Developing algorithm for estimating chlorophyll-a concentration in the Thac Ba Reservoir surface water using Landsat 8 Imagery

Abstract

This study aims at developing a regional algorithm to quantify chlorophyll-a concentration (Chla) in the Thac Ba Reservoir surface-water using Landsat 8 imagery basing on in-situ data of Chla and above-water reflectance taken in both dry and rainy seasons 2018. In situ datasets obtained from 30 water sampling sites show a strong correlation (R² =0.73) with the reflectance ratio of two Landsat 8 (L8) bands, the green band (band 3: B3) versus the red band (band 4: B4), B3/B4, by an exponential equation. The algorithm for estimating Chla using this ratio was well­ matched up the validation using multiple-dates in-situ datasets (R2 = 0.82; RMSE ~ 5 %) and was then applied to L8 images level 2 acquired in both dry and rainy seasons to understand the spatiotemporal distribution of Chla over the reservoir. Obtained maps of Chla present clearly two trends: (1) Chla in the reservoir water in the dry season (averaged at 15.3 mg/m³) is relatively lower than those in the rainy season (averaged at 17.0 mg/m³); (2) In both seasons, Chla increased from water area in front of the Chay River mouth to the center of the reservoir. The algorithm and method outlined in this study could be applied to monitoring Chl in other inland waters having similar features as the Thac Ba Reservoir water.