FLOODWATER PERCOLATION AND GROUNDWATER RECHARGE IN ARID LANDS

The principal source of water in arid environments around the world is related to transmission loss of floodwater in ephemeral streams. Though floodwater is not considered a sustainable water source, both natural ecosystems and human societies exploit this scarce water source to maintain life. While natural ecosystems have adapted to the natural hydrologic regime, human requires development of efficient scientifically-based methods to exploit the ephemeral availability of water. Flood water is usually exploited through two main practices: (1) pumping the local alluvial aquifers that are occasionally and infrequently replenished by flood water percolating, and (2) damming the stream channel to store large volumes of flood water for direct use or to enhance percolation into the local alluvial aquifers. This study focuses on the critical relations between floodwater and recharge processes of shallow alluvial aquifers from both natural undisturbed ephemeral streams and percolation/storage reservoirs. The experimental setup simultaneously monitors and analyzes all three hydrological domains controlling the recharge process: (1) the flood hydrograph, (2) the deep vadose cross section and (3) the groundwater response to the flood hydrograph. The study implements a new technique for the installation of Time Domain Reflectomtry probes (TDR), to monitor continuously the vadose zone moisture and temperature profiles. These probes were installed in specially drilled slanted boreholes. Water level and EC sensors were installed in piezometers. Flood events impact on groundwater recharge was recorded through three main natural floods, in December 2003, January 2004, October 2004 and in a controlled percolation experiments under ponded conditions that was carried out by May 2004. Preliminary analysis of the data from both controlled experiment and natural floods shows quick response of the vadose zone and groundwater to the flood event in open stream channels. The measured percolation flux rates from the bottom of the stream channel were ~0.03 to ~0.2 m/h. On the other hand a significant decrees in percolation rates was observed under the reservoir due to sealing of the reservoir bottom by silt. Consequently a major portion of the water collected by the reservoir is lost to evaporation and never reaches the groundwater.