The study of hydraulic head changes over time is a common task for groundwater hydrologists. Groundwater signatures are numerical metrics, or statistical aggregates, that quantify the behavior observed in hydraulic head hydrographs. Signatures can be helpful in a number of classical hydrological tasks, such as hydrograph classification, clustering, change detection, and model evaluation, selection, and calibration. Despite the potential benefits of using signatures in groundwater studies, their application has not yet been thoroughly explored. To support research into the application of signatures in groundwater studies, we introduce the new groundwater signatures module from the Pastas software. The signatures module is written in Python, fully tested and documented, and available as open-source software under the MIT license. In this paper, it is shown how the signatures are tested and can be used in practical applications through two examples. In the first example, signatures are used to characterize and cluster monitoring wells in a nationwide monitoring network in Switzerland. In the second example, signatures are used to evaluate how well different groundwater model structures simulate the heads. Future research opportunities involving groundwater signatures are discussed.

A novel time-lapse modelling scheme for airborne electromagnetic (AEM) monitoring data sets is presented, using data from multiple surveys applied to study the hydrorelated evolution of the Bookpurnong floodplain in South Australia. Additionally, it introduces a new wide-ranging approach for this type of study, incorporating new processing, validation and interpretation tools. Time-lapse studies are widespread in the literature but are not commonly applied to model electromagnetic (EM) data, particularly AEM data. This is linked to the challenges of performing overlapping data acquisition with inductive systems. The key features of the present time-lapse scheme include the definition of independent forward and model meshes, essential for considering discrepancies in the location of soundings which arise in multitemporal AEM data acquisition. Moreover, the incorporation of system flight height in the inversion revealed important for achieving satisfactory data fitting and limiting artifact propagation in the time-lapse models. A novel processing workflow for AEM multitemporal data sets is also presented. This has proven important for effectively processing the multitemporal data sets, which presents new challenges in identifying noise coupling arising from the use of different systems across vintages of data, possible variations in acquisition settings operated by different field crews, and changes in subsurface resistivity in the survey area. Results generated from the time-lapse modelling are evaluated with an independent hydrogeological validation (IHV), designed to support the geophysical models validation and interpretation by providing a first-step hydrogeological evaluation. At Bookpurnong, along a sector of the Murray River floodplain, multitemporal AEM surveys were collected in 2015, 2022 and 2024, to study natural and engineered changes in the groundwater system over time. The time-lapse models show significantly smaller variations compared to those determined with individually modelled survey data sets, while delineating sharply bounded changes in resistivity across the floodplain. This highlights the effectiveness of the new time-lapse scheme in minimizing inversion variations typically encountered with independently modelled results affected by larger equivalence issues. Here, AEM models are first compared with resistivity borehole measurements, revealing a close match between the two methodologies and spatial variations in resistivity consistent with a meandering river across the floodplain. These variations are further validated and interpreted using the IHV approach, which revealed a direct correlation between the hydrological stress of the Murray River and the response of shallow aquifers. Additionally, time-lapse geophysical models, combined with a hydrostratigraphic analysis, allow for a direct correlation between shallow and deep hydrogeological responses. We believe that the time-lapse methodology described here can be widely applied to multitemporal studies using AEM data sets, enabling the study of a broad range of natural processes with great accuracy and at the basin scale.

The performance of time series models is assessed using synthetic head series simulated with a numerical model that solves Richards' equation for variably saturated flow. Heads were simulated in a homogeneous unconfined aquifer between two parallel canals; measured daily precipitation and potential evaporation are specified at the land surface and root water uptake is simulated. The head response to a precipitation event is nonlinear and depends on the saturation degree and rainfall before and after the precipitation event while evaporation reduction occurs during summers. Synthetic series were generated for 27 years and three different soil types; the unsaturated zone thickness varies between 0 and >5 m. The synthetic head series were simulated with a linear and nonlinear time series model. Performance of a linear time series model with four parameters, using a scaled Gamma response, gave R² values ranging from 0.67 to 0.96. The nonlinear time series model with five parameters simulates recharge using a root zone reservoir after which the head response to recharge is simulated with a scaled Gamma response function. The nonlinear time series model was able to simulate all synthetic head series very well with R² values above 0.9 for almost all models. The head response of the nonlinear model to a single precipitation event compares well to the response of the variably saturated groundwater model. The provided scripts may be used to simulate synthetic head series for other climates or for systems with additional complexity to assess the performance of other data-driven models.