A new saturated-unsaturated three-dimensional (3D) groundwater flow model (SU3D) has been developed that calculates the pressure distribution over the entire groundwater flow domain in response to rainfall and evapotranspiration. Recent advances in solving the saturated-unsaturated groundwater flow equation are incorporated into SU3D, which solves the nonlinear, 3D, modified mixed form of the Richards equation continuously throughout the groundwater flow domain. The block-centered, finite-difference method with a variably sized grid is employed to solve the governing partial differential equation. The non-linear terms of the governing equation are linearized using a modified Picard iteration scheme, and the preconditioned conjugate gradient method is used to solve the linearized system of equations. SU3D can simulate the effects of pumping from an aquifer, and it has an option to calculate potential evapotranspiration (PET) from meteorological data. The PET is partitioned into potential evaporation and potential transpiration as a function of the leaf area index, and then the actual evaporation and transpiration are calculated. Overland flow and seepage face calculations are not included in SU3D.