Advancing estimates of groundwater recharge by integrating multi-sensor observations across the vadose zone in a drying climate

Understanding the impact of climate change on groundwater recharge is crucial for the sustainable management of groundwater systems, especially when regulatory agencies are managing aquifers already fully allocated. Recharge emerges as the outcome of Critical Zone (CZ) processes such as interception, runoff, or plant water uptake that use or store water from rainfall as it traverses the soil-plant-atmosphere continuum. Consequently, recharge is best understood and observed through multiple observations that can characterise storage, potentials and transport of water both in the soil and in the groundwater. Understanding how these CZ processes respond to a variable climate is essential for informing groundwater allocation management and decision-making. We present the results of field observations and a meta-analysis of recharge studies spanning the last 50 years in the Mediterranean climate area around Perth (Australia). This period coincides with a 15% reduction in winter rainfall, with the impacts on recharge partly revealed by the meta-analysis, but confounded by varying observation methods and sites. Seven field observation sites with consistent, multi-sensor instrumentation were therefore established to reveal recharge dynamics and estimate recharge fluxes. Electromagnetic soil moisture sensors provide vertical information across the soil profile (up to 10 meters below ground), complemented with soil water potential sensing at the surface and capillary fringe. ERT observations and manual soil moisture measurements in ancillary access tubes extend this information laterally (i.e. from 1D to 2D).  Groundwater depth, meteorological and remotely sensed information enables contextualisation of the observations.  Historical data analysis shows that rainfall reductions lead to nonlinear (3 to 4 times higher), decreases in recharge. The installed monitoring stations reveal how the dynamics of wetting fronts are influenced not only by the climatic variables but also by the types of vegetation and their response to a drying climate. This suggests the presence of distinct local recharge mechanisms operating within the transient systems of the area. The insights obtained from these monitoring sites can be benchmarked against broader observations, such as data provided by remote sensing or borewell measurements, to generate databases of recharge estimates useful for models.