Groundwater recharge assessment in a data-scarce semi-arid volcanic island using evapotranspiration-constrained process-based modelling

Groundwater recharge estimation in semi-arid volcanic islands is critical to water-management; however, it is often hindered by sparse monitoring networks and the lack of observed records suitable for conventional calibration. Within the GENESIS project, this study presents a transferable workflow to quantify groundwater recharge in data-scarce island settings by combining field-based infiltration information with satellite-derived actual evapotranspiration (AET) constraints to refine the catchment water balance. The workflow is demonstrated on El Hierro (Canary Islands, Spain), a small oceanic volcanic island characterized by steep relief, sharp climatic gradients, heterogeneous land cover, and limited hydrometric infrastructure. Model inputs combine multi-source datasets, including daily observed precipitation and temperature complemented by gridded meteorological variables, and locally soil hydraulic properties derived from infiltration tests to represent effective near-surface hydraulic conductivity at the model support scale. A Sobol global sensitivity analysis is applied to identify the most influential parameters controlling AET and soil–aquifer fluxes, supporting a parsimonious calibration design. Calibration proceeded in two stages, combining soft constraints from previously reported hydrological ratios with a hard calibration against island-mean AET aggregated to island resolution to minimize scale-mismatch artifacts. Groundwater recharge is computed as percolation reaching the shallow aquifer within the Soil and Water Assessment Tool (SWAT), and uncertainty is characterized through post-calibration parameter sampling, reported as ensemble ranges given the absence of independent recharge observations. The workflow is designed to be transferable to other data-scarce basins where stream-based calibration is not feasible, while explicitly documenting key assumptions (e.g., omission of horizontal precipitation due to lack of observations).