Evaluation of opportunities to characterize and monitor moisture in the unsaturated zone above the Western Mountain Aquifer

The GRaCCE project (Groundwater Recharge and Climate Change Effects - Quantification of resilience of water resources in carbonate aquifers to drought conditions) aims to develop process-based integrated and data-driven surrogate methods for determining groundwater recharge and predicting droughts in order to support water management in semi-arid regions such as Israel, Palestine and Jordan. Previous studies have shown that the thick vadose zones (several hundred meters) prevalent in the region can be relevant for water management as long-term reservoirs and, if considered as a dynamic water resource, can contribute to mitigating supply shortages during long-term droughts. In order to evaluate this water resource, it is necessary to characterize and monitor the moisture distribution in the vadose zone. In principle, borehole- and surface-based geophysical methods as well as remote sensing data can be used for this purpose. In order to assess the possibilities of the various methods for the specific site conditions of the Western Mountain Aquifer, the water balance of the area was investigated for the period from 1950 to 2020 using a double permeability variably saturated HydroGeoSphere model. Moreover, the distribution of soil moisture content at four intervals up to a cumulative depth of two-meter was inspected utilizing FLDAS2 NASA daily dataset. The temporal development of vertical moisture profiles was extracted from the HydroGeoSphere and FLDAS2 models for some selected locations. The profiles show a strong “intra-annual variation” at soil level which is strongly dampened by a depth two meter. This variability is generally not observed in higher depth profiles, as generated by HydroGeoSphere, where the shift from wet to dry periods made some “inter-annual variation” of moisture content. This result further supports the former studies claiming the importance of vadose zone on regulation of drought periods at aquifer level. Moreover, based on this assessment, a site-specific initial assessment of the suitability of the measurement methods such as cosmic ray neutron sensing, ground penetrating radar, resistivity measurements, nuclear magnetic resonance and remote sensing was carried out.