Nonlinear least squares has proven to be a very useful tool for the estimation of parameters in groundwater modeling. The reason for this success is the power of the approach: the only fundamental requirement is to posses one numerical simulator able to calculate the output ~(.,c) of the model (for example the computed pressure history) once a value x (for example the hydraulic conductivity map) has been assessed to the unknown parameter (hence cp denotes the parameter --* output or modelling operator). Given a value z of the experimental data (for example the measured pressure history), nothing can prevent you from forming the least-squares error: