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Recent advances and opportunities in data assimilation for physics-based hydrological modeling
Full text linkData assimilation applications in integrated surface-subsurface hydrological models (ISSHMs) are generally limited to scales ranging from the hillslope to local or meso-scale catchments. This is because ISSHMs resolve hydrological processes in detail and in a physics-based fashion and therefore typically require intensive computational efforts and rely on ground-based observations with a small spatial support. At the other end of the spectrum, there is a vast body of literature on remote sensing data assimilation for land surface models (LSMs) at the continental or even global scale. In LSMs, some hydrological processes are usually represented with a coarse resolution and in empirical ways, especially groundwater lateral flows, which may be very important and yet often neglected. Starting from the review of some recent progress in data assimilation for physics-based hydrological models at multiple scales, we stress the need to find a common ground between ISSHMs and LSMs and suggest possible ways forward to advance the use of data assimilation in integrated hydrological models
Stream Network Dynamics of Non‐Perennial Rivers: Insights From Integrated Surface‐Subsurface Hydrological Modeling of Two Virtual Catchments
No full textUnderstanding the spatio-temporal dynamics of runoff generation in headwater catchments is challenging, due to the intermittent and fragmented nature of surface flows. The active stream network in non-perennial rivers contracts and expands, with a dynamic behavior that depends on the complex interplay among climate, topography, and geology. In this work, CATchment HYdrology, an integrated surface–subsurface hydrological model (ISSHM), is used to simulate the stream network dynamics of two virtual catchments with the same, spatially homogeneous, subsurface characteristics (hydraulic conductivity, porosity, water retention curves) but different morphology. We run two sets of simulations to reproduce a sequence of steady-states at different catchment wetness levels and transient conditions and analyze the joint variations of the stream length (L) and discharge at the outlet (Q) with high spatio-temporal resolutions. The shape of the L(Q) curves differs in the two catchments but does not depend on the climate forcing, as it is mainly controlled by the underlying topography. We then analyzed the suitability of the topographic wetness index and the contributing area to identify the spatial configuration of the maximum stream length in the two catchments. These two morphometric parameters provided a good estimate of the spatial distribution of the maximum flowing network in both the study catchments. Our numerical simulations indicate that ISSHMs have the potential to accurately describe the spatio-temporal variations of the stream networks and the processes driving such dynamic behavior and that, overall, they can be useful tools to gain insights into the main physical drivers of non-perennial streams
FINITE ELEMENT SIMULATION OF A RAINFALL INDUCED SHALLOW LANDSLIDE IN AN EXPERIMENTAL HILLSLOPE WITH A MULTIPHASE POROUS MEDIA MODEL
Full text linkHow do different runoff generation mechanisms drive stream network dynamics? Insights from physics-based modelling
Full text linkNon-perennial river catchments are characterized by an ever-changing spatial configuration of their flowing streams. A combination of empirical data and simplified analytical frameworks has been frequently used in the literature to analyse the co-evolution of the total active stream length (L) and the catchment discharge at the outlet (Q). However, despite the increasing availability of field data, understanding how runoff generation processes drive the spatio-temporal dynamics of non-perennial river reaches remains challenging. In this paper we use CATHY, an integrated surface-subsurface hydrological model (ISSHM), to investigate the impact of saturation-excess (Dunnian) and infiltration-excess (Hortonian) runoff generation on the stream network dynamics of two virtual catchments with spatially homogeneous subsurface properties but different morphology. The numerical simulations show that when surface runoff is triggered by saturation-excess mechanisms, the subsurface domain is slowly saturated, and the stream network gradually expands upstream from the outlet towards the catchment divides. In these conditions, the specific inflow per unit contributing area is relatively uniform along the network, thereby implying that L and Q display a monotonically increasing one-to-one relationship. On the other hand, infiltration-excess mechanisms lead to more heterogeneous saturation patterns in the subsurface domain. In particular, during the wetting phase, Hortonian processes originate highly transient conditions and a non-uniform spatial distribution of the specific inflow along the stream network. This is reflected by a hysteretic LQ relation and a marked asymmetry between the wetting and drying phases of the event. The application of an ISSHM proved to be a useful tool to elucidate the processes that drive stream network expansion and retraction in non-perennial rivers.Schematic representation of the catchment response (i.e., saturated surface areas and wet stream channels spatio-temporal dynamics) driven by saturation-excess (Dunnian) mechanisms (a, b) and infiltration-excess (Hortonian) mechanisms (c-e). imag
Modeling hydrological impacts of afforestation on intermittent streams
