In terms of surface water, the most important hydrological basin is Sakia El Hamra, which covers an area of 81,000 km².

The rest consists of poorly individualized diffuse networks, due to the almost horizontal topography, infrequent precipitation, and high evaporation.

The hydrogeology of the region is characterized by the presence of two distinct aquifer domains: the sedimentary domain, which includes the majority of the region’s groundwater, and the crystalline basement domain, which can contain limited but highly valuable water flows for supplying water to the area.

This domain is composed of Precambrian or Paleozoic rocks that occupy the eastern, southeastern, and northern zones of the basin. It lacks widespread aquifers and water circulation is limited to fracture zones and riverbeds. Water is generally encountered at shallow depths with low flow rates and is of good to moderate quality.

This domain occupies the western part of the basin, along the Atlantic Ocean. It covers an area of approximately 110,000 km² on the continent and extends under the sea. It is relatively rich in widespread underground water reservoirs. The scarcity of precipitation results in little or no recharge of the aquifers.

Isotopic analyses confirm that the majority of groundwater infiltrated more than 5,000 years ago, and therefore, these aquifers are considered fossil.