FIG. 3. Flowchart describing the coupled modeling approach used in the HYDRUS package for MODFLOW: (a) steps shown in gray correspond to the treatment of variably saturated water flow (i, stress period; j, time step; k, soil profile number; n_t, number of time steps; n_s; number of stress periods; n_p, number of HYDRUS profiles); (b) calculations carried out by the HYDRUS package during one MODFLOW time step.