Shallow Groundwater Dynamics in Urban Environments: The Crucial Role of Capillary Forces

In urban areas, a high groundwater table can cause problems such as groundwater flooding, unintended infiltration into sewer and drainage systems, and infrastructure damage. These challenges are further intensified by climate change, underscoring the need for improved water management in low-elevation urban areas. Developing solutions depends on a detailed understanding of groundwater‑level dynamics, which constitute the core of this research.Recent research based on urban groundwater monitoring sites in Denmark has shown that shallow groundwater varies seasonally far more than deeper groundwater. Furthermore, it is shown that shallow groundwater levels may abruptly rise by 50–150 cm as an immediate response to rainfall. The explanation for the extreme dynamics is identified as the capillary fringe zone, which becomes fully saturated in response to infiltrating rain. In the capillary fringe zone just above the normal groundwater level, the capillary forces are stronger than the gravitational forces, leaving this transition zone nearly saturated. Under the impact of infiltrating rain, the saturation of the capillary fringe zone results in a change in pore water pressure from negative to positive and thus an observable change in the groundwater table.In areas with shallow groundwater, the magnitude of these significant groundwater level increments can be critical for triggering groundwater flooding or unintended infiltration into sewer systems and drainage infrastructure. Moreover, observations have shown that in clayey soil types, the groundwater level can remain elevated for days to weeks after rainfall, whereas in sandy soils, the groundwater levels return to their original state much more quickly.Understanding these dynamics requires greater knowledge of infiltration processes, the unsaturated zone, and, in particular, the capillary fringe zone, which in many cases is neglected in the estimation of groundwater level variability.In this work, we present analyses of time series of groundwater head, soil moisture, and rainfall, and link observed rainfall-response to physical and hydraulic soil properties. Furthermore, we propose the development of a one-dimensional modelling concept of the vadose zone based on the Darcy flow equation integrated with respect to soil depth and combined with an explicit finite-difference solution of the continuity equation. The model is demonstrated to simulate the dynamics of groundwater head as a response to rainfall for different soil types. Accordingly, the study aims to model potential groundwater variability as a function of multiple soil‑properties, providing a basis for subsequent risk assessment related to shallow groundwater in urban areas.