POTENTIALS OF VHF SOUNDING RADARS FOR LARGE-SCALE CHARACTERIZATION OF GROUNDWATER MOUNDING UNDER HYPER-ARID CONDITIONS

Groundwater mounding is the localized rise of groundwater table against the regional hydraulic gradient, when a focused recharge rate to an aquifer exceeds the rate at which water is conveyed away from the recharge zone. Groundwater mounds (GM) commonly occur in shallow aquifers under humid conditions by the infiltrating storm water through high conductive vadose zones. However, occurrence of GM in hyper-arid deserts with very low precipitation and infiltration rates is a paradox. Here, we report the occurrence of numerous GM in the Saharan-Arabian Desert from the Nubian Aquifer System in Egypt, Rus Aquifer in Qatar, Umm Er Radhuma Aquifer (UER) in Saudi Arabia and Liwa Aquifer in UAE using head data and hydrological maps. These GM occur in different aquifer types including sandstones, carbonates and sandy aquifers and range in height from few meters in Qatar to > 50 meters in Egypt and range in maximum horizontal extent from 10 km in UER to > 100 km in Liwa Aquifer. Integrated ALOS PALSAR digital elevation models with head data indicate that the majority of these GM are shallow with < 40 meters below the ground surface. A comprehensive understanding of the origin, spatial extent, response to climate variability and subsurface geometry of the GM is impeded by the paucity of wells and the underestimation of the GM in the map production by using different interpolation methods to avoid anomalous groundwater rising and/or using wide contour interval (i.e. typically of 50 m). Integrated isotopic and hydrological data relate the origin of the GM in the Nubian, UER and Rus aquifers to upward vertical recharge from deeper aquifers, yet the origin of GM in Liwa Aquifer is still questionable. Moreover, the subsurface geometry and dynamics of the GM are poorly understood. Future airborne and Earth orbital VHF sounding radar mission’s concepts such as the Orbiting Arid Subsurface and Ice Sheet Sounder (OASIS) that aims at probing shallow aquifers (<100 m deep) under arid conditions (using a 45 MHz center frequency radar sounder with 10 MHz bandwidth), can provides a unique opportunity to perform a large-scale and time-coherent geophysical propping to delineate the GM geometry as well as assessing their origin and dynamics.