M A Geyh1 D Ploethner2

1Riibeland 12, 29 308 Winsen/Aller, Germany (e-mail: [email protected]) 2Bundesanstalt fiir Geowissenschaften und Rohstoffe (BGR), Hannover, Germany

Abstract: The Thar Desert of Pakistan stretches along the border to India and is one of the most densely populated deserts in the world. Brackish to saline groundwater prevails. A locally restricted fresh groundwater resource was discovered by a comprehensive hydrogeological, geophysical, and isotope hydrological survey conducted from 1986 to 1991. The origin, recharge mechanism and age of the fresh groundwater resource were assessed. There is only fossil ground-water and this must be mined. Sodium is the predominant cation. Present groundwater recharge is absent or extremely low as the annual precipitation rate and the potential evapotranspiration rate amount to less than 200 mm/a and about 2700 mm/a, respectively. The investigations comprised a hydrogeological well inventory, electrical resistivity transects on the ground and an air-borne electromagnetic survey, followed by a test-hole drilling programme combined with geophysical borehole logging, aquifer testing, and groundwater sampling for both chemical and environmental isotope analyses. The results of this study delivered a hydrogeological concept on the origin and recharge of the fresh groundwater body. We found that the fresh groundwater was indirectly recharged during flash floods in low lands during the last pluvial period rather than directly replenished in the high mountain areas far in the east.

Cholistan, in the northern part of the Thar Desert in Pakistan, is a vast sandy area of about 26 000 km2. A fresh groundwater resource was discovered during our study. It is located within the area 29°0' to 29° 15' N and 72°0' to 73°0' E, between Fort Abbas in the east and Fort Mojgarh in the SW (some 60 km SE of Bahawalpur). It stretches along the southern bank of the Old Hakra River traced by a chain of forts: Phulra (Abbas), Mirgarh, Jamgarh, Marot, Mojgarh, Dingarh and Derawar, remnants of the Moghul Period (AD 1526 to 1857).

In the north, the irrigated land of the Hakra Right canal system borders the project area. The canal water comes from the Sutlej River, about 90 km north of Fort Abbas. The southwestern part of the study area (some 450 km2) was under irrigation between 1927 and 1932. The land surface slopes from 140 m a.s.l. in the east to 125 m a.s.l. in the SW. The target area represents a sub-recent stream bed and meander-belt floodplain built up by the Old Hakra River (Geological Survey of Pakistan 1964; Fig. 1). Between 4000 and 3500 years BP the discharge of the Old Hakra River gradually decreased due to the southward shift of its headwaters. The river stopped running perennially around 2500 years BP while flash floods reached the target area until mid-thirteenth century. The river eventually ceased running in the early sixteenth century (Wilhelmy 1969). This former fluvial environment exhibits a flat, hard pan surface on which dunes irregularly rest up to 10 m high.

The top sediments of the floodplain consist of silty to sandy loam or silty clays. They are of low permeability. Thus, rainwater accumulates in natural depressions and may remain for a couple of days, or is stored in artificial ponds for stock watering over a period of up to four months. The pans have no vegetation, while shrub, grass or single small trees grow on the dunes. The vegetation is typically xerophytic.

The study area receives a mean annual rainfall of less than 200 mm. Two-thirds of the precipitation, in the form of high intensity downpours, takes place during the monsoon period lasting from July to September. The rainfall is, however, so erratic that continuous droughts are often experienced for two to five years at a time. The potential evaporation rate amounts to about 2700 mm/a.

The mean annual maximum and minimum air temperatures average 34°C and 18°C, respectively. The daily maximum temperature rises above 408C during the hottest months of May and June.

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