1,721,022 research outputs found
Assessment of adaptive vs. heuristic time stepping for variably saturated flow
No full textThe performance of improved initial estimates and 'heuristic' and 'adaptive' techniques for time step control in the iterative solution of Richards equation is evaluated. The so-called heuristic technique uses the convergence behaviour of the iterative scheme to estimate the next time step whereas the adaptive technique regulates the time step on the basis of an approximation of the local time truncation error. The sample problems used to assess these various schemes are characterized by nonuniform (in time) boundary conditions, sharp gradients in the infiltration fronts, and discontinuous derivatives in the soil hydraulic properties. It is found that higher order initial solution estimates improve the convergence of the iterative scheme for both the heuristic and adaptive techniques, with greater overall performance gains for the heuristic scheme, as could be expected. It is also found that the heuristic technique outperforrns the adaptive method under strongly nonlinear conditions. Previously reported observations suggesting that adaptive techniques perform best when accuracy requirements on the numerical solution are very stringent are confirmed. Overall both heuristic and adaptive techniques have their limitations, and a more general or mixed time stepping strategy combining truncation error and convergence criteria is recommended for complex problem
Comparative Analysis of Kinematic Approximation and Richards Equation Models for Subsurface Flow on Complex Hillslopes
No full textGeneralized solutions for the kinematic wave equation for subsurface flow have recently been derived for hillslopes of arbitrary geometry by introducing two dimensionless geometric parameters α and ε which define the hydrologic similarity between hillslopes with respect to their characteristic response (Norbiato and Borga, 2008). These solutions are derived by using a second order polynomial function to describe the bedrock slope and an exponential function to describe the variation of the width of the hillslope with hillslope distance. In this presentation we assess the behavior of this simple, one- dimensional model in comparison with a fully three-dimensional Richards equation model for a series of free drainage scenarios. For different values of saturated hydraulic conductivity, we specify the range of values of the two dimensionless geometric parameters α and ε for which the generalized solution is valid. Special attention is given to the discretization and setup of the boundary and initial conditions
Three-Dimensional Model of Coupled Density-Dependent Flow and Miscible Salt Transport in Groundwater
No full textChap. 1
Mass-conservative reconstruction of Galerkin velocity fields for transport simulations
No full textAccurate calculation of mass-conservative velocity fields from numerical solutions of Richards’ equation is central to reliable surface–subsurface flow and transport modeling, for example in long-term tracer simulations to determine catchment residence time distributions. In this study we assess the performance of a local Larson-Niklasson (LN) post-processing procedure for reconstructing mass-conservative velocities from a linear (P₁) Galerkin finite element solution of Richards’ equation. This approach, originally proposed for a-posteriori error estimation, modifies the standard finite element velocities by imposing local conservation on element patches. The resulting reconstructed flow field is characterized by continuous fluxes on element edges that can be efficiently used to drive a second order finite volume advective transport model. Through a series of tests of increasing complexity that compare results from the LN scheme to those using velocity fields derived directly from the P₁ Galerkin solution, we show that a locally mass-conservative velocity field is necessary to obtain accurate transport results. We also show that the accuracy of the LN reconstruction procedure is comparable to that of the inherently conservative mixed finite element approach, taken as a reference solution, but that the LN scheme has much lower computational costs. The numerical tests examine steady and unsteady, saturated and variably saturated, and homogeneous and heterogeneous cases along with initial and boundary conditions that include dry soil infiltration, alternating solute and water injection, and seepage face outflow. Typical problems that arise with velocities derived from P₁ Galerkin solutions include outgoing solute flux from no-flow boundaries, solute entrapment in zones of low hydraulic conductivity, and occurrences of anomalous sources and sinks. In addition to inducing significant mass balance errors, such manifestations often lead to oscillations in concentration values that can moreover cause the numerical solution to explode. These problems do not occur when using LN post-processed velocities.</p
A Simulation/Optimization approach to manage groundwater resources in the Gaza aquifer (Palestinian Territories) under climate change conditions
No full textThe Gaza aquifer is the main source of water for agricultural, domestic, and industrial uses in the Gaza Strip. The
rapid increase on water demand due to continuous population growth has led to water scarcity and contamination
by seawater intrusion (SWI). Furthermore, current projections of future climatic conditions (IPCC, 2007) point to
potential decreases in available water, both inflows and outflows.
