Gridded versus on site monthly rainfall centennial dataset for climate variability assessment in Campania Region (Southern Italy)

The Mediterranean basin is characterised by significant spatial and temporal precipitation variability and is recognised as one of the major climate change hotspots on a global scale, where recent studies have documented an intensification of the hydrological cycle with reduced seasonal precipitation but increased frequency of extreme events. This inherent variability is further compounded by non-climatic factors related to data availability and quality. Historical hydro-climatic databases are often affected by discontinuities and gaps, while recent monitoring networks provide more accurate observations but shorter records, which are frequently insufficient for robust climate trend analyses. In order to address these limitations, gridded datasets have been developed. However, these products may introduce additional uncertainties and potential distortions into the results.

The present study focuses on the Campania region, in Southern Italy, which is characterised by a complex orography and significant discontinuities in the available historical time series. In the 2000s, the regional hydro-climate monitoring network underwent structural restructuring. New sensors were installed, while the existing ones (historical network) were upgraded with new technology. However, only a few stations retained their original location and characteristics, resulting in a discontinuity between the historical database (1918-1999) and the current one. This discontinuity was overcome by reconstructing a single continuous monthly scale precipitation dataset through geostatistical interpolation on a regular grid with a spatial resolution of 10 × 10 km. This reconstruction enables the analysis of an almost centennial (1918-2023) continuous precipitation time series to investigate long-term variability in regional precipitation regimes, which represents furthermore a key driver to investigate changes in hydrological processes. To evaluate climate trends and the reliability of gridded reconstructions, the non-parametric Mann-Kendall and Sen's tests were applied to precipitation at the annual scale in parallel on both datasets (in situ observations and gridded reconstructions).

The results highlight trends indicative of the complexity documented at the Mediterranean scale. For the historical period (1918-1999), in situ observations reveal a predominantly negative trend, consistent with the pattern observed across the Mediterranean basin during the second half of the 20th century, which may indicate a potential reduction in regional water availability. In contrast, the most recent period reveals an inversion of this tendency, with a prevalence of positive trends. Analysis of the gridded reconstructions also confirms these patterns across most of the region, although some local discrepancies are present. Nevertheless, the majority of detected trends, in both datasets and particularly in the most recent period, are not statistically significant.