Inferring groundwater response time at regional scale by following a spectral approach

Groundwater is the main component of river discharge during low-flow periods. The aquifer response to recharge typically depends on the catchment hydro-geological characteristics, such as the hydraulic conductivity, the storage coefficient and the aquifer dimensions. Moreover, such a response time is key to buffer drought propagation during dry periods. As a consequence, it is of paramount importance to evaluate how fast an aquifer will react to an external perturbation. Here, we apply a spectral approach to evaluate the aquifer response time. At the regional scale, the aquifer behaves as a low-pass filter, which modifies the input signal (e.g., the recharge) in the output signal (e.g., the baseflow) according to its properties. For instance, the groundwater response will be faster when the aquifer transmissivity is high or the storage is low. We tested our method across a wide range of German catchments using stream flow datasets. Spectral analysis across catchments of different sizes can provide insight into the spatial aggregation of groundwater response, thereby indicating the scaling rule in large heterogeneous catchments. This approach can help to evaluate the response time in humid regions, which are characterized by frequent interruptions of recession periods. Moreover, response time can serve to quantify the effect of possible external perturbations (climate, irrigation or land management changes) on aquifer resilience.