A numerical assessment of SWI in the Gaza coastal aquifer under climate induced changes has been carried out by
means of the CODESA-3D model of density-dependent variably saturated flow and salt transport in groundwaters.
After integrating available data on climatology, geology, geomorphology, hydrology, hydrogeology, soil use, and
groundwater exploitation relative to the period 1935-2010, the calibrated and validated model was used to simulate
the response of the hydrological basin to actual and future scenarios of climate change obtained from different
regional circulation models. The results clearly show that, if current pumping rates are maintained, seawater
intrusion will worsen.
To manage sustainable aquifer development under effective recharge operations and water quality constraints, a
decision support system based on a simulation/optimization (S/O) approach was applied to the Gaza study site.
The S/O approach is based on coupling the CODESA-3D model with the Carroll’s Genetic Algorithm Driver.
The optimization model incorporates two conflicting objectives using a penalty method: maximizing pumping
rates from the aquifer wells while limiting the salinity of the water withdrawn. The resulting coastal aquifer
management model was applied over a 30-year time period to identify the optimum spatial distribution of pumping
rates at the control wells. The optimized solution provides for a general increase in water table levels and a
decrease in the total extracted salt mass while keeping total abstraction rates relatively constant, with reference to
non-optimized conditions
A simulation/optimization study to assess seawater intrusion management strategies for the Gaza Strip coastal aquifer (Palestine)
No full textSeawater intrusion is one of the major threats to freshwater resources in coastal areas, often exacerbated by groundwater overexploitation. Mitigation measures are needed to properly manage aquifers, and to restore groundwater quality. This study integrates three computational tools into a unified framework to investigate seawater intrusion in coastal areas and to assess strategies for managing groundwater resources under natural and human-induced stresses. The three components are a three-dimensional hydrogeological model for density-dependent variably saturated flow and miscible salt transport, an automatic calibration procedure that uses state variable outputs from the model to estimate selected model parameters, and an optimization module that couples a genetic algorithm with the simulation model. The computational system is used to rank alternative strategies for mitigation of seawater intrusion, taking into account conflicting objectives and problem constraints. It is applied to the Gaza Strip (Palestine) coastal aquifer to identify a feasible groundwater management strategy for the period 2011–2020. The optimized solution is able to: (1) keep overall future abstraction from municipal groundwater wells close to the user-defined maximum level, (2) increase the average groundwater heads, and (3) lower both the total mass of salt extracted and the extent of the areas affected by seawater intrusion
Investigating parameter transferability across models and events for a Semiarid Mediterranean Catchment
Full text linkPhysically based distributed hydrologic models (DHMs) simulate watershed processes by applying physical equations with a variety of simplifying assumptions and discretization approaches. These equations depend on parameters that, in most cases, can be measured and, theoretically, transferred across different types of DHMs. The aim of this study is to test the potential of parameter transferability in a real catchment for two contrasting periods among three DHMs of varying complexity. The case study chosen is a small Mediterranean catchment where the TIN-based Real-time Integrated Basin Simulator (tRIBS) model was previously calibrated and tested. The same datasets and parameters are used here to apply two other DHMs-the TOPographic Kinematic Approximation and Integration model (TOPKAPI) and CATchment HYdrology (CATHY) models. Model performance was measured against observed discharge at the basin outlet for a one-year period (1930) corresponding to average wetness conditions for the region, and for a much drier two-year period (1931-1932). The three DHMs performed comparably for the 1930 period but showed more significant differences (the CATHY model in particular for the dry period. In order to improve the performance of CATHY for this latter period, an hypothesis of soil crusting was introduced, assigning a lower saturated hydraulic conductivity to the top soil layer. It is concluded that, while the physical basis for the three models allowed transfer of parameters in a broad sense, transferability can break down when simulation conditions are greatly altered
Combined assimilation of soil moisture and streamflow data by an ensemble kalman filter in a coupled model of surface–subsurface flow
