DYNAMICS OF INFILTRATION FROM RESERVOIRS IN ARID LANDS

Groundwater is a fundamental source of water in arid environments. It is increasingly exploited through movement into marginal lands due to population growth and agricultural requirements. Hence, evaluation of the water potential in such environments requires improved understanding of floodwater infiltration and groundwater recharge mechanisms. Though groundwater is considered a reliable water resource, it is recharged through floodwater infiltration – an unreliable entity when considering the high temporal and spatial variability of precipitation in arid environments. A common method applied in arid environments to increase floodwater infiltration is percolation reservoirs. These are designed to capture floodwater and increase the residence time, facilitating greater infiltration to the local alluvial aquifers. In recent years, the infiltration efficiency of percolation reservoirs versus undisturbed natural stream beds has been questioned.

This study compares the infiltration mechanisms during and after a natural flood event in a percolation reservoir and the associated ephemeral stream channel. Monitoring stations were established in the upstream ephemeral channel and in the percolation reservoir in Tzukim Reservoir, Arava Valley, Israel. Deep infiltration through the vadose zone of each site was monitored continuously using the Vadose Zone Monitoring System (VMS). The monitoring system hosts flexible time domain reflectometry probes (FTDR) and pore water sampling ports (VSP) installed in a slanted borehole. The VMS allowed continuous monitoring of the vadose zone water content variation and frequent sampling of the vadose zone pore water for chemical analysis. In addition a tracer experiment was conducted in the reservoir to further enhance understanding of the infiltration process.

Analysis of the data shows that the vadose zone and groundwater responded quickly to the natural flood event in the ephemeral stream channel. Initially the percolation reservoir displayed similar results, however, a decrease in percolation rate within the reservoir was promptly observed. This is due mainly to the clay deposits that occur in the reservoir. Consequently, a substantial amount of the captured water is lost to evaporation and does not enter the groundwater storage as per the design.