Antecedent groundwater depletion amplifies hydrological drought severity: aquifer-river connectivity and cascading recovery in the 2022 Po Plain extreme event

Abstract. The 2022 Po River drought produced a 78 % reduction in annual discharge at Pontelagoscuro despite a precipitation deficit of only 20–25 %, a discrepancy not attributable to meteorological anomalies alone. Using GRACE-FO groundwater storage anomalies (GWSA), ERA5-Land soil moisture fields, and a 25-year daily discharge record, we show that antecedent aquifer depletion altered aquife-river connectivity, amplifying the hydrological response beyond what meteorological forcing alone would predict.

GWSA had reached −149 mm by August 2021, more than a year before the drought peak. During the 2022 irrigation season, groundwater abstraction substantially exceeded recharge once Alpine-sourced surface water became unavailable, producing the largest single-season storage loss in the 22-year satellite record (−246 mm, Z = −2.32). The recession coefficient k increased by 90.7 % between 2021 and 2022, exceeding the storage-dependent trend by 90.5 % and consistent with a threshold transition in connectivity near GWSA = −150 mm, a level corresponding to roughly the lowest quintile of the 2002–2024 August GWSA record. Direct runoff collapsed by 72 %, leaving baseflow as the dominant hydrograph component.

Recovery proceeded in three tiers: meteorological indices within 8 months, river discharge within 12 months, and soil moisture within 14 months; GWSA had not returned to historically normal levels within the observational record, leaving a period of persistent hydrological drought of at least 7 months undetected by standard precipitation-based indices. Antecedent groundwater depletion thus alters river discharge sensitivity to meteorological forcing, with recovery in groundwater-dependent irrigated basins extending well beyond the meteorological signal. We propose a diagnostic framework based on GWSA, k, and BFI as an early-warning tool for data-sparse irrigated basins.