In this paper, we introduce an innovative method to map small reservoirs in semi-arid environment. The proposed technique is based on the exploitation of the recently introduced Level-1α RGB SAR composite, and it is based on change-detection. In particular, we introduce a user-oriented water index defined by a weighted ratio of the bands composing the RGB product. The performance of the method are evaluated by comparison with other popular segmentation techniques of the same complexity in order to demonstrate its reliability. Finally, an applications concerning hydrology, i.e the estimation of reservoir bathymetry starting from area measurements, is presented to underline the usefulness of remote sensing in multidisciplinary contexts.