Recharge systems for protecting and enhancing groundwate

V
Large scale recharge modeling
in the arid area of the eastern Sahara
W. Gossel, A. Sefelnasr, A. Ebraheem and P. Wycisk
Abstract
A GIS-based (Geographic Information System) hydrogeological database was established for the development of
a numerical three-dimensional groundwater flow model for the Nubian Sandstone Aquifer System in Egypt and the
adjacent countries in the eastern Sahara. It was calibrated under steady-state and transient conditions. The
model was then used to simulate the response of the aquifer to the impact of the climatic changes that took place
during the last 25,000 years, to estimate the recharge of the aquifer and to approximate and locate the palaeolakes
that contributed to the aquifer in past times. From the results of the simulation, it is indicated that the infiltration
during the wet periods 20,000 and 5,000 years before present (b.p.) formed the groundwater in this
aquifer. The present recharge of groundwater due to regional groundwater flow from the more humid regions
in the southern areas of the aquifer is negligible, demonstrating that the groundwater within the Nubian Aquifer
System is fossil water, and the aquifer performed unsteady state condition after the last wet period ending
3,000 years b.p.
Keywords
Arid area, Eastern Sahara, Egypt, GIS-based modeling, Nubian Aquifer System, Recharge modeling.
INTRODUCTION
The sustainable management of water resources especially in the arid regions like eastern Sahara- the most arid
region in the world- constitutes a significant issue today due to severe water deficiency, increased demand on water
and ecological troubles caused by the non-rational utilization of water storages. The problem of water resources is
most critical for the development in this area, as the only available water resource is the groundwater gained from
the Nubian Aquifer System (Fig. 1), which consists of continental and shallow marine sediments with a thickness of
up to 4,500 m and holds an enormous water reserve. The Nubian Aquifer System in the eastern Sahara, considered
the largest groundwater system in Africa, is formed by two major and two minor basins: a) The Kufra Basin, which
comprises the southeastern area of Libya, the northeastern area of Chad and the northwestern corner of Sudan,
b) The Dakhla Basin of Egypt, c) The southernmost strip of the Northwestern basin of Egypt, d) The Sudan
Platform (Klitzsch and Wycisk 1999; Wycisk 1993; Wycisk 1994). The total area is about 2.35 million square kilometers.
In the center and north of the System, where a hyperarid climate prevails, the average precipitation ranges
from 0–5 mm/year. Obviously, there is no groundwater recharge in most parts of the System since thousands
of years. Thus, the aquifer has been under unsteady state conditions for thousands of years before 1960, and the
climatic changes including wet periods, 20,000 and 5,000 years ago, supplied plenty of precipitation to meet
the requirements for adequate local groundwater formation (Ebraheem et al. 2002; Ebraheem et al. 2003; Ebraheem
et al. 2004; El Sayed et al. 2004; Gossel et al. 2004; Heinl and Thorweihe 1993).
In practice, since the beginning of the increasing agricultural development of the Egyptian and Libyan Oases in
1960, there is a common agreement that the aquifer is under an unsteady state condition. The current groundwater
exploitation is approximately ten times the maximum present recharge for this aquifer in the Egyptian part (Heinl
10 – 16 June 2005, Berlin ■ 5th International Symposium ■ AQUIFER RECHARGE ■ ISMAR 2005