Spatial and temporal variation of moisture flux in soil over a 1 year period was studied in the Thar desert of India. Tritium was injected at a depth of 0.8 m at 18 sites distributed over an area of 50 km2 and at 20 adjacent points at a single site over a sand dune, before the 1994 monsoon. Vertical soil profiles were collected from the 18 sites after the end of monsoon and at the end of the year and from the multiple-injections site at different time intervals throughout the year. Moisture flux values, computed from tritium and moisture distribution in soil profiles, show that the moisture influx at a site depends upon the grain size of the top 20 cm layer and that about 68% of the total influx of water below the injection depth is lost during the summer months. The data for the multiple-injections site show that the tritium spike was displaced to a depth of 4 m by the end of the monsoon, and it remained stationary during the postmonsoon period of 8 months, although about 64% of the soil moisture was lost. The spiked layer showed enrichment in the specific activity of tritium during summer months. These observations suggest that vapor diffusion is the dominant process for moisture loss from the Thar desert soil. The net input (natural recharge) to groundwater due to 1994 precipitation was 62 mm. ¿ 1998 American Geophysical Union