Int. J. Environ. Res. Public Health 2010, 7 2444 Figure 5. The correlation coefficients between first-derivative of reflectance and Chl-a. It is shown in Figure 4 that various bands with higher absolute correlation coefficients exist between first-derivative reflectance and Chl-a concentration. The number of correlation coefficients greater than 0.8 is more than that of the single-band analysis result. The strongest correlation appears for the bands at 446.9 nm, 793.4 nm, 819.4 nm and 868.1 nm, with correlation coefficients of −0.929, −0.902, 0.902, and 0.906, respectively, indicating that the optimal bands for Chl-a concentration estimation by first-derivative algorithm are adjacent to the blue and NIR region. The absolute value of the correlation coefficient for the band near 446.9 nm is the largest, consistent with the conclusions concerning the absorption valley caused by Chl-a absorption [34]. 3.2.3 Reflectance ratio VS Chl-a concentration The reflectance ratio of characteristic bands is widely used to estimate Chl-a concentration. The reflectance ratio of the peak and valley spectra near 700 nm or 570 nm was applied in various studies. To validate the feasibility of the algorithm on the reflectance ratio of characteristic bands for Chl-a concentration estimation in Tangxun Lake, two combinations of R705.2/R682.1 and R705.2/R572.4 were applied in linear modeling. The results of linear regression models with the two combinations are shown in Figure 6.