Dynamics of Floodwater Infiltration and Groundwater Recharge Under Ephemeral Channels in Arid Regions

Shallow alluvial aquifers underneath ephemeral streams are often the only reliable source of water that can sustain human habitation in arid environments (e.g. Arava Valley, Israel; Rio Andarax, Spain; Kuiseb River, Namibia). The main source of replenishment of these alluvial aquifers is recharge from floodwater infiltration. Accordingly, effective management of surface water and groundwater in arid regions requires a better understanding of the processes controlling floodwater infiltration and recharge of alluvial aquifers. This study focuses on understanding the dynamic process of floodwater infiltration from ephemeral channels while implementing innovative methods specifically designed to quantify the recharge fluxes. The monitoring system provides real-time continuous measurements of the hydraulic conditions in all three domains involved in the recharge process: (a) the flood, (b) water-content variations along the unsaturated profile, (c) the groundwater response to the recharge event. Water-content variations along the unsaturated profile were monitored using flexible TDR (FTDR) probes installed along slanted boreholes underneath the stream channel. Water levels and salinity of both the flood and the groundwater were measured simultaneously. Two study sites were selected for this work: the Buffels River, South Africa and the Kuiseb River, Namibia. The monitoring stations installed at those sites recorded several flood events during 2005/2006. Data collected during this period revealed the dynamic process in which floodwater percolates through the vadose zone and recharges the groundwater. Each flood initiated an infiltration event expressed by wetting of the vadose zone and a rise in the water table. The sequential wetting of the vadose zone allowed direct calculations of the wetting-front propagation velocities and percolation fluxes from land surface down to the groundwater. With the arrival of the wetting front to the water table, groundwater began to rise, indicating an increase in groundwater storage in response to the recharge event. Water fluxes were calculated using several independent methods: (a) combining the calculated wetting-front propagation velocity with the change in moisture profile, (b) the rate at which the water table rises as an indication of the percolation rate, and (c) the final increase in groundwater storage through the measured change in groundwater levels. Interestingly, the calculations performed for all of the floods yielded corresponding flux values of approximately 1 cm/h. Aquifer dimensions, as well as total recharge estimations, were also derived from the data. Salt-transport dynamics at each site and the positive influence of the flood events on groundwater quality were revealed from the EC measurements.