Evolution of MODFLOW: Past and Future

Evolution of MODFLOW – Past and Future

MODFLOW was released in 1983, and it has evolved significantly through the present. The design incorporated the capability to be easily enhanced by others, and the quantity of enhancements is a testament to the success of this design; however, the original design incorporated a number of decisions that limit the possibility for enhancements. The most significant decision that limits enhancements is the form of the flow equation. MODFLOW is designed to solve a discretized flow equation in the form Ah=b where A is a symmetric matrix with 7 diagonals. Ideally A and b do not depend on h, but allowance for minor dependencies are made through Picard iteration (lagging the h dependence). Picard iteration works well when the h dependence has a small impact on flow; however, the h dependence is large in an increasing number of situations being simulated. Other limitations that must be addressed are the data sharing among the components of MODFLOW and the predefined time step lengths. This paper presents examples of the need to modify these design decisions.