Safe disposal of toxic wastes in geologic formations requires minimal water and gas movement in the vicinity of storage areas. Ventilation of repository tunnels or caverns built in solid rock can desaturate the near field up to a distance of meters from the rock surface, even when the surrounding geological formation is saturated and under hydrostatic pressures. A tunnel segment at the Grimsel test site located in the Aare granite of the Bernese Alps (central Switzerland) has been subjected to a resaturation and, subsequently, to a controlled desaturation. Using thermocouple psychrometers (TP) and time domain reflectometry (TDR), the water potentials &psgr; and water contents &thgr; were measured within the unsaturated granodiorite matrix near the tunnel wall at depths between 0 and 160 cm. During the resaturation the water potentials in the first 30 cm from the rock surface changed within weeks from values of less than -1.5 MPa to near saturation. They returned to the negative initial values during desaturation. The dynamics of this saturation-desaturation regime could be monitored very sensitively using the thermocouple psychrometers. The TDR measurements indicated that water contents changed close to the surface, but at deeper installation depths the observed changes were within the experimental noise. The field-measured data of the desaturation cycle were used to test the predictive capabilities of the hydraulic parameter functions that were derived from the water retention characteristics &psgr;(&thgr;) determined in the laboratory. A depth-invariant saturated hydraulic conductivity ks=3.0¿10-11 ms-1 was estimated from the &psgr;(t) data at all measurement depths, using the one-dimensional, unsaturated water flow and transport model HYDRUS . For individual measurement depths, the estimated ks varied between 9.8¿10-12 and 6.1¿10-11 ms-1. The fitted ks values fell within the range of previously estimated ks for this location and led to a satisfactory description of the data, even though the model did not include transport of water vapor.¿ 1997 American Geophysical Union