Diagnosing catchment hydrological memory in the Po, Arno, Ticino and Adda basins (north-central Italy) through a multi-scale standardized drought index

Study region North-central Italy: the Po, Arno, Ticino and Adda basins, spanning Mediterranean pluvial, snow-dominated Alpine and lake-regulated regimes. Study focus Drought propagation depends on how catchments integrate precipitation anomalies across accumulation timescales. Existing approaches summarize this memory with a single accumulation scale, or with transfer functions that are hard to compare across basins and seasons. We extend the D (SPI) index to test whether alternative attenuation patterns of precipitation memory can be distinguished against standardized streamflow anomalies (SQI1), characterizing the extent, seasonality and uncertainty of catchment memory. Classical transfer-function models serve as benchmarks. New hydrological insights for the region The Po and Adda show stable precipitation-streamflow relationships through the year, whereas the Arno and Ticino show stronger seasonality and periods of weak coupling. Alternative attenuation patterns, each at its own accumulation window K , often explain streamflow equally well, so neither the pattern nor K is identifiable alone. The data do constrain the effective memory (mean age of the precipitation months driving the index), from 0.4 to 1.5 months in the Arno to 1.9–4.5 months in the Adda. Benchmarks and D (SPI) reach comparable explanatory power and weight the same recent months. At monthly resolution, memory is better characterized by its effective extent than by a unique attenuation pattern. Results are in-sample; the Ticino record is short (1964–1994), and the Ticino and Adda are Po tributaries.