IGW-Desktop Tutorial 8a - Coupled surface water-groundwater modeling

This video illustrates the use of IGW-Desktop to perform coupled surface water-groundwater modeling. A watershed with a lake is modeled. The lake’s water level is determined by performing a simple mass balance of the lake, accounting for all the lake’s sources and sinks. In this example, the only source and sink for the lake are groundwater influx and outflux, as this lake is a “flow-through” lake. The coupled transient model is run until steady state is reached, i.e., until the lake’s water level stabilizes. Multiple computational layers are also used in this example to illustrate the vertical groundwater dynamics around the lake. The steps to create the model are outlined briefly: (1) A pre-made polygon defines the aquifer extent. There is a lake-polygon in the top-left portion of the model domain. Create a 2nd polygon near the center of the domain polygon. • ‘Source and Sinks’ tab - check ‘Two-way head dependent flux’, and assign a constant stage and constant bottom elevation, and use the default leakance value. The stage value that is entered is used as the starting lake level, which is then updated when solving the coupled models. • Check the ‘From coupled SW/GW modeling’ box to enable coupled modeling, and open the Surface Water Dynamics window • QSW represents direct additions/subtractions of water to the lake via an external source, such as the outflow from a pumping well used in a lake augmentation project. In this example, we assign a value of 0 to Qsw (and thus no external source of water to the lake). (2) Draw a cross-section through the lake that was just created. Edit the cross-section in Attributes Explorer. The options that can be changed are: • The number of nodes along the cross-section (in both the direction of the cross-section and in the vertical direction); the horizontal and vertical grid space; the cross-section thickness; and the vertical exaggeration factor (by default set to the square root of Kx/Kx) • Uncheck the assignment of the default vertical exaggeration, and input ‘10’ (3) Discretize the model. Note the windows that appear: one is the cross-section model view (‘Cross-section 1’, while the other is a profile chart that tracks key parameters, such as head along the cross-section (Profiles Display…’) (4) Open ‘Simulation Time Parameters’ and select a ‘Transient State’ model. Use the time-step of 100 days (5) Click on the ‘Numerical Solver Settings’ icon in the icon toolbar. In the window that opens, go to the ‘Flow’ tab, and ensure that the ‘Explicit’ scheme is selected under ‘Surface Water Modeling Numerical Scheme’. • This scheme will first compute the groundwater head in each “lake cell”, and then compute lake level, which is used to compute the groundwater head in that cell for the next time step. • The implicit scheme solves the coupled groundwater-surface water equations simultaneously. (6) Still within in the ‘Flow’ tab of ‘Solver Settings’, check the box next to ‘Lakes’ in ‘Coupling Control’. This allows lake levels to be updated by and used as feedback for the groundwater heads in the aquifer. (7) Run the model forward. Note the ‘Surface Water’ model window that appears displaying the lake level as a function of time, as well as the terms in the lake mass balance. (8) Continue running the model until the lake levels are no longer changing, that is, the lake level is at steady-state. The steady-state lake stage is reported in the ‘Surface Water’ window, as well as the groundwater flux into the lake. (9) Right-click within the ‘Cross-section’ model window, and select ‘Draw Options’ - Click on the ‘Velocity’ options. Change the options so that a velocity vector is shown every 4 grid points in the horizontal and vertical direction. (10) Click on the ‘Deep discretization’ icon, and add 4 computational layers to the conceptual layer (11) Run the model, and note that the velocity vectors show upwelling of groundwater into one portion of the lake (along the cross-section), while the other part of the lake is losing water to the aquifer. Eventually, the steady-state solution is reached More details on IGW: please go to https://www.magnet4water.net/GW_home.aspx - Hydrosimulatics Inc. https://www.magnet4water.net