The high precision achieved with continuous geodetic instruments makes it necessary to take ambient factors into account. Among these, one of the most disturbing is the hydrologic agent. After giving a characterization of the induced signals in the specific case of subsurface tilt and extensometric measurements, the techniques of predictive filtering are shown to solve the problem of modeling the induced signals. The results obtained here may be applied also to other continuous geodetic and gravity measurements. ¿ 1999 American Geophysical Union