Subsurface storage drives drought propagation and recovery across climates and catchment properties

Extensive knowledge of hydrological processes occurring during droughts is required for a sustainable water resources management, especially in a changing climate. Large-sample analyses are particularly informative in this sense, because they allow us to extend the understanding beyond specific catchments. Data from experimental catchments and observatories showed that water stored within the catchment can sustain evapotranspiration and discharge during dry periods and previous multi-catchments studies on droughts highlighted the storage control on hydrological drought characteristics, through the quantification of hydrological signatures and catchment properties. However, few studies have explicitly quantified across different climates and catchment types the contribution of subsurface storage changes (ΔS) in the annual water balance, in drought propagation (from the meteorological to the hydrological one), and in drought recovery. Here, we assembled a dataset blending ground-based precipitation and discharge data, and remote-sensed actual evapotranspiration data to study drought propagation and recovery in a water-balance and data-based perspective for 102 catchments across various climatic and morphological properties in Italy. This region experienced severe drought years over the study period (hydrological years 2010 - 2019), as detected by the Standardised Precipitation Index for an accumulation period of 12 months. This large-sample analysis revealed that (i) subsurface storage is a non-negligible term in the annual water balance, as ΔS mean annual value represents on average the 11% of precipitation across the catchments, (ii) its depletion sustains discharge during drought years (median annual ΔS anomaly equal to -97 mm for catchments attenuating the hydrological drought with respect to the meteorological one), and (iii) it recovers from precipitation deficits over shorter time scales than evapotranspiration, but similar as those of discharge. These findings emphasize the need of explicitly considering subsurface storage in drought analyses to properly inform policy makers and water managers, as it is a key driver in drought propagation and recovery across climates and catchment properties.