Abstract
Alluvial aquifer systems occurring beneath wadis valleys (ephemeral streams) in Saudi Arabia are used to meet the water supply needs of over 8 million people and additionally, supply the irrigation requirements for small farms. Historically, the use of water from these alluvial freshwater aquifers was relatively small and used to supply low populations of Bedouin farmers. In the last two decades pumping of alluvial freshwater aquifers has increased exponentially, resulting in water levels receding to over 25 m below surface. Also, pheatophytes, such as Acacia sp., have been planted in the wadis. A series of groundwater investigations and some one-dimensional unsaturated zone modeling has demonstrated that infiltration during flood events is no longer reaching the water table to recharge the aquifer beneath the channels in many of the wadi systems. The causes of the disrupted recharge are: 1) lowering of water levels has caused in increase in the thickness of the unsaturated zone which requires longer-duration flood events to allow sufficient infiltration to wet the system and allow progressive downward movement of water, 2) vertical unsaturated zone movement of water is diverted along the top of mud layers that occur in fining-toward beds causing re-emergence in downstream areas where the water evaporates at land surface, and 3) the extensive root systems of the Acacia plants removes the infiltrated water before it can reach the saturated zone. The only recharge that can occur is along the perimeter of the wadi systems where fractures in the surrounding rock intersect with the permeable parts of the aquifer. This recharge rate is quite small compared to normal channel-loss infiltration. Over-pumping and the lower rate of recharge have combined to create a non-sustainable use of the only available source of freshwater in vast Middle East regions. Re-establishing sustainability of the wadi aquifer resources can be achieved by reducing pumping and creating new artificial recharge schemes, such as stormwater or domestic ASR systems. The only other means supplying freshwater would be to desalinate seawater and pump it to the users. The economic impact of using desalinated seawater for supplying small population villages and low yield crops is economically unsustainable.