As part of an interdisciplinary analysis of the economic evaluation of the water resources of the Rio Grande Region of New Mexico, a conjunctive use surface water-groundwater simulator was developed. The simulator was used to determine the dynamic availability of groundwater and its relationship to flow in the Rio Grande. The structure and use of the simulator are described. The simulator is a computer model in which the governing nonlinear parabolic partial differential equation, used to simulate unconfined aquifer flow in two dimensions, was solved by implicit finite differences. The set of difference equations in residual form are solved by the line successive ovex relaxation method or Gauss Elimination depending upon the size of the problem.