Why is the Ratio of Reflectivity Effective for Chlorophyll Estimation in the Lake Water?

Kazuo Oki

doi:10.3390/rs2071722

Abstract

The reasons why it is effective to estimate the chlorophyll-a concentration with the ratio of spectral radiance reflectance at the red light region and near infrared regions were shown in theory using a two-flow model. It was found that all of the backscattering coefficients can consequently be ignored by using the ratio of spectral radiance reflectance, which is the ratio of the upward radiance to the downward irradiance, at the red light and near infrared regions. In other words, the ratio can be expressed by using only absorption coefficients, which are more stable for measurement than backscattering coefficients. In addition, the band selection is crucial for producing the band ratio when the chlorophyll-a concentration is estimated without the effects of backscattering. I conclude that the two wavelengths selected must be close, but one must be within the absorption range of chlorophyll-a, and the other must be outside of the absorption range of chlorophyll-a, in order to accurately estimate the chlorophyll-a concentration.

Highlights

A number of spectral vegetation indices have been developed [1,2]
Spectral measurement was carried out using Lake Kasumigaura as a test site to find out why the ratio of the reflectivity is effective for making a chlorophyll-a estimation
It was found that the chlorophyll-a concentration could be accurately estimated by using the ratio of spectral radiance reflectance at two different wavelengths, that is, with a red light region around 675 nm as the denominator and a near infrared region within the range of 700 nm through 730 nm as the numerator

Summary

Introduction

A number of spectral vegetation indices have been developed [1,2] General vegetation indices such as the Simple Ratio Index (SR) can be calculated using the ratio of spectral reflectance at two different wavelengths, with a red light region as the denominator and a near infrared region as the numerator. It has generally been considered effective to use the wavelength ranges of chlorophyll-a-absorbing light (red light region) and strongly reflected light (near infrared region) in leaves. It has not been clearly explained why the chlorophyll-a concentration in leaves or in bodies of water can be precisely estimated. Lake Kasumigaura in Japan was selected as the test site because a lot of data on the lake, such as the chlorophyll-a concentration and spectral signature, has already been obtained

Radiative Transfer Model at the Water Surface

Measurement of the Spectral Signature

Chlorophyll-a Estimation Using the Ratio of the Reflectivity

Conclusions