Geological Survey groundwater model visualization tool is now available to help users visualize the inputs and outputs of complex groundwater models across the country. Groundwater models are computer codes that simulate the physics of groundwater motion within an aquifer system. This new web-based mapping tool, called GWWebFlow , allows water managers and the public to visualize complex groundwater models in a more understandable way. The tool allows users to quickly and easily view computer model inputs, such as information about aquifer properties, as well as the outputs, such as groundwater levels and flow direction.
Easy access to this information will help resource managers make informed decisions about resources. The USGS plans to add other existing groundwater models from around the country to this web application. Houston area: Hydrogeology and simulation of groundwater flow and land-surface subsidence in the northern part of the Gulf Coast aquifer system, Texas, San Antonio region: Updated numerical model with uncertainty assessment of drought conditions on brackish-water movement within the Edwards aquifer.
The organization imposed by a model helps alert the modeler to errors in assumptions and to processes not previously considered. For this reason, mathematical modeling should be performed at the beginning of every hydrogeological study that addresses nontrivial questions e. Today, groundwater models are accepted as essential tools for addressing groundwater problems.
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Similarly, a conceptual model of a groundwater system simplifies and summarizes what is known about the hydrogeology in the form of written text, flow charts, cross sections, block diagrams, and tables. A conceptual model is an expression of the past and current state of the system based on field information from the site, and knowledge available from similar sites. A more powerful groundwater model is one that quantitatively represents heads in space and time in a simplified representation of the complex hydrogeologic conditions in the subsurface.
Broadly speaking, groundwater models can be divided into physical laboratory models and mathematical models. Learn more about these models in the introductory chapter to Modelling Fundamentals. Anderson, William W. Woessner and Randall J. This second edition is extensively revised throughout with expanded discussion of modeling fundamentals and coverage of advances in model calibration and uncertainty analysis that are revolutionizing the science of groundwater modeling.
A DIY Groundwater Model
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Model input data include a global topography model, a long-term time series of global groundwater recharge, and a global soil model. However, no New Zealand data were used in that calibration.
Metropolitan Area Groundwater Model | Minnesota Pollution Control Agency
As the EWT method is well described in other studies, we will not provide more detail in the main text of our study. Generally, they found that the model correctly estimated shallow water tables in New Zealand's alluvial aquifer systems, such as the Hauraki Plains, Heretaunga Plains, and the Canterbury Plains Fig. Overall, water tables have a tendency to be too shallow, and Westerhoff and White concluded that was mostly because the model does not incorporate pumping or other human draining features, e.
However, they also found several issues in the Canterbury Plains, which included a bias towards calculation of shallow water table depth — i. EWT depths were too shallow compared to ground observations at many locations — that was possibly driven by wrong model input data e. Therefore, Westerhoff and White recommended improvement in the method and its input data, which included better model convergence criteria; representation of terrain; estimation of hydraulic conductivity, i.
Our model improvements include a national terrain model; national recharge estimates; hydraulic conductivity estimates based on the geological map of New Zealand; and model resolution, initial conditions, and convergence criteria. More accurate elevation data are available in New Zealand, but only piecemeal at the local scale of catchments.
For the purpose of feeding recharge into the NWT model with daily time steps, mean daily recharge was calculated for each NWT model cell Fig. These steps are described below.
The NWT method applies geology data, i. Permeabilities of main rock type and secondary rock type were averaged, with main rock type weighted twice as much as secondary rock type. For local testing purposes, a simple calibration procedure was built in. This procedure was only used in the case where ample ground-observed water level data were available for this study Canterbury and Waipa; see Sects. Before calibration, the ground-observed water level point data were gridded to an interpolated surface within the area containing ground observations.
All input data were gridded at this cell size, either by averaging if the cell size of the original input data was smaller than the model grid cell size or by its nearest value if cell size of the original input data was larger than the model grid cell size. Locally, the model was run at different spatial resolutions for the sake of more in-depth research on model behaviour see Sect. Initial estimates of water table depth depth and elevation for the NWT model were set equal to the EWT water table depth and elevation, i. The EWT method had a convergence criterion, where the iterative calculation stops when the water table reaches an equilibrium, i.
Additionally, feeding in recharge that contains uncertainty causes the equilibrium to probably never be reached.
