1,721,059 research outputs found
Processes space-time variability and hydrological response of headwater catchments: role of rainfall, vegetation and antecedent conditions
Full text linkThis thesis aims to enhance the understanding of the hydrological functioning of headwater catchments by focusing on i) how rainfall patterns controls spatial and temporal variability of soil moisture, and ii) how the soil moisture variability provide a control to the catchment response.
A first analysis of the spatial variability of soil moisture was carried out for data at 0-30 and 0-60 cm depth collected on a plot in Grugliasco (Po Plain, Northern Italy), characterized by two land uses (meadow and vineyard). Results showed that the differences in spatial mean and variability of soil moisture for the meadow and the vineyard are likely due to the different vegetation cover. Evaluation of the main physical controls on the spatial mean and the variability of soil moisture was carried out by using a simple bucket model. The model was calibrated by using spatial mean soil moisture and it had a relatively good prediction capability. The model was also shown to be able to capture the main differences between the two sites in terms of spatial variability of soil moisture.
The spatial and temporal variability of soil moisture was also analyzed in relation to throughfall spatial patterns in plot on a forested hillslope in the Italian pre-Alps. Throughfall was measured using two types of throughfall collectors: buckets and rain gauges. The collectors differed in size, number and spatial arrangement. Results showed that buckets and rain gauges measured similar throughfall amounts during rainfall events. However, findings indicate that different collectors can lead to differences in the quantified spatial variability of throughfall and presence of local clusters and outliers.
Near-surface soil moisture was measured upslope of each bucket, at 0-7 and 0-12 cm depth before and after rainfall events. Throughfall and soil moisture spatial patterns were not significantly or only weakly correlated, likely due to the lateral and vertical redistribution of water in the soil profile during the 2-36 hour period between the end of the rainfall event and the start of the soil moisture measurements. The temporal stability of soil moisture was larger than the temporal stability of throughfall and they were also not significantly correlated. The patterns of temporal stability were also not related to canopy characteristics (i.e., canopy openness and leaf area index). The application of the simple bucket model revealed that a large spatial variability in saturated hydraulic conductivity that is correlated with the spatial variability in leaf area index and root fraction weaken the correlation between throughfall and soil moisture patterns. The analysis of field data combined with the model application suggests that in this specific forested hillslope the spatial organization of soil moisture is dominated by a combination of soil properties and vegetation characteristics, rather than by the throughfall spatial patterns.
Saturation at the soil-bedrock interface or the rise of shallow groundwater into more permeable soil layers results in subsurface stormflow and can lead to hillslope-stream connectivity. Networks of spatially-distributed piezometers in five small headwater catchments in the Italian Dolomites and the Swiss pre-Alps were used to quantify and compare the spatial and temporal variability of subsurface connectivity and its relation to streamflow dynamics. Results showed that the time that piezometers were connected to the stream was significantly correlated to the topographic wetness index, for two Swiss pre-alpine catchments, or to the distance to the nearest stream, for the dolomitic catchment with the largest riparian zone. During rainfall events, mainly anti-clockwise hysteretic relations between streamflow and the area that was connected to the stream were observed. Threshold-like relations between maximum connectivity and total stormflow and between maximum connectivity and the sum of total rainfall plus antecedent rainfall were more evident for the dolomitic catchments, where the riparian zone is characterized by a groundwater table near the soil surface. These preliminary results suggest that the delayed increase in subsurface connectivity relative to streamflow is likely not affected by the presence of a riparian zone. However, further analyses are needed to determine if morphology of the catchments affect the observed relations between subsurface connectivity and total stormflow.
Finally, this thesis attempted to develop an index for the quantification of hysteretic loops between hydrological variables at the runoff event timescale. The index provides information on the direction, the shape and the extent of the loop. The index was tested with synthetic data and field data from experimental catchments in Northern Italy. Hysteretic relations between streamflow and soil moisture, depth to water table, isotopic composition and electrical conductivity of stream water were correctly identified and quantified by the index. The sensitivity of the index to the temporal resolution of the measurements and measurement errors was also tested. The index can successfully quantify hysteresis, except for very noisy data or when the temporal resolution of the measurements is not well suited to study hysteresis between the variables. Overall, this metric can be used to test if models reproduce temporal variability in hysteresis or to compare hydrological responses in different catchments or at different spatial scales
Use of major anions and cations as tracers to analyse runoff generation processes in a mountain catchment
No full textUsing 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
Space-time soil moisture variability for two different land use types: analysis at the plot scale
No full textRunoff 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
Understanding the spatial and temporal variability of water sources in a humid forested catchment
No full textRunoff generation processes and fraction of young water for streamflow and groundwater in a pre-alpine forested catchment
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