العيون المائية و علاقتها بأوديه الحوض الأوسط لوادي السهباء

Abstract EN

The southern area of Riyadh city and Alkharj plain have many sinkholes in the form of pits and caves.

The area forms the upper catchment of Wadi Al-Sahba.

Alkharj plain is the start of the main channel of Wadi Alsahba which receives water from 11 major wadi systems with the mean annual runoff is estimated at 57.5x106 m3/annum.

There are four sinkholes (pits and caves) locations: firstly, the area at Jabal Al-Jubail which have two sinkholes, Heet cave and Jubail nearby Wadi AlSulyi.

Secondly, the area at Jabal Farzan which has two sinkholes, north Ain and South Ain nearby Wadi Nisah.

Thirdly, , the area at Jabal Qusa'iah which has three sinkholes, Ain Dhila, Semha, and Umm Khisa nearby Wadi AlKharj.

Fourthly, the area at Jabal Khafs which has two sinkholes, north Ain and South Ain nearby Wadi Aqaimi.

Samples collected from Ain AlDhali and Heet showed contains a little tritium (0±2 T.U.) which is caused by infiltration through the wadi surface.

The occurrence of these pits formed by the solution of anhydrite from the natural limestone and containing vast quantities of water at the Arab, Sulai and Heet formations.

These aquifers are confined because the water is flowing toward the sinkholes.

Chemically, the Firzan water is the highest quality water amongst those sinkholes.

The mechanical water pumped for agricultural usage from these pits can be divided into two periods: the first period is from 1938-1980, where the water level is lowered from 6m to 18m (12m difference) within 42years.

The second period is from 1980-1995, where the water level is lowered from 18m to 100m (82m difference) within 15years.

The dramatic drop in water level in second period can be ascribed to the water is depleted by expansions of wheat farms.The twin pits of Ain Dhila and Ain Semha are connected by underground caverns is shown by the fact that when one sinkhole is pumped, the water level in the other shows a corresponding decrease.