Transient response in large groundwater basins: Implications for sustainable management

Regional confined aquifers are becoming increasingly strategic as water pressures intensify worldwide. These aquifers are often found as part of multi-layer systems within extensive groundwater basins. Long response time scales challenge conventional approaches of sustainable management: hydraulic perturbations might propagate slowly and management decisions must be informed by the timing on which the impacts of climatic or anthropogenic stresses fully materialize.

Using a synthetic numerical model of a multi-layer system, we investigate response times across a wide range of hydraulic diffusivity scenarios spanning the diversity of real-world contexts. In particular, we highlight the critical role of leakage through confining units, an often-overlooked process that strongly governs how the groundwater basin adjusts to a given stress. To connect these dynamics with management decisions, we apply a constrained optimization framework to estimate sustainable yields. Here, we define the sustainable yield as the maximum pumping rate that meets specified drawdown or flow constraints within a given planning horizon.

Our findings show that the vertical diffusivity of confining units exerts an important control on response times of confined aquifers and that, in some cases, analytical solutions may significantly overestimate these times by ignoring leakage processes. In low-diffusivity regional systems, response times can span well beyond typical planning horizons, making them relevant for long-term management decisions. Crucially, the choice of planning horizon becomes a determining factor in defining sustainable abstraction volumes. Our approach explicitly quantifies this relationship, providing an informative basis for sustainable management decision-support. 

This work underscores the need for a paradigm shift away from steady-state notions of groundwater sustainability. Effective management of regional groundwater systems requires adaptive strategies that recognize delayed responses and the long-term consequences of our decisions, which may unfold over decades, centuries, or even longer. What is considered sustainable depends not only on the magnitude of pumping impacts, but also on the timing of those impacts relative to human and management timescales.