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Heavy rainfall sometimes results in flash floods of short duration. River beds are dry for the rest of the time. Part of the surface runoff percolates through the sedimentary layers in the valleys and recharges the groundwater, while some is lost through evaporation. The largest quantity of runoff occurs in the western region, which represents 60 percent of the total runoff although it covers only 10 percent of the total area of the country. The remaining 40 percent of the total runoff occurs in the far south of the western coast (Tahama), which only covers 2 percent of the total area of the country. Total renewable surface water resources have been estimated at 2.2 km3/year, most of which infiltrates to recharge the aquifers. Total renewable groundwater resources have been estimated at 2.2 km3/year and the overlap at 2 km3/year, which brings the total Internal Renewable Water resources (IRWR) to 2.4 km3/year. Total groundwater reserves (including fossil groundwater) have been estimated at about 500 km3 of which 340 km3 are probably abstractable at an acceptable cost in view of the economic conditions of the country.
Groundwater is stored in six major consolidated sedimentary old-age aquifers located in the eastern and central parts of the country. This fossil groundwater, formed some 20 000 years ago, is confined in sand and limestone formations of a thickness of about 300 m at a depth of 150 – 1 500 m. Fossil aquifers contain large quantities of water trapped in fissures. For example, the Saq aquifer in the eastern part of the country extends over 1 200 km northwards. Nevertheless all of these aquifers are poorly recharged (water entered these aquifers thousands of years ago), yet continuously ‘mined’. The natural recharge of these aquifers is only about 3.5 million m3/day, or 1.28 km3/year. These resources are precious as they are not the product of an ongoing hydrological cycle. According to the Water Atlas of Saudi Arabia, these resources are estimated at 253.2 km3 as proven reserves, while the probable and possible reserves of these aquifers are 405 and 705 km3 respectively. In a similar study the Ministry of Planning (MOP) showed that the reserves amount to 338 km3 with secondary reserves reaching 500 km3 (probable). Estimates made by the Scientific Research Institute’s Water Resources Division at Dahran city of 36 000 km3 are more than seventy times higher than the above estimates. However, they estimated 870 km3 as being economically abstractable which is somewhat closer to the above figures. Furthermore, they stressed that with technological advances more amounts could be utilized. An engineering firm, the Saudi Arabia Engineering Consult, gave an estimate of about 2 175 km3. These studies may indicate that the estimates of the ministries are very conservative (Al-Mogrin, 2001). In total, an estimated 394 million m3/year flow from aquifers from Saudi Arabia to Jordan (180), Bahrain (112), Iraq (80), Kuwait (20), and Qatar (2).
In 2004, there were approximately 223 dams of various sizes for flood control, groundwater recharge and irrigation, with a collective storage capacity of 835.6 million m3. A major dam, the King Fahd dam in Bisha in the southwest with a capacity of 325 million m3, was built in 1997 and there are plans to build another 17 dams.
Saudi Arabia is the largest producer of desalinated water from the sea. In 2004 there were 30 desalination and power plants. There were 24 plants on the west coast and six on the east coast. In 2006, 1.03 km3 of desalinated water were produced (Table 2). The water produced is used for municipal purposes. The quantities produced cover some 48 percent of municipal uses. In fact, the desalinated water produced is sometimes exported to distant cities. For instance, in 2004 some 528 million m3 were produced on the western coast of which over 50 percent was exported to the city of Jiddah, while 536 million m3 were produced on the eastern coast, of which over 65 percent was exported to the city of Riyadh, which is located in the centre of the country at about 400 km from the sea on both sides. The total length of pipelines used for the transmission of desalinated water is about 4 156 km. The capacity of desalinated water reservoirs amounted to 9.38 million m3.
In 2002 total treated wastewater reached almost 548 million m3, of which 123 million m3 were reused. In 2003 70 sewage treatment plants were in operation. The use of treated wastewater is still limited at present (166 million m3 in 2006), but it represents a potentially important source of water for irrigation and other uses.