Combining the measure of water availability and the socio-economic capacity to access it gives new insights in the fields of water resources management and poverty alleviation. This approach lets researchers think about new multidimensional water scarcity indexes that have contributed to the definition of the water poverty index (WPI). The methodology first used for the calculation of this aggregate index was based on an equally weighted average of its five components (resources, capacity, access, use and environment). The main objective of this paper is to improve this procedure by using an objective weighting scheme. For this purpose we use principal component analysis to give more weight to components with larger variance and to discard components with very small ones. This improved WPI is applied in the case of Tunisia. We have obtained relevant results which could help policy makers to devise better policies to alleviate water poverty in the Inland region which was where the Tunisian revolution began.