Revisiting hydraulic response times in regional multilayer groundwater systems: the role of aquitards and surface boundaries

Abstract. Hydraulic response times describe how fast groundwater systems adjust to stresses, such as climatic changes or groundwater abstraction, making them relevant for sustainability, renewability, and vulnerability assessments. These are typically estimated for step changes in boundary conditions using analytical solutions derived from simplified, homogeneous, one-dimensional flow systems, depending on the hydraulic diffusivity and spatial extent of individual hydrogeological units. While unconfined systems are widely recognized to adjust via surface–groundwater interactions, confined systems are often treated as hydraulically isolated from surface boundaries assuming fully impermeable confining units. Here, we revisit these assumptions using a simplified analytical model that represents a multilayer groundwater system as an equivalent homogeneous medium with anisotropic diffusivities in the horizontal and vertical directions, to derive an analytical formulation for its dominant time constant. Different analytical approaches are evaluated against a synthetic numerical model of a regional multilayer groundwater basin subject to step reductions in recharge rates, considering a broad range of hydraulic diffusivities. Results show that one-dimensional formulations can overestimate response times of confined aquifers by several orders of magnitude when the role of aquitards and active surface boundaries is neglected. When inter-layer hydraulic coupling is strong, vertical diffusion through aquitards can significantly accelerate pressure propagation between deeper aquifers and surface boundaries. We introduce a dimensionless criterion comparing horizontal and vertical response timescales to determine whether a multilayer system responds as a hydraulically coupled continuum or as a set of weakly coupled layers. For weakly coupled systems, each layer responds independently and one-dimensional approaches remain appropriate. Conversely, in strongly coupled systems, response times emerge as basin-scale properties generally increasing with depth, and a two-dimensional formulation is needed. These findings delineate the hydrogeological contexts under which different analytical formulations provide reliable order-of-magnitude estimates of hydraulic response times in regional multilayer systems.