The role of precipitation and irrigation on groundwater droughts in the Piemonte Plain, Italy

Study region: Piemonte Plain (upper Po plain), northwestern Italy. Study focus: The relationship between meteorological and groundwater droughts of shallow aquifers is investigated in the Piemonte Plain, characterized by widespread irrigation, mainly supplied by Alpine-fed streamflow distributed through a complex irrigation network. Groundwater-level trends (2000–2023) were analyzed considering also seasonal behaviors. Anomalies in precipitation and groundwater levels were studied through the Standardized Pre- cipitation Index (SPI) and Standardized Groundwater Index (SGI) across multiple time-windows and lags. A correlation-weighted lag was introduced to assess SPI–SGI response times during and outside the irrigation period. A conditional frequency analysis was carried out to study the propagation of meteorological drought into groundwater drought. New hydrological insights for the region: The aquifer system exhibits a widespread decline over the 2000–2023 period. The analysis of the SPI-SGI correlations shows different results for the irri- gation and non-irrigation periods. Irrigation weakens the relationship between precipitation and groundwater levels and the rice-cultivated area, mostly irrigated with flooding, shows the lowest SPI-SGI correlation values. The newly introduced weighted lag allows for a better characteriza- tion of groundwater response time to precipitation, overcoming the use of a single lag corre- sponding to maximum correlation. The propagation of meteorological drought into groundwater drought is disentangled during the irrigation period, mitigating SGI negative values in case of scarce precipitation.