INFLUX AND EFFLUX OF MOISTURE IN A DESERT SOIL DURING A 1 YEAR PERIOD

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 a t a depth of 0.8 m at 18 sites distributed over an area of 50 km(2) an d 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 multi ple-injections site at different time intervals throughout the year. M oisture flux values, computed from tritium and moisture distribution i n 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 mo nths. 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 i t remained stationary during the postmonsoon period of 8 months, altho ugh about 64% of the soil moisture was lost. The spiked layer showed e nrichment in the specific activity of tritium during summer months. Th ese observations suggest that vapor diffusion is the dominant process for moisture loss from the Thar desert soil. The net input (natural re charge) to groundwater due to 1994 precipitation was 62 mm.