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

Piemonte Plain (upper Po plain), northwestern Italy. 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 Precipitation 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. The aquifer system exhibits a widespread decline over the 2000–2023 period. The analysis of the SPI-SGI correlations shows different results for the irrigation 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 characterization of groundwater response time to precipitation, overcoming the use of a single lag corresponding 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. • Groundwater levels declined in last twenty years in the region (according to monthly trend analysis). • The SPI-SGI correlation describes the propagation of meteorological to groundwater drought. • A novel weighted-lag approach is developed to analyze SPI-SGI relationship. • Current irrigation practices recharge shallow aquifers thus buffering groundwater droughts.