From minutes to millennia, from plots to planet: A spatiotemporal view on groundwater recharge

Groundwater recharge is a key yet highly uncertain component of the hydrological cycle and is characterized by pronounced variability across temporal and spatial scales. Recharge processes are commonly represented using long-term averages and spatially aggregated concepts, by which the episodic, heterogeneous, and scale-dependent nature of recharge is often obscured. In this presentation, a multiscale perspective on groundwater recharge is presented, covering field observations, regional analyses, and global modeling approaches to illustrate how recharge occurs on timescales ranging from minutes to millennia and from local plots to the planetary scale.

The temporal dimension of recharge is illustrated using examples from arid environments, with a focus on Saudi Arabia. Under current climatic conditions, diffuse groundwater recharge in these regions is typically very low. However, rapid and focused recharge can be generated within minutes during intense rainfall events in areas with exposed karst features, where surface runoff is efficiently channeled into the subsurface. But there are also important processes related to groundwater recharge on much longer timescales. In many arid regions, fossil groundwater is a vital resource that was replenished under past climatic conditions. These paleo-groundwater recharge events represent hydraulic impulses that continue to influence today's groundwater levels and flow patterns, resulting in non-stationary groundwater systems.

The spatial variability of groundwater recharge is examined at local and global scales. Based on exemplary case studies from South Asia, recharge is found to be highly heterogeneous and is controlled by climatic gradients, geological conditions, and intensive human water use, including irrigation return flows. At the global level, groundwater recharge is estimated using neurol networks with eXplainable AI, identifying the dominant factors for groundwater recharge for different climate zones and showing that the control and sensitivity of predictors for groundwater recharge are highly region-specific.

Building on these temporal and spatial perspectives, the implications of groundwater recharge for water quality are highlighted. Using the Hessian Ried (Germany) as an example, a control of recharge rates and residence times in the unsaturated zone on reactive solute transport to groundwater is demonstrated, thereby directly linking recharge dynamics to groundwater quality.

Overall, groundwater recharge is shown to be inherently multiscale and dependent on specific environmental conditions, with important implications for the selection of recharge estimation methods, the temporal and spatial aggregation of groundwater models, and sustainable groundwater management under changing climatic and land-use conditions.