Al-Ahmadi, Masoud Eid; (1992) Hydrogeology of Wadi ad Dawasir Basin, South Saudi Arabia. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Wadi Ad Dawasir is located in the southern part of Saudi Arabia. It is about 600 km south of Riyadh and 350 km north of the city of Najran at the junction of the Riyadh-Najran and Abah road. This Wadi is considered to be important agricultural development because it contains a major water resource in the Wajid Aquifer. The rock types in the area are Precambrian with a cover of sedimentary rocks. Since the Wajid formation which covers most of the area consists mainly of sandstone and poorly consolidated quartz gravel conglomerate, this formation forms an important aquifer known as the Wajid Aquifer from which numerous wells presently take their water. The hydrology of the general area was investigated using the available climatological data and the mean annual rainfall during the period 1983-1986 was calculated and found to range from 30 to 40 mm. The total potential evapotranspiration was determined during this study by several methods and was found to lie between 1755 and 2860 mm at the Assulayyil station. The hydraulic properties of the aquifers were determined from pumping test data using both analytical methods and numerical methods. The transmissivity of the Wajid Aquifer was found to be in the range from about 400 to 2000 m2/d, while the storativity was calculated to vary from 2.3 x 10 -4 to 4.8 x 10 -3 Horizontal and vertical permeability was determinedin the laboratory for a number of samples. In addition, an area was selected and the wells were investigated more closely during a long field trip to the area when the water levels were measured where possible and a piezometric map was constructed. About forty-five water samples were analyzed in the laboratory for major cations and anions, and it was found that there are two types of water in the area according to TDS values; the first with a range between 391 and 1009 mg/1, while the second ranges from 1218 to 8000 mg/1. The ground water quality evaluation shows that the first type could be suitable for drinking and for irrigation purposes, while the latter is not suitable for drinking by humans but can be used by livestock except for the water from wells W2, W3, W7, W8. Nevertheless, all can be used for irrigation purposes. The total amount of water storage in Wajid Aquifer was estimated to be 4.0 x 10 11 cubicmetres. The recharge to the aquifer was impossible to calculate, this is due to the lack of data but is thought to be negligible. On the other hand, an estimate of the quantity of water abstracted from the area was about 4.0 x 10 9 cubic metres per year. Since the number of wells is increasing exponentionallyin the area, numerical ground water modelling was attempted with a PC version of the MODFLOW finite-difference program to simulate the aquifer behaviour and attempt predictions of the head response. The results show that the water level will decline rapidly, and the present confined condition will change to unconfined unless the pumping rate is reduced.

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