No full textHydrologic models can largely benefit from the use of data assimilation algorithms, which allow to update the modeled system state incorporating in the solution of the model itself information coming from experimental measurements of various quantities, as soon as the data become available. In this context, data assimilation seems to be well fit for coupled surface--subsurface models, which, considering the watershed as the ensemble of surface and subsurface domains, allow a more accurate description of the hydrological processes at the catchment scale, where soil moisture largely influences the partitioning of rain between runoff and infiltration and thus controls the flow at the outlet. The need for a better determination of the variables of interest (streamflow at the outlet section, water table, soil water content, etc.) has led to a many efforts focused on the development of coupled numerical models, together with field and laboratory observations. Nevertheless, uncertainty in the schematic description of physical processes and inaccuracies on source data collection induce errors in the model predictions. The ensemble Kalman filter (EnKF) represents an extension to nonlinear problems of the classic Kalman filter by means of a Monte Carlo approach. A sequential assimilation procedure based on EnKF is developed and integrated in a process-based numerical model, which couples a three-dimensional finite element Richards equation solver for variably saturated porous media and a finite difference diffusion wave approximation based on a digital elevation data for surface water dynamics. A detailed analysis of the data assimilation algorithm behavior within the coupled model has been carried out on a synthetic 1D test case in order to verify the correct implementation and derive a series of fundamental parameters, such as the minimum ensemble size that can ensure a sufficient accuracy in the statistical estimates. The assimilation frequency, as well as the effects induced by assimilation on the surface and/or subsurface system state, was tested on a 3D synthetic test case represented by a 1.62 km2 tilted v-catchment, for which observations of pressure head and streamflow data are assimilated in order to retrieve the true watershed state in 2 scenarios: i) starting from a drier initial condition and ii) intentionally imposing a biased atmospheric forcing. In general, streamflow prediction is improved by assimilation of both pressure head and streamflow individually and by coupled assimilation. However, assimilation of streamflow data only does not improve the subsurface system state, leading to a deficit in soil moisture compared to both the true and the open loop simulations. Combined assimilation is therefore more adequate for the description of the entire surface—subsurface system state. The sensitivity analysis to the assimilation frequency yields contradictory results: as expected, a higher assimilation frequency improves the true state retrieval in the drier initial condition scenario, while for the biased atmospheric forcing scenario an analogous improvement is not manifest
Seawater intrusion risk analysis under climate change conditions for the Gaza Strip aquifer (Palestine)
No full textSeawater intrusion (SWI) has become a major threat to coastal freshwater resources, particularly in the Mediterranean basin, where this problem is exacerbated by the lack of appropriate groundwater resources management and with serious potential impacts from projected climate changes. A proper analysis and risk assessment that includes climate scenarios is essential for the design of water management measures to mitigate the environmental and socio-economic impacts of SWI. In this study a methodology for SWI risk analysis in coastal aquifers is developed and applied to the Gaza Strip coastal aquifer in Palestine. The method is based on the origin-pathway-target model, evaluating the final value of SWI risk by applying the overlay principle to the hazard map (representing the origin of SWI), the vulnerability map (representing the pathway of groundwater flow) and the elements map (representing the target of SWI). Results indicate the important role of groundwater simulation in SWI risk assessment and illustrate how mitigation measures can be developed according to predefined criteria to arrive at quantifiable expected benefits
Ensemble Kalman Filter Data Assimilation for a Coupled Model of Surface and Subsurface Flow
No full textIn this study the ensemble Kalman filter (EnKF) is implemented in a detailed catchment-scale hydrological model that couples a three-dimensional finite element Richards equation solver for variably saturated porous media and a finite difference diffusion wave approximation based on a digital elevation data for surface water dynamics. In data assimilation, the Kalman filter (KF) updates the system state based on the relative magnitudes of the covariances of both the observations and the model state estimate. EnKF has been demonstrated to be a valid alternative to KF for nonlinear filtering problems, and is based on the approximation of the conditional probability densities of interest using a finite number of randomly generated model trajectories. We describe the implementation of EnKF for our coupled groundwater--surface water model, and will examine issues of robustness and computational efficiency, important for such a detailed numerical model characterized by strong nonlinearities in the pressure--moisture and pressure--conductivity relationships and by complex interactions across the land surface boundary. The implementation is tested for a synthetic soil moisture profile retrieval experiment described in Entekhabi et al. (IEEE Trans. Geosci. Remote Sensing, 1994). In this column experiment surface observations are assimilated to retrieve the true moisture profile starting from a poor estimate of the initial moisture state of the system
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