The impact of geomorphology on groundwater recharge in a semi-arid mountainous area

Abstract

The impact of geomorphological structures on rainwater infiltration and groundwater recharge in the mountainous terrain of a semi-arid region was studied in the southern part of the Judean Western Mountain Aquifer (WMA) in Israel and the Palestinian Authority. The study was conducted in two main typical geomorphological landscape units which include exposed rock outcrops on the hillslope, and human-made agricultural soil terraces in a first-order stream channel. We measured rainwater infiltration through the unsaturated zone in the two geomorphological setups using vadose zone monitoring systems (VMS), that enabled continuous tracking of rainwater percolation and tracer transport through the unsaturated zone with respect to the rain patterns.The results from three years of continuous monitoring of water percolation through the unsaturated zone indicated a quick and intense percolation pattern in the rocky terrain compared to that measured in the deep soil of the terrace. The precipitation patterns, as reflected from the rain distribution and intensities, appeared to have a significant impact on deep water percolation and ultimately on groundwater recharge. In the rocky terrain, rain events of high intensity, even during drought years of low cumulative precipitation, resulted in deep water infiltration, which ultimately turned into groundwater recharge. On the other hand, infiltration through deep soils, as prevailed in terraces, valleys, and high plains, was found to be very limited, subjected to intense evaporation and ultimately result by reduced groundwater recharge. It appears that the value of annual cumulative precipitation is of a lower importance to groundwater recharge while high rain intensity is the most dominant factor responsible for deep water infiltration.Tracer migration patterns in the unsaturated zone revealed that precipitation on the rock outcrops promotes the generation of local runoff that rapidly drains into the subsurface through infiltration hot spots. It has been observed that these hot spots are often marked by prickly burnet shrubs (Sarcopoterium spinosum), which thrive year-round on the water abundancy in the soil pockets beneath the impervious rock on land surface. An important outcome of this study is the suggested strong coupling between geomorphological features and groundwater recharge. Application of variable infiltration functions to the different geomorphological characteristics of each terrain may improve groundwater recharge models. Climate changes in recent years, expected to lead to decreased annual precipitations and increased evaporation in parallel with an increase in rainfall intensities in the semi-arid mountainous areas of the eastern Mediterranean. Yet according to our results, the expected increase in rain intensity may actually result in an increase in groundwater recharge.