Satellite data for Improved Water Quality Products in Mulargia reservoir

Earth observation (EO) imagery gathered from Landsat-8 and Sentinel-2 are operationally used with H2020 SPACE-O project (Space Assisted Water Quality Forecasting Platform for Optimized Decision Making in Water Supply Services) to observe a set of key variables for water quality monitoring in Mulargia reservoir. This serves as drinking water source for the towns of Cagliari and hinterland, and 20 different villages place in the surrounding area (700.000 inhabitants). The products, based on the physics-based Modular Inversion and Processing System image-processing-chain, provide time-series data of the water quality parameters from 2013 to 2017. The water quality obtained was Chlorophyll-a (chl-a), Secchi Disk Depth, Turbidity and some ecological and warning indexes (Trophic Status Index, HAB Probability). Moreover, the TIR Sensor on board of Landsat-8 data was used to investigate the lake surface temperature and evaporation rate. The validation of the products show a good agreement between EO vs in situ data (turbidity R2 = 0.82; chl-a R2 = 0.72), in particular the trend of the results during the years shows the same tendency. For Lake Mulargia, 138 clouds free images produce many information about the water quality: the average chl-a was 10.8 mg m-3, and average turbidity was 5.9 NTU. Our results show that in spring and autumn period the quality of the water is worse. Especially, in concomitance with low water levels and intense precipitation that foster a high run-off of particulate matter from the basin. Furthermore, the results verified a patchy distribution of red cyanobacteria in April and October 2015 with a coverage of 2.6 and 1.4 hectares, respectively. All the products obtained are used in the SPACE-O modules to support the implementation of the Mulargia monitoring as well as for the water forecasting for local populations.