover maps to reduce predictive streamflow uncertainty in snow-dominated catchments. The well-known GLUE methodology is applied with a multi-objective approach, combining streamflow observations recorded at the outlet section and satellite-derived snow cover maps, aggregated to fractional values of the catchment area. The hydrological model used in this study includes a snowpack routine which exploits a statistical representation of the distribution of clear sky potential solar radiation - a significant advantage when parameter sensitivity and uncertainty estimation procedures are carried out. The study provides an assessment of this approach based on operational quality data from two medium-size mountainous basins (a nested one included in a larger parent basin) located in the eastern Italian Alps. The nested basin is considered as ungauged, thus allowing a spatial assessment of the multi-objective approach. Results show a positive feedback between streamflow and snow cover area likelihoods, highlighted by means of the Pareto plot. Moreover, a better identifiability of the parameters driving snowmelt rate is found and consequently a shrink of the predictive streamflow uncertainty is observed. A containing ratio of 0.54 and a mean sharpness of 0.11 are found at the outlet of the parent basin, while a containing ratio equal to 0.65 and a mean sharpness equal to 0.17 are estimated at the nested basin, used as a validation test. These results confirm the potential of MODIS snow cover maps as additional data to inform hydrological models leading to more reliable and sharper streamflow simulations. This approach might be also appealing when streamflow simulations are required for ungauged basins
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