This chapter deals with the formulation of the optimisation problem, starting with the analysis of the encoding of decision variables. The design of static precipitation sensor networks have been traditionally driven by aspects such as the minimisation of interpolation estimations, the even spatial distribution of the sensors and the maximisation of information content they provide. The problem of designing an optimal sensor network consists of defining optimal network configurations such that minimise a predetermined objective function. Although precipitation sensor networks provide data to simulation models, an aspect worth exploring is how the precipitation sensor networks would look like if they were designed to improve these models. An analysis of robustness of the optimal solutions is carried out. This analysis aims to understand how robust are each of the previously found optimal solutions to the location of the sensors.