The Pre-conditioned Conjugate Gradient (PCG) Solver

As an alternative to the SOR-solver, MIKE SHE’s groundwater component also includes the pre-conditioned conjugate gradient solver (Hill, 1990). The PCG solver includes both an inner iteration loop, where the head dependent boundaries are kept constant, and an outer iteration loop where the (non-lin­ear) head dependent terms are updated. The PCG solver includes a number of additional solver options that are used to improve convergence of the solver. The default values will generally ensure good performance. For the majority of applications, there is no need to adjust the default solver settings. If, on the other hand, non-convergence or extremely slow convergence is encountered in the SZ component, then some adjustment of the solver set­tings may help.

The PCG solver in MIKE SHE, which is identical to the one used in MOD­FLOW (McDonald and Harbaugh, 1988), requires a slightly different formula­tion of the hydraulic terms when compared to the SOR solver.