IGW-Desktop Tutorial 6a - 3D groundwater modeling and mass balance analysis

This video demonstrates the use of IGW-Desktop to create a 3-dimensional groundwater model with multiple layers. First, a 2-dimensional model is created, which is then converted to a 3-dimensional model by adding layers and additional features in an interactive manner. As each layer or feature is added, the impact of each incremental change is visualized as the model’s complexity increases. Simultaneously, a mass balance analysis is performed that also shows the impact of each change. Finally, this video shows how the “conceptual” layers can be discretized into “computational” layers to provide greater vertical resolution of processes. The steps to create the model are outlined briefly: (1) Set the workspace extent to 10000m by 7500m (not shown in video) (2) Create a polygon covering the entire modeling workspace; edit properties in ‘Attributes Explorer’ (AE) • ‘Flow Properties’ - check ‘Conductivity’ and assign a constant value • ‘Elevations’ tab - check ‘Top Elevation’ and ‘Bottom Elevation…’ and assign constant values to both. • ‘Sources/Sinks’ tab - ‘Prescribed Flux’ tab - check the ‘Recharge’ box and assign a constant value (3) Select the ‘Zone Budget’ button at the bottom of the Attributes Explorer window to enable mass balance analysis. (4) Create a polygon at the eastern edge of the model to represent the river; edit properties in ‘Attributes Explorer’. • ‘Sources/Sinks’ tab - ‘Head-Dependent Flux’ - select the ‘Head-dependent flux (Two-way)’ • Select ‘River’ and enter constant values for Stage, Bot Elevation and Leakance. (5) Discretize and run the model. (6) Go to the Attributes Explorer window, and in the bottom right corner select the ‘Water Balance’ button under ‘Zone 2’ (the aquifer polygon). This opens the mass balance window for the aquifer. Note that all the recharge entering the aquifer is flowing out of the aquifer into the river as base-flow. (7) To add a layer go to File - Create New - Layer and select ‘OK’ to add one layer below the existing layer. (8) Now create a polygon in the second layer; edit properties in ‘Attributes Explorer’. • ‘Flow Properties’ - check ‘Conductivity’ and assign a constant value. • ‘Elevation’ - check ‘Bottom Elevation/Thickness’ - select ‘Thickness’ and enter a constant value. (9) Discretize and run the model. Note that the mass balance window has not changed much although additional terms representing the flux between the two layers can be seen. The relatively minor impact with the addition of the second layer is because of the lower hydraulic conductivity. (10) Now, add another layer below the second layer. (11) Create a polygon in the third layer; edit properties in ‘Attributes Explorer’. • ‘Flow Properties’ - check ‘Conductivity’ and assign a constant value. • ‘Elevation’ - check ‘Bottom Elevation/Thickness’ - select ‘Thickness’ button and enter a constant value. (12) Discretize and run the model. Note that the mass balance window now shows more significant changes, although the total flux entering the river does not change. The terms show an increase in the flux through the bottom of the first layer, which flows through the confining unit (2nd layer), enters the third layer and flows horizontally, before upwelling back through the confining unit to the first layer and finally discharges to the river. Note that this flux is exactly balanced, since no additional source/sink are present in the second or third layers. (13) Now, a pumping well is added to the third layer. (14) The model is discretized and solved. The mass balance shows that the discharge to the river has now decreased. The fluxes through the bottom of the first layer are no longer balanced, as more flux leaves the first layer than upwells back into it. The difference in flux is “captured” by the pumping well in the third layer. (15) Now, a polygon is inserted in the second layer with a higher hydraulic conductivity to represent a “break” in the confining layer. 15a) The impact of this break is relatively minor on the mass balance, but the flow contours show a change because the break in the confining layer offers a path of less resistance to vertical flow towards the pumping well in the third layer. (16) Now, click on the discretize button, and click on ‘Define number of computational layers’ to increase the number of vertical layers in each “conceptual” layer. Note that the layers are discretized by using the water table, which discretizes the saturated thickness of the unconfined aquifer rather than the entire thickness. (17) The model is then solved – the mass balance analysis window now shows mass balance for each computational layer individually. The overall mass balance for the conceptual layer can also be computed. More details on IGW: please go to https://www.magnet4water.net/GW_home.aspx - Hydrosimulatics Inc. https://www.magnet4water.net