In arid lands, the coarse porous materials of the beds of ephemeral streams (locally called Wadis in Oman) are conduits for the freshwater that infiltrates-exfiltrates through these beds during irregular rainfall events. The percolating water through the wadi bed forms perched or localized aquifers that severs as a source of water supply for the scattered oases and settlements along the Wadis in arid areas. During long dry spells, the floating wadi aquifers keep flowing for 2-4 months after the rainfall event. The water table of the regional aquifers beneath wadi bed is often located at >10 m below the wadi bed. In this study, we hypothesized that the porous media of the wadi aquifer act as an intricate hydrologic reservoir, which sustains the surface flow. In a selected wadi, we monitored the Hortonian flow pattern and measured the total dissolved solids (TDS) of surface water samples at different locations. Besides, the water table levels in nearby wells tapping the regional aquifer, rainfall, and wadi discharge data were analyzed. The analysis of the results showed there is a limited hydraulic connection (during and post flash floods) between the water table variations and surface water in the wadi course. The TDS measurements illustrate an increasing trend downstream of the wadi channel (increased from 600 ppm in the upstream to 1800 ppm in the downstream (the locations are 51.3 km apart). Consequently, a classical "base flow" in a vertical plane perpendicular to the wadi axis is highly unlikely to explain the resilience of the surface flow. We put forward a Toth-type model of U-turning paths of water particles between the surface compartment and a Darcian flow in the hyporheic zone and wadi aquifer. The vertical planes of this flow are perpendicular to ones studied for a losing channel in Al-Shukaili et al. (2019). The in-wadi-aquifer residence time of this water is much longer than that for a normal (Manning-type, Chow, 1959 ) surface flow. High downstream TDS in our wadi is hypothesized to be caused by this Tothian flow topology.
|دورية||American Geophysical Union, Fall Meeting 2020|
|حالة النشر||Published - ديسمبر 1 2020|