Application of satellite images to monitor river turbidity in the catchment of Shimen reservoir

To face the extreme weather, appropriate and effective water quality management is necessary. The current watersheds monitoring stations around Taiwan are mostly fixed-point measurements, therefore the collected data hardly represent the entire catchment area. Simultaneously, the current data collection frequency of these monitoring stations is only once in a month or even longer, which limited the number of samples. In this study, hydrological model (SWAT) and satellite imagery (SPOT and Landsat series) were used to study water turbidity in the upstream and reservoir areas of Shimen Reservoir. The prediction model of Shimen reservoir water surface area turbidity was established by satellite imagery, and calibration and verification were carried out. SWAT was employed to identify high turbidity events and the satellite image was utilized to estimate turbidities. Discussion on the accuracy of turbidity estimation model is provided. According to the results, the application of satellite images to estimate the turbidity of Shimen reservoir water surface area is highly feasible, with the relative error for approximately 30%. The turbidity estimation in the water surface of reservoir is more accurate than that in the channel water surface area as the area is open, exempt from impacts of surrounding sandbar or vegetation. The accuracy of upstream channel water surface area is limited, for the water depth is shallow and easily affected by the noises due to high reflection. Furthermore, if the monitoring is carried out during the drought period, it is likely to be error-prone as the water level is less than 1 m which results in high reflectivity and a misjudgment of high water turbidity.