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Lakes are a fundamental component of the environment and the territory and represent a precious source of fresh water for various uses. The area of the Prealps north of the Po valley in Italy is characterized by the presence of lakes which represent almost 80% of the total volume of fresh water in Italy (Rogora et al., 2018). The Insubric lakes (Lugano, Maggiore and Como) have their shared basins between Italy and Switzerland, and they are the objective of the SIMILE project, a cross-border Italian-Swiss project that aims to improve their coordinated management and strengthen stakeholder participation in the processes of knowledge and monitoring of water resources (Brovelli et al., 2019) by analyzing data acquired from in-situ to satellite sensors. The present work refers to data collected by remote sensing methods which offer the possibility to obtain synoptic views of water bodies to monitor water quality parameters (WQPs) such as the chlorophyll-a (Chl-a), the total suspended matter (TSM) and the lake surface water temperature (LSWT) (Giardino et al., 2013). This work presents an extensive evaluation of the space-time trends of the parameters based on the SIMILE remote sensing database.

This paper describes the semi-automated procedure implemented for the production of Water Quality Parameters (WQP) maps obtained processing Sentinel-3 and Landsat-8 imagery in the framework of SIMILE Interreg project. The processing chain includes the use of the C2RCC processor to obtain Chl-a (Chlorophyll-a) and TSM (Total Suspended Matter) and the Barsi method to produce maps of water surface temperature. The maps were filtered to exclude anomalous values due for example to clouds, water reflection (such as glint), or mixed pixels and compared to in-situ data. The filtering included an outlier rejection performed with the 3σ rule. The values singled out as local anomalies where checked with respect to possible local behaviours, such as the presence of very small gulfs and inflow/outflow streams and providing guidelines with visual examples, to support the operator. The idea of a procedure as much as possible automated and guided is to foster the WQP maps production after the end of SIMILE project.

SIMILE is an INTERREG Italy-Switzerland project that aims to preserve water quality of the subalpine lakes Como, Lugano and Maggiore (Northern Italy), through an integrated innovative monitoring system. For this purpose, satellite images are processed to map and monitor Chlorophyll-a (CHL-a), Total Suspended Solids (TSM) and Lake Water Surface Temperature (LWST). This study combines these remotely sensed water quality parameters (WPQs) maps, produced for the SIMILE project during 2019–2020, to propose and discuss a zonation approach that can support the monitoring of the study lakes through the analysis of spatial and temporal dynamics of the selected parameters. The approach consists in performing a cluster analysis on a combined sample of WQPs maps, on a monthly basis, for each lake; then the different lake clusters are compared over time, through time series analysis of the WQPs patterns. Finally, the clusters patterns are aggregated over time to map the lakes’ areas that have experienced higher or lower WQPs values during 2019–2020. The results show a high spatial variability for the lakes under study, both during the different seasons and years; a North-South gradient has been identified for all WQPs pattern, requiring for further investigation.