Infrared Space Observatory observations demonstrated that water (H2O) and other species like carbon monoxide (CO) are supplied in giant planet upper atmospheres by external sources: interplanetary dust particle, icy rings and/or satellites, large comet impacts. These sources have different spatio-temporal properties that we can use to discriminate them. At Saturn, stratospheric CO was probably deposited by an ancient comet impact. For water, the situation remains unclear, even if Enceladus and its water geysers are promising candidates. In this paper, we model altitude-latitude distributions of water in Saturn's stratosphere and compare them with Herschel PACS and HIFI observations to determine its source. Herschel observations have already enabled the detection of a water torus at the orbital distance of Enceladus. A fraction of this water is predicted to rain on Saturn. We test this prediction and compare it to other possible sources.