ABSTRACTA model to solve the inverse problem in ground water has been developed. The model consists of an unconstrained multivariable optimization algorithm and a ground‐water simulation model. The optimization algorithm is a modified version of Newton's second derivative method. It is a ground‐water‐specific algorithm and was found to be more efficient than other more general unconstrained optimization algorithms requiring first‐ and/or second‐order derivatives and those requiring no derivatives. The groundwater simulation model is based on a finite‐difference technique but it can be replaced easily by a finite‐element model. The results of an application of this model to a real aquifer are discussed in a companion paper titled “Inverse Problem in Ground Water: Mod