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National maps of NWT water table depth Fig. The NWT water table depth clearly follows the aquifer delineations of White , e. Many very small shallow-water-table features are found in between areas where water tables are deeper. These features resemble stream valleys, as the more detailed terrain model allows the model to discharge more water to the surface in these valleys. The finer spatial detail is best appreciated when zooming in to smaller regions. Therefore, evaluation analyses are described for two regional case studies Canterbury region, Sect.
The Canterbury Plains is New Zealand's largest alluvial aquifer system.
Surface geology in the Canterbury region is dominated by metamorphic rock and floodplains Fig. Metamorphic rock includes a large area of greywacke that forms much of the Southern Alps. Aquifers in the higher plains are mostly unconfined; in the lower plains they are typically confined and supply groundwater to extensive networks of spring-fed streams around Lake Ellesmere and the city of Christchurch.
These confining conditions are due, in part, to the occurrence of marine sediments deposited near the current coast during interglacial periods. Most of these wells are located in the Canterbury Plains. It is assumed that these observations represent the water table and can thus be compared to NWT water table depth and water table elevation. Median observed groundwater level for all wells was calculated.
The national Geographx elevation model was used for conversion of groundwater depth to groundwater elevation. For example, 3. Westerhoff and White showed that evaluation of the EWT model revealed higher discrepancies in the same area i. Although the discrepancies are smaller for the NWT model than for the EWT model in these areas, they occur at the same locations.
Possible explanations for these discrepancies are considered in Sect. Increasing agricultural land use and deforestation, mainly in the low-lying Hamilton Basin, could potentially result in the deterioration of water quality in the Waipa River catchment, according to Rawlinson , who performed a review on existing information in the catchment. However, these observations are sparse because, temporally, data mostly consist of few measurements and, spatially, data are concentrated more in some areas than others; i.
The NWT model can thus provide a better spatial insight into the water table in the catchment, provided that they correlate well enough with the ground observations. NWT water table depths clearly demonstrate the location of shallow water tables in the low-lying basin area Fig. The spatial pattern of NWT water table elevation is a good visual match to the interpolated ground-observed water table elevation Fig. In addition, the NWT map shows more detail than the interpolated ground-observed surface and calculates water table depth at many places where there are no ground observations.
The dashed line is the relation. The effect of calibration towards the ground observations was also taken into account. Calibration towards hydraulic conductivity, albeit a simple implementation, improves the correlation of the modelled water table with ground-observed water level model output. Inclusion of the lidar DEM does not significantly improve the results of the uncalibrated run, but it does improve calibrated model runs. Water table depth shows significantly lower correlation than water table elevation, as was also the case in the evaluation in the Canterbury region.
The Discussion section describes possible causes for these differences. The study area is shown in red. Results are shown in Fig. Spatial plots of two runs Fig. Model equations are based on the global-scale EWT model, with adjusted model parametrisation and national input datasets.
Because of these improvements, NWT-derived water table depths show the areas of alluvial aquifers, including their variations, with higher spatial resolution than the EWT model. The NWT water table elevation shows significantly better correlation with ground-observed water level data than EWT model results. The NWT model is currently the only dedicated high-resolution nationwide groundwater model for New Zealand, and it is able to estimate the water table at places where there are no ground observations. In addition, it shows more detail than most other interpolated water table surfaces and the EWT model results.
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Because water is primarily regulated at the regional level, regional models can show different results e. These inconsistencies can lead to trans-boundary issues, such as catchment boundary definition and source protection. A fundamental role of groundwater science is to identify and characterise groundwater catchments of water supplies. Better delineation of groundwater catchments is therefore essential for source protection. This makes the NWT model relevant to solving trans-boundary issues and to assisting in provision of nationally consistent groundwater data.
The advantage of the NWT model, compared to catchment-scale groundwater flow models, is that its simple algorithm makes computation of the water table across all catchments relatively fast. The NWT model can therefore provide useful preliminary water table information for catchment-scale numerical groundwater flow models, in both data-sparse and data-rich regions. Furthermore, the NWT model inputs can provide other initial estimates that are useful to other models.
For example, data of hydraulic conductivity and recharge are also nationwide. The disadvantage of simplifying assumptions in the NWT model is that more complex groundwater features are not handled well.