What do the boundary conductances GNUP and GNUU do, and why are they important?
For reasons that have to do with matrix structure and efficient calculation of boundary flows, specified pressure boundary conditions are implemented somewhat indirectly in SUTRA. The flow equation is formulated as though each pressure boundary node were connected to an external node set to the specified pressure. The connection between each pressure boundary node and its external partner is assigned a “conductance,” GNUP. If GNUP is zero, there is no hydraulic communication between each pressure boundary node and its external partner, and the pressures computed at the pressure boundary nodes will be independent of the pressure specifications. On the other hand, if GNUP is large enough, each pressure boundary node is in perfect hydraulic communication with its external partner, and the pressures computed at the pressure boundary nodes will match the pressure specifications perfectly (i.e., to the full precision provided by the computer). Intermediate values of GNUP produce partial matc