No full textAlthough the majority of river networks across the world are intermittent or ephemeral, afforestation management of these catchments is mostly founded on studies in perennial catchments. The hydrological model CATHY (CATchment HYdrology) was used here to simulate the effects that different degrees of progressive conversion from pasture to plantation have on the streamflow generation in intermittent streams. The model was applied to two rural catchments with different size and topographic features in southwest Victoria, Australia. Simulated scenarios included different levels of plantation establishment in pasture areas planting gradually from downslope to upslope and vice versa. Different models for root water uptake were compared to account for water stress, oxygen stress, and root water compensation. A function of root growth over time was also explored to see how it affected model results. The model results show that complex interactions between topographic features and afforestation patterns are crucial in controlling catchments hydrological behavior. In particular, results show that planting in the prone-saturation areas has the largest effects on streamflow. Oxygen stress has a more significant impact than root water compensation on streamflow changes. A time dependent root growth results in smaller streamflow reduction on average, although with different impacts on the two catchments, also due to the interplay between topography and plantation patterns. Overall, our results show that there are multiple factors affecting the water balance when a catchment is partially or completely afforested and those must be taken into account when implementing forestry management strategies
Prime evidenze di uno studio sperimentale per la definizione di un approccio modellistico multiscala. Il caso dell’acquifero di Settolo (TV)
No full textLa memoria presenta le prime evidenze emerse da uno studio sperimentale attualmente in corso su un acquifero naturale ubicato a Settolo (TV), nella fascia pedemontana veneta. Trattasi di un acquifero freatico indifferenziato sfruttato per
usi idropotabili, che si estende su un’area di circa 6 km2, caratterizzato da suoli prevalentemente ghiaiosi - ma con marcate eterogeneità - e da forti interazioni con il fiume Piave, che delimita l’acquifero stesso dal lato sudovest. Allo scopo di validare la successiva attività modellistica, il sito è stato oggetto, a partire dall’estate 2009, di un monitoraggio continuo mediante la misura della precipitazione, del livello del Piave e del livello di falda in un numero relativamente elevato di pozzi di osservazione. È inoltre stata condotta una campagna di misure geofisiche tramite la tecnica dell’ERT (Electrical Resistivity Tomography) superficiale per la caratterizzazione della conducibilità elettrica delle formazioni e
la verifica della presenza di facies diverse. Le prime elaborazioni dei dati raccolti evidenziano la complessa dinamica della falda, caratterizzata da due regimi diversi:
il primo, legato alle variazioni di livello del fiume Piave; il secondo, legato alla ricarica del bacino del Rio di Funer, un piccolo corso d’acqua che confluisce in sinistra Piave subito a valle dell’area oggetto di studio. L’acquifero è soggetto all’estrazione di portate di una qualche entità ad opera della locale società di gestione dell’acquedotto e, in misura minore, di privati. Le prove geofisiche indicano la presenza di formazioni porose caratterizzate da resisitività elettrica elevata, probabilmente tracce di paleo-alvei del fiume Piave che potrebbero rappresentare importanti percorsi preferenziali in caso di contaminazione dell’acquifero
Numerical dispersion of solute transport in an integrated surface–subsurface hydrological model
No full textIntegrated surface–subsurface hydrological models (ISSHMs) are increasingly being used for the assessment of contaminant transport in the environment, in addition to their more common use in water flow applications. However, the subsurface solute transport solvers in these models are prone to numerical dispersion errors. Numerical dispersion is a well-known issue in groundwater modeling, but its impacts on the results of ISSHM simulations are still poorly understood. In this study, the CATchment HYdrology (CATHY) model is used to assess the potential impacts of numerical dispersion on the simulation of coupled surface–subsurface solute transport. We first simulate the subsurface transport of a nonreactive tracer in two soil column test cases (1D and 3D) with known analytical solutions. The subsurface solute transport solver in CATHY adopts a computationally efficient time-splitting technique whereby the advection component of the governing equation is solved on elements and the hydrodynamic dispersion component is solved on nodes. Comparison between simulation results and analytical solutions with different mesh discretizations and different values for the hydrodynamic dispersion coefficients allows for accurate quantification of the numerical dispersion error and yields insights into the parameters and other factors that control it. It is shown that, taken alone, the advection and dispersion solvers are very robust, but their combination can result in significant numerical dispersion, stemming from the exchange of concentration information from elements to nodes and vice versa in the time-splitting procedure. The tests also show that these errors can be kept under control by ensuring that the grid Péclet number is in the range 0.5-1.0 or smaller. We then apply CATHY in a third test case involving two synthetic hillslopes (concave and convex) in fully coupled surface–subsurface mode, in order to examine the impact of this subsurface numerical dispersion on simulated streamflow hydrographs, in particular with reference to pre-event water contributions to runoff. Here as well the results show that the effect of numerical dispersion can be controlled by keeping the grid Péclet number sufficiently small. This work provides a new set of benchmark test cases for integrated surface–subsurface hydrological models, extending to solute transport the flow-only suite of benchmarks recently published in two intercomparison studies
Is it possible to use different data types and scales to reduce flow and transport uncertainty in natural heterogeneous formations? The experimental setup of the Settolo aquifer (Italy)
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