Estimation of Chlorophyll Concentration in Lakes and Inland Seas
From Near-infrared and Red Spectral Signature
Kazuo Oki* and Yoshifumi Yasuoka**
National Institute for Environmental Studies,
16-2 Onogawa, Tsukuba, Ibaraki, 305 Japan
K. Tokumura
*Institute of Socio-Economic Planning, University of Tsukuba
**National Institute for Environmental Studies
Contact : Y.Yasuoka National Institute for Environmenal Studies
16-2 Onogawa, Tsukuba, Ibaraki 305, Japan
Tel. +81-298-50-2543, Fax +81-298-51-4731,
E-mail yyasuoka@nies.go.jp
Abstract
A remote sensing method to estimate distribution of rich chlorophyll concentration in takes or inland seas in proposed. First, the basic relationship the chlorophyll concentration and the spectral reflectance of water was investigated. As a result, chlorophyll estimation model was derived using the ratio of spectral reflectance at two different wavelengths of 675 nm (red range) and 700 nm (near-infrared range). It was found that the spectral signature of near infrared range is investigate the behaviour of the proposed model was used in rich chlorophyll water types. Furthermore, the amount of specular reflection from the water surface was assessed based on the spectral signature data measured above and below the water surface. The percentage of specular reflection was evaluated at least 20% of the total radiance at the surface within the range of 400nm. Finally a method to remove the effect of specular reflection at the water surface was investigated for the proposed model. The model for specular reflection was proposed to eliminate its effect and to improve chlorophyll estimation accuracy.
1. Introduction
Environmental degradation in lakes and inland seas has been very serious due to extensive land cultivation or urbanization around area. In particular. Eutrophication has caused massive growth the phytoplankton or zooplankton in water and gives serious physical, social and economic impacts to the area. Measuring chlorophyll contents in the plankton is of significant importance in tackling with the water pollution problem. It is, however, not easy to monitor water quality distribution over a large area periodically based on a conventional water sampling and analysis method. In this papper, a remote sensing method to estimate the distribution of chlorophyll concentration is proposed. Lake Kasumigaura is selected as a test site for the study, and spectral signatures of water, spectral reflectance below and above water surface was measured together with several water quality parameters including chlorophyll, suspended solids and transparency.
Several models has been proposed for the estimation of chlorophyll concentration of lakes and oceans however, most of them utilized only the spectral signatures in visible range (e.g., Gordon et al. 1983; Okami et al. 1982). It was because the water body usually absorbes the near-infrared spectral radiation and there is no reflectance from water in that range. In general, their models don't operate well to estimate rich chlorophyll concentration. In this study, chlorophyll estimation model which is effective in remote sensing of rich chlorophyll water area like lakes or inland seas was proposed.
The effect of specular reflection at the water surface was also one of important problem for estimating chlorophyll concentration by remote sensing. Takashima et al. (1986) developed model in order to remove effect of specular reflection at the water surface based on Cox and Munk model. However, the Takashima model has not been verified, if it would oerate well in rich chlorophyll water area like lakes or inland seas. In this study, the model was verified based on the spectral signature data measured above and below the water surface.
2. Chlorophyll-a Estimation Model
2.1 Radiative Transfer Model at the Water Surface
The spectral reflectance R of radiance of irradiance at the water surface is defined by
Where I, is the up welling spectral radiance from the water surface. E is the downward irradiance onto the water surface from total solar radiation. I is the upwelling spectral radiance just above water surface. I, is the upwelling spectral radiance reflected from water surface due to total solar radiance. E, and E, are the downward irradiance onto the water surface due to direct sun and the diffuse sky light, respectively. In this report, it is assumed that the water body is a lambertian reflector and the angular distribution of radiance in the lower hemisphere of the water surface is uniform for radiance traveling up ward. Then I, can be expressed as
Where t and n are transmittance and refractive index from water to air, respectively. 1(0) is the upwelling spectral radiance just below the water surface. E, (0) is the upwelling spectral irradiance just below the water surface. We cannot, however, measure it directly, therefore, E. (0) was estimated from the upwelling iiradiance at the depth of Zas
Ew(0)=ew(Z) expk.Z (5)
Where k is the extinction conefficient for the upwelling irradiance of the water, expressed as
2.2 Measurement of Spectral Reflectance at Lake Kasumigaura
The upwelling radiance of the Lake Kasumigaura was measured by using spectroradiometer at the water surface and at several depth layers at the each points (Pt1 Pt18). The Pt 10, Pt11 to Pt 16, and Pt17 to Pt18 were measured on 10 September 1993, 22 April 1994 and 27 July 1994, respectively. The measurements covers the wavelength interval 400nm to 850nm with a resolution of 2nm. At the each points, were measured as follows.
- Upwelling spectral radiance from the water surface (I)
- Upwelling spectral irradiance of the underwater at depths of 10 and 40cm
From the water surface (Ew, (0.1), Ew (0.4))
- Spectral irradiance of white board (Es)
The spectral signature of water obtained by averaging over ten times scanning. The upwelling spectral up welling spectral radiance (E
w (0.1), E
w
(0.4)) of the underwater was measured one time (ten times scanning) and upwelling spectral radiance (It) at the water surface was measured three times (thirty times scanning). Average of It, over three times measurements is expressed as It, Water quality parameters such as chlorophyll-a, suspended solids and transparency were also measured at the same point.
