Beyond climatic-driven groundwater drought: extracting anthropogenic signatures fromstandardized groundwater indices

Groundwater abstraction can substantially reshape how climatic drought propagates into aquifer storage in semiarid Mediterranean settings. Here we propose an attribution framework to separate hydroclimatic and anthropogenic controls on standardized groundwater anomalies in two hydraulically connected aquifers of Spain’s Ebro Basin: the Plio-Quaternary of Alfamén and the Miocene of Campo de Cariñena.

We first reconstruct temporally continuous groundwater-level series for 1980–2025 using transfer-function noise (TFN) models in Pastas. Models are driven by daily precipitation and Penman–Monteith potential evapotranspiration, and include reconstructed monthly abstraction stresses. From these reconstructions we compute monthly Standardized Groundwater Indices (SGI) under current, pumped conditions and compare them to multiscale Standardized Precipitation–Evapotranspiration Index (SPEI) to quantify climate–groundwater coupling and identify spatial response types. We then isolate the effect of abstraction by building counterfactual “no-pumping” simulations through linear decomposition of calibrated TFN models and removal of pumping contributions, enabling within-piezometer comparisons against a reference-consistent baseline. Focusing on 2010–2025, we evaluate how abstractions alters anomalies beyond frequency using an SGI < −1 threshold, including month-level reclassification, event structure, peak timing, exceedance probabilities, and the instantaneous abstraction effect defined as ΔSGI = SGI_pumped − SGI_nopump.

Under pumping, climate–groundwater coupling strengthens monotonically with climatic accumulation, mean SGI–SPEI correlations increase from ~0.07–0.10 at 1-month SPEI to ~0.43 (Alfamén) and ~0.52 (Cariñena) at 48-month SPEI scales. Long-window coupling and response types show coherent spatial organization across intensively cultivated areas, particularly along the valley floor and lower piedmont. Persistent SGI declines under pumping concentrate in the central parts of both aquifers, broadly coinciding with irrigation hotspots, whereas piezometers near aquifer margins more often exhibit transient or non-significant declines. A key exception occurs in the shallow Plio-Quaternary of Alfamén near ephemeral streams, where episodic focused infiltration can temporarily offset local drawdown. Removing abstraction fundamentally shifts the apparent drought timescale. SGI without pumping shows no declining trends and aligns most strongly with annual climate variability (around SPEI12), with correlation peaks up to ~0.8 and network means near ~0.45 in both aquifers, indicating that observed downward SGI trends largely reflect externally imposed abstraction.

Counterfactual diagnostics reveal temporal reorganization. Pumping produces longer, more persistent anomalies episodes and seasonally biased onsets (late autumn/early winter, plus a June onset cluster in Cariñena), while peak timing of the events remains partly climate-governed. Exceedance probabilities of crossing SGI < −1 are higher at every monitoring point under pumping; the largest increases appear in central sectors, where sustained pumping and thicker saturated zones amplify cumulative stress on storage, but elevated likelihoods and ΔSGI also extend beyond the main abstraction hotspots into areas without raw drawdown signals. Over the monitoring network, we observe that pumping increases the likelihood and persistence of moderate groundwater anomalies, delays recovery, and lengthens the effective memory of the system, implying that SGI derived from observed heads in heavily exploited aquifers reflects a compound climate–management signal and should be complemented with counterfactual baselines and month-resolved persistence metrics for attribution and management.