Thursday, December 31, 2020

So...How Does AnAqSim Compare to MODFLOW?

Here at flexAEM, one of the questions that often comes up is “How does AnAqSim compare to MODFLOW?” Although this question is somewhat broad, and can be interpreted in different ways, two ways in which AnAqSim and MODFLOW can be compared are in their capabilities and their accuracy. As to a comparison of capabilities, AnAqSim includes many of the key features that have made MODFLOW so popular (and one of the most widely used groundwater modeling software programs in the world today). For instance:

  • Like MODFLOW, AnAqSim has the ability to incorporate multiple layers into a model, which allows for better representation of more complex aquifer systems. 
  • The subdomain method that AnAqSim is based on allows you to apply this mult-layer capability only in the area of interest within a broader, less detailed regional model domain.  This flexible layering scheme is similar to nested grid models that can be generated using more recent versions of MODFLOW, such as MODFLOW-USG and MODFLOW-6.
  • The area of the aquifer for which you build an AnAqSim model can be hydraulically linked to extended areas of the aquifer you are analyzing using the same types of specified head, specified flow, or general head boundary conditions that are applied in MODFLOW.
  • AnAqSim, like MODFLOW, can simulate both steady-state and transient flow conditions.
  • With regard to Transient simulations, similar to MODFLOW, AnAqSim allows the user to specify stress periods and time steps, and vary several different model inputs through time, including aquifer recharge, well pumping rates, and river stages. 
  • AnAqSim (like MODFLOW) also allows for the incorporation of anisotropy.
  • AnAqSim can be used to trace particle pathlines in three dimensions (which can be done using MODPATH upon generation of a flow field using MODFLOW).
  • Like MODFLOW, PEST can be used in conjunction with AnAqSim to automate and greatly streamline the model calibration process (see our Blog post on using PEST with AnAqSim HERE). 

But what about accuracy?  How do modeling results from AnAqSim compare to those generated by MODFLOW?  Since the AEM method is based on exact analytical solutions, whereas MODFLOW uses the finite difference method to approximate those solutions, the answer is that AnAqSim is typically more accurate than MODFLOW. To illustrate this, a simple box model with three pumping wells was generated using AnAqSim and MODFLOW. As shown in Figures 1 and 2 below, the MODFLOW results become increasingly more accurate as the model is further discretized, and to achieve a similar level of accuracy as the results generated by AnAqSim, a grid spacing of 2.5’ x 2.5’ must be utilized in the MODFLOW model, which can be computationally taxing.

Figure 1 – Comparison of simulated head field using AnAqSim and MODFLOW with Variable Grid Spacing

Figure 2 – Comparison of simulated heads using AnAqSim and MODFLOW with Variable Grid Spacing (with MODFLOW grids turned off and heads at pumping wells displayed)

How about particle pathline tracing?  Again, a comparison of particle pathlines generated using AnAqSim and MODFLOW/MODPATH in a model with a “stepped” base elevation indicates that the results match almost exactly (see Figure 3 below).

Figure 3
 – Comparison of AnAqSim and MODFLOW particle pathlines with a 5-ft stepped base elevation

For additional comparisons of AnAqSim to MODFLOW (and other analytical solutions), please visit Fitts Geosolutions’ “Checks of AnAqSim” page.As demonstrated above, a comparison of the features and accuracy of results generated by AnAqSim and MODFLOW indicates that AnAqSim incorporates many of the key features and capabilities that make MODFLOW such a versatile, widely used tool, and produces results that are typically more accurate than those generated by MODFLOW. These capabilities make AnAqSim a great alternative to MODFLOW for most simple to moderately complex modeling projects.