Run Model The Run Model window contains the Start button that actually implements the MODFLOW solution routines. Before clicking on the Start button, note the Tolerance (acceptable error), Maximum Iterations, and Accelerator text boxes. You can always stop a model by clicking on any key during execution, so the maximum iterations is not that important. The accelerator either slows convergence (if less than one) or speeds convergence (if greater than one). Be careful. In general, an accelerator set to one is adequate. The conjugate gradient solution algorithm does not use an accelerator. The Run Model window includes a list box where you can observe the maximum error in any iteration. If the model does not appear to be converging, you can stop execution by pressing any key. The Run Model window also includes the Water Balance Modeling check box. Selecting this box bypasses the solution routines in MODFLOW. Instead, Graphic Groundwater only implements the budget routines in MODFLOW. In addition, all active cells are treated as constant head cells during the solution. Stoertz and Bradbury (1989) described water balance modeling for determining the distribution of areal recharge in a model. Consult Anderson and Woessner (1992) or Stoertz and Bradbury (1989) for more information.