A new method is presented to efficiently estimate daily groundwater level time series at unmonitored sites by linking groundwater dynamics to local hydrogeological system controls. The presented approach is based on the concept of comparative regional analysis, an approach widely used in surface water hydrology, but uncommon in hydrogeology. The method uses regression analysis to estimate cumulative frequency distributions of groundwater levels (groundwater head duration curves (HDC)) at unmonitored locations using physiographic and climatic site descriptors. The HDC is then used to construct a groundwater hydrograph using time series from distance-weighted neighboring monitored (donor) locations. For estimating times series at unmonitored sites, in essence, spatio-temporal interpolation, stepwise multiple linear regression, extreme gradient boosting, and nearest neighbors are compared. The methods were applied to ten-year daily groundwater level time series at 157 sites in alluvial unconfined aquifers in Southern Germany. Models of HDCs were physically plausible and showed that physiographic and climatic controls on groundwater level fluctuations are nonlinear and dynamic, varying in significance from “wet” to “dry” aquifer conditions. Extreme gradient boosting yielded a significantly higher predictive skill than nearest neighbor and multiple linear regression. However, donor site selection is of key importance. The study presents a novel approach for regionalization and infilling of groundwater level time series that also aids conceptual understanding of controls on groundwater dynamics, both central tasks for water resources managers.

Characterization of karst systems, especially the assessment of structure and geometry of conduits along with forecast of state-variables, are essential for groundwater quality/quantity management and implementation/rehabilitation of large-scale engineering projects in karst regions. These objectives can be fully met by utilizing process-based discrete-continuum models, such as MODFLOW-2005 CFPv2, as employed here. However, such tools should be used with the caveat of the potential non-uniqueness of results. This research focuses on the joint-inversion of discharge, water temperature, and solute concentration signatures of Freiheit Spring in Minnesota, USA, in response to a spatiotemporally small-scale hydraulic and transport experiment. Adopting the multi-model concept to address conceptual uncertainty, seven distinctive model variants were considered. Spring hydro-chemo-thermo-graphs for all variants were simultaneously simulated, employing joint-inversion by PEST. Subsequently, calibrated models were compared in terms of calibration performance, parameter uncertainties and reasonableness, as well as forecast capability. Overall, results reveal the reliability of the discrete-continuum flow and transport modeling, even at a spatiotemporally small-scale, on the order of meters and seconds. All conceptualized variants suggest almost identical conduit tracer passage sizes which are close to the flood-pulse method estimates. In addition, the significance of immobile conduit-associated-drainable storages in karst hydrodynamic modeling, which is uniquely provided in our model code, was highlighted. Moreover, it was demonstrated that the spring thermograph and hydrograph carry more information about the aquifer characteristics than the chemograph. However, this last result can be site-specific and depends on the scale of the experiment and the conceptualized variants of the respective hydrological state.