Two groundwater stories, one drought: Standardized Groundwater Index and baseflow proxies under climatic forcing in near-natural aquifers in southern Spain

Groundwater dynamics shape how drought is experienced in landscapes: they regulate the persistence of streamflow, controlling the duration and magnitude of ecological stress linked to low flows, and govern recovery trajectories long after rainfall deficits ease. Despite this, groundwater is often weakly represented in routine drought characterization, largely because piezometric records are sparse, discontinuous, and unevenly distributed, and because groundwater responses are filtered through storage, geology, and time-lagged recharge processes that obscure simple attribution to atmospheric anomalies. Robust and comprehensive drought diagnostics and early warning need methods that link meteorological forcing to interpretable indicators of groundwater storage and release.

The analysis is conducted in near natural headwater catchments in southern Spain, thereby reducing the influence of pumping. We propose a triangulation approach to characterize drought propagation using three complementary components: meteorological drought forcing measured with Standardized Precipitation- Evapotranspiration Index (SPEI), groundwater drought state obtained from piezometric data, measured with the Standardized Groundwater Index (SGI), and groundwater-controlled discharge behaviour captured through a simple baseflow proxy extracted from gauged streamflow and Terraclimate modelled runoff data (Abatzoglou et al., 2018).

Meteorological drought is represented by the SPEI evaluated across accumulation windows from 1 to 48 months. Observed groundwater head series are quality-controlled, filled in and regularised using transfer-function noise timeseries modelling with the Pastas software (Collenteur et al., 2019) to obtain continuous records, from which SGI is computed using a month wise non-parametric standardisation. Baseflow is derived from observed discharge and runoff data using a consistent separation approach, and standardised to enable direct comparison with SGI as a second, catchment-integrated representation of groundwater state.

We explore drought propagation by mapping correlations and response lags between SPEI and both groundwater anomaly indicators, SGI and standardized baseflow, identifying the dominant memory windows and seasonality of sensitivity. Predictive performance is then assessed using regressions for interpretable relationships between groundwater response and the most informative SPEI scales, and Random Forest regression to capture further interactions. We stratify and interpret these relationships by lithology, aquifer properties and catchment size. We further test whether SPEI–groundwater relationships exhibit structural changes over time, via moving-window correlations, wavelet analysis and segmented analyses across sub-periods and seasons.

Across sites, the triangulation reveals coherent but aquifer-dependent propagation patterns, which are presented with narratives and diagrams of drought propagation pathways. SGI and baseflow-based state indicators consistently align with SPEI at intermediate to long accumulation windows, reflecting nuanced modulation in storage and recession dynamics. Importantly, baseflow proxies complement SGI by providing a continuous, integrated signal of groundwater release that can support and strengthen monitoring, especially where piezometric data are sparse. The combined framework delivers operationally relevant SPEI trigger windows and predictive models for anticipating groundwater-related anomalies in Mediterranean environments.

References
Abatzoglou, J. T., Dobrowski, S. Z., Parks, S. A., & Hegewisch, K. C. (2018). TerraClimate, a high-resolution global dataset of monthly climate and climatic water balance from 1958–2015. Scientific data, 5(1), 1-12.
Collenteur, R. A., Bakker, M., Caljé, R., Klop, S. A., & Schaars, F. (2019). Pastas: Open source software for the analysis of groundwater time series. Groundwater, 57(6), 877-885.