Monitoring the trophic state of shallow urban lakes using Landsat 8/OLI data: a case study of lakes in Hanoi (Vietnam)
Lakes in the Hanoi urban areas are highly vulnerable to serious eutrophication and algae bloom due to anthropogenic pollution and climate change. This study aims at monitoring the trophic state of lakes in Hanoi by developing an empirical model for directly estimating the trophic state index (TSI) from Landsat 8 (L8) level 2 data, which has been atmospheric corrected by the Land Surface Reflectance Code (LaSRC) algorithm and provided freely by the US Geological Survey (USGS).
Regression analysis of a 138-point data set of in situ TSI measured in 13 lakes in Hanoi on seven dates in the 2015–2020 time period with the simultaneously acquired L8 reflectance data set showed a significant correlation between TSI and L8 spectral ratio of the near-infrared band (band 5) versus the green band (band 3) by a logarithmic equation (the coefficient of determination, R2 = 0.65). Validation results demonstrated that the model was appropriate for estimating TSI in highly trophic waters (the root-mean-square error, RMSE = 6.6).
The model then was applied to six selected L8 images to observe an increasing trend in TSI of 25 lakes in the Hanoi urban area during the 2015–2020 time period. The L8-LaSRC performed better than the Landsat 8 Provisional Aquatic Reflectance Product in providing data for monitoring shallow urban lakes. Our proposed model can be applied to monitor the TSI of worldwide lakes with similar features as lakes in the Hanoi urban areas.
Vinh, Pham Quang; Ha, Nguyen Thi Thu; Thao, Nguyen Thien Phuong; Linh, Nguyen Thuy; Oanh, La Thi; Luong, Phuong Thi; Huyen, Nguyen Thi Thu.