Int. J. Environ. Res. Public Health 2010, 7 bands from 380 nm to 900 nm. The curve of correlation coefficients of 384 bands is shown in Figure 4. Figure 4. The curve of correlation coefficients between reflectance and Chl-a concentration. 2443 It is shown in Figure 3 that reflectance between 380 and 500 nm is positively correlated with Chl-a concentration. A negative correlation occurs when the wavelength is larger than 500 nm. The largest negative coefficient appears between 726.5 nm and 734.4 nm (Table 1). Table 1. The correlation between spectral reflectance and Chl-a concentration from 726.5–734.4 nm. Wavelength (nm) Correlation coefficients 726.5 727.8 −0.80652 −0.82203 729.1 −0.815 730.5 731.8 733.1 734.4 −0.83498 −0.83097 −0.83998 −0.82338 All seven correlation coefficients between reflectance and Chl-a concentration between 726.5 nm and 734.4 nm are larger than 0.8. For Chl-a estimation based on remote sensing in Tangxun Lake, the optimal bands of the single-band model are near-infrared and the most relevant band is around 733.1 nm. 3.2.2. First-derivative of reflectance VS Chl-a concentration The first-derivative of reflectance can be calculated as [30-33]: R(λi ) ' = R(λi+1) − R(λi−1) λi+1 −λi−1 (2) In the formula, R(λ2)′ is first-derivative of reflectance in band λ2; R(λ2+1) and R(λ2-1) are the reflectance of band λ2+1 and band λ2-1, respectively; and λ2+1 and λ2-1 are the band wavelengths. Correlation analysis was conducted between first-derivative of reflectance and Chl-a concentration with Formula 2, and the results are shown in Figure 5.