Mapping spatio-temporal dynamics of main water parameters and understanding their relationships with driving factors using GF-1 images in a clear reservoir

Due to eutrophication and water quality deterioration in clear reservoirs, it is necessary to monitor and manage the main water parameters: concentration of total phosphorus (C-TP), chemical oxygen demand (C-COD), chlorophyll-a (C-Chla), total suspended matter (C-TSM), and Secchi disk depth (SDD). Five random forest (RF) models are developed to estimate these parameters in Xin'anjiang Reservoir, which is a clear drinking water resource in Zhejiang, China. Then, the spatio-temporal distributions of the parameters over 7 years (2013-2019) are mapped using GaoFen-1 (GF-1) images and the relationships with driving factors are analyzed. Our study demonstrates that the parameters' distributions exhibited a significant spatio-temporal difference in Xin'anjiang Reservoir. Spatially, relatively high C-TP, C-COD, C-Chla, and C(TSM)but low SDD appear in riverine areas, showing strong evidence of impact from the incoming rivers. Temporally, C(Chla)and C(TSM)reached high values in summer and winter, whereas SDD and C(TP)were higher in the summer and autumn, respectively. In contrast, no significant seasonal variations of C(COD)could be observed. This may be why C(COD)is not sensitive to hydrological or meteorological factors. However, precipitation had a significant impact on C-Chla, C-TP, SDD, and C(TSM)in riverine areas, though these parameters were less sensitive to meteorological factors. Moreover, the geomorphology of the reservoir and anthropogenic interference (e.g., tourism activities) also have a significant impact on the water quality parameters. This study demonstrates that coupling long-term GF-1 images and RF models could provide strong evidence and new insights to understand long-term dynamics in water quality and therefore support the development of corresponding management strategies for freshwater reservoirs.