Ground-based measurements on 15 Minnesota lakes with wide ranges of optical properties and Landsat TM data from the same lakes were used to evaluate the effect of humic color on satellite-inferred water quality conditions. Color (C 440), as measured by absorbance at 440 nm, causes only small biases in estimates of Secchi disk transparency (SDT) from Landsat TM data, except at very high values (> ∼ 300 chloroplatinate units, CPU). Similarly, when chlorophyll a (chl a) levels are moderate or high (> 10 μg/L), low-to-moderate levels of humic color have only a small influence on the relationship between SDT and chl a concentration, but it has a pronounced influence at high levels of C 440 (e.g., > ∼200 CPU). However, deviations from the general chl a-SDT relationship occur at much lower C 440 values (∼ 60 CPU) when chl a levels are low. Good statistical relationships were found between optical properties of lake water generally associated with algal abundance (SDT, chl a, turbidity) and measured brightness of various Landsat TM bands. The best relationships for chl a (based on R 2 and absence of statistical outliers or lakes with large leverage) were combinations of bands 1, 2, or 4 with the band ratio 1:3 (R 2 = 0.88). Although TM bands 1-4 individually or as simple ratios were poor predictors of C 440, multiple regression analyses between ln(C 440) and combinations of bands 1-4 and band ratios yielded several relationships with R 2 ≥ 0.70, suggesting that C 440 can be estimated with fair reliability from Landsat TM data.
Bibliographical noteFunding Information:
This work was supported in part by NASA RESAC project and a grant from the Minnesota Department of Natural Resources. We thank David Wright, MDNR project officer, for his interest and assistance. KDM was supported in part by a Department of Civil Engineering Sommerfeld Fellowship.
- Humic color
- Secchi disk transparency