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This paper presents four different studies from arid regions intended to highlight applications of isotopic studies in arriving at specific hydrologic understanding. These studies employed environmentally stable isotopes 2H, 18O, 13C and radioactive isotopes 3H and 14C.

The Thar Desert extends from the western side of the Aravalli mountain ranges in India up to the limit of the Indus valley in Pakistan (Fig. 1). It covers about 60% of the area of Rajasthan state in the northwestern part of India. Having c. 38% of the state's population with a density of 84 persons/km2, this is one of the most populated desert region of the world. The constantly increasing human and livestock population exerts tremendous stress on the available natural ground-water resources.

The land is characterized by sand dunes with interdunal plains in the north, west and south, and alluvium in the central and eastern parts. Streams are very few, ephemeral in nature and confined mostly to the rocky parts of the desert; the most prominent is the Luni river in the SW region. The average annual precipitation is low (100 to 300 mm per year) and erratic with frequents droughts. The average annual maximum and minimum temperatures are 45°C and 8°C, respectively.

The main source of water in the area is ground-water. The region, with mean annual potential evapotranspiration ranging from 180 to 200 cm, faces an acute shortage of potable water during the summer months. At many places the ground-water is brackish or saline. Efforts are being made

Fig. 1. Physical map of India showing the Thar Desert.

by the state groundwater department to study known groundwater resources and explore potential ones in the region.

The possible groundwater recharge processes in arid areas like Rajasthan are: (a) direct recharge of precipitation through the unsaturated zone; (b) indirect recharge through river channels from flash floods or from irrigation canals etc. and (c)

palaeoclimatic recharge during humid episodes in the past.

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