Groundwater in desert areas

Field observations

Humid areas have typically high groundwater recharge and small catchment sizes; in contrast drylands have extended catchment areas with very low groundwater recharge.

In many dry lands, low salinity lakes occur as well as freshwater springs adjacent to salt lakes or sabkhas. In the Badain Jaran Shamo (Gobi desert) more than 70 lakes with a total surface of area about 27 km2 are known, having either high (180 g/l) and low salinities (<1 g/l) (Chen et al. 2004); the same area also hosts fresh water in dug wells adjacent to saline lakes. In Jordan the Azraq desert freshwater springs yielded about 1.2m3/s to the Azraq Sabkha before pumping by gravity wells close to these springs started to supply Amman city with drinking water (Almomani & Seiler 1996). The Chott area of south Tunisia represents a terminal discharge area and sabkha, which is fed by fresh water.

-▼_

Q = 100

Î

86

14

14

conductivities in m/s

Q = 100

t

55.5

1

43.5

31.5

12

12

G = 100 -T ,

Q = 100

Î

85.5

1.5

1.5

13

13

Fig. 2. Selected examples of frequently occurring aquifer systems (left columns) and the distribution of the groundwater recharge (in percentage of infiltration) upon the individual aquifers. G, Infiltration; Q, discharge to rivers.

Radiocarbon and tritium dating of such fresh waters revealed that they are mostly free of tritium, hence older than 50 to 100 years, and carbon-14 ages range from several thousand to 10 000 years. Statistic evaluations of radiocarbon groundwater ages in north Africa and the Arabian peninsula resulted in two frequencies of water ages (25 000 years BP and 5000 years BP), which have been interpreted as periods with significant groundwater recharge (Verhagen et al. 1987). Between these two periods only limited ground-water recharge is expected.

Groundwater chemistry from samples collected from dug wells and springs around lakes in the

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