Abstract
Wadi aquifers in Saudi Arabia historically have been recharged primarily by channel loss (infiltration) during floods. Historically, seasonal groundwater levels fluctuated from land surface to about 3 m below the surface. Agricultural irrigation pumping has lowered the water table up to 35 m below the surface. The geology surrounding the fluvial system at Wadi Qidayd consists of pelitic Precambrian rocks that contribute sediments ranging in size from mud to boulders to the alluvium. Sediments within the wadi channel consist of fining upward, downstream-dipping beds, causing channel floodwaters to pass through several sediment sequences, including several mud layers, before it can reach the water table. Investigation of the wadi aquifer using field observation, geological characterization, water-level monitoring, geophysical profiles, and a hypothetical model suggests a critical water level has been reached that affects the recharge of the aquifer. The wetted front can no longer reach the water table due to the water uptake in the wetting process, downstream deflection by the clay layers, and re-emergence of water at the surface with subsequent direct and diffusive evaporative loss, and likely uptake by deep-rooted acacia trees. In many areas of the wadi system, recharge can now occur only along the channel perimeter via fractured rocks that are in direct horizontal hydraulic connection to the permeable beds above and below the water table.
Highlights
In drylands regions, most stream discharges are of an ephemeral nature with no perennial discharges of surface water that are locally produced [1]
A series of field studies were conducted in Wadi Qidayd that is located about 120 km north of Jeddah in western Saudi Arabia (Figure 3)
The geology of the Wadi Qidayd consists primary of schists, and meta-sedimentary rock types that produce a wide variety of sediments including boulders, cobbles, pebbles, gravel, sand, and mud [15]
Summary
Most stream discharges are of an ephemeral nature with no perennial discharges of surface water that are locally produced [1]. Degree of surface water ponding within the wadi channel during an event, aquifer underlying a wadi channel is recharged primarily by infiltration and percolation (3) vertical hydraulic conductivity of the alluvial sediments, degree horizontal stratification coupled with transmission loss (infiltration) in the channel during(4). Depending geology basin in which awith wadimud channel lies, the alluvialseparation sediments between significant quantities ofupon mudthe that formsofathe stacked pattern layers creating constituting the underlying aquifer can consist primarily of boulders, gravel, and sand, or can the high contain hydraulic conductivity gravel beds ([12], Figure 1). A critical position of the water table will be reached whereupon recharge from channel flood loss will no longer occur It is the purpose of this research to explore the issue of anthropogenic lowering of water levels by pumping of wells in a wadi aquifer system that severely reduces or eliminates channel flood loss as a means of recharge. Geophysical measurements, and a theoretical assessment using one-dimensional unsaturated zone modeling were made
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