In this paper the principle of radar commonly used in chipless RFID is implemented to detect the presence of water in contact with a resonant radar target. It is shown how it is possible to determine very precisely the time at which the water on the tag has completely evaporated. Based on this observation, a method for determining the volume of a drop of water of a few µl on a resonant scatterer is introduced. The measurements carried out in a real environment are in very good agreement with the simulations and make it possible to consider the detection of the presence of water in contact with an RFID tag without a chip and to evaluate its volume.