RADAR DETECTION OF MACROPHYTE STANDS USING L BAND AND C BAND DATA 
Evlyn M.L.de Moraes Novo 
evlyn@ltid.inpe.br 
Maycira P.F. Costa 
maycira@ltid.inpe.br 
National Institute for Space Research, Brazil. 
Remote Sensing Division 
P.O. Box 515 
12210-970 - Säo José dos Campos - Säo Paulo - Brazil 
Commission VII, Working Group 5 
KEY WORDS: Aquatic Vegetation, SAR digital data; Monitoring. 
ABSTRACT 
This paper describes a study performed to assess the suitability of multiband SAR data for discriminating stands of macrophytes 
with differences in height and density. In this study L band HH JERS-1 SAR and C HH band CCRS SAR 580 data were acquired 
for the Tucuruí reservoir in March 7, 1994 and April 14, 1992, respectively. In spite of the two year difference in data acquisition, 
both sets were obtained during the beginning of the rising water season. Therefore it was assumed that differences in the detection 
of macrophyte stands and dead tree trunks were related to changes in the microwave interaction between macrophyte canopy in the 
L and C band. The SAR data were georeferenced and resampled to 12 m by 12 m resolution. The SAR data were submitted to 
digital processing as follows: image filtering, contrast enhancement to produce a multiband composition in which differences in 
color were related to stand height and density. Aerial photographs were used for selecting macrophyte stand classes and the 
following image variables obtained: average digital number and standard deviation. The digital data for both L and C band were 
normalized against the mean digital number obtained for water surface since absolute calibration data were not available. The 
results show that band C is more sensitive to differences in the macrophyte canopy height and density than L band data. 
1. INTRODUCTION 
Amazon river system is the main source of hydroelectricity for 
the northern Brazil. Because of that, large areas of forest land 
have been converted into large reservoirs. Three of them 
(Tucuruí in Pará state, Balbina in the Amazon state, and 
Samuel, in the Rondonia state) are responsible for the 
conversion of over 5 000? km of forest land into aquatic 
environments. The environmental problems brought by such 
transformations in the natural landscape are discussed at length 
in the literature (Junk and Howard-Willians, 1984, Junk and 
Mello, 1990; Kelman, 1990; Novo and Tundisi, 1994). The 
increase in the amount of aquatic vegetation is one of the most 
widespread environmental impacts of these large tropical 
reservoirs. The spread of aquatic vegetation brings about the 
following threatens to the environment: a) spread of endemic 
diseases; b) deterioration of the water quality; c) increase in 
the flux of gases to the atmosphere; d) disruption of the 
geomorphologic and hydrological balance of the local river 
basin. 
Almost nothing is known about reservoir as sources or sinks of 
the greenhouse gases such as carbon dioxide (CO,) and 
methane (CH4). A better assessment of the role of the 
reservoirs as source of methane to the atmosphere, however, is 
still dependent on the knowledge of the area occupied by the 
macrophyte stands (Ruddy and Harris, 1994). 
TM/Landsat data have been used to map the distribution of 
aquatic vegetation in the amazon region (Abdon and Meyer, 
1990). Aquatic vegetation biomass changes both seasonally and 
from year to year according to the nutrient availability, 
hydrological and hydrodynamic factors, etc. However, cloud 
cover did not allow the monitoring of these changes along the 
year, by using optical sensors. On the other hand, synthetic 
aperture radar (SAR) data are not affected by cloud cover and 
can provide information on the temporal distribution of the 
aquatic vegetation stands. 
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The interaction between the active microwave radiation and 
the earth surface is affected by two sets of variables: the 
variables related to the radiation field and the variables related 
to the target. The variables related to the radiation field are the 
frequency, the incidence angle and the polarization. The 
frequency of the incident wave is a key factor in the 
penetration depth and in the scattering from rough surfaces 
(Ulaby et al, 1986). The penetration depth varies linearly 
with A in the radar sensors spectral region. The penetration 
depth is larger for smaller frequencies. The L-band (24 cm 
wavelength) signal penetrates deeper than the C-band (5 cm 
wavelength) signal. 
The influence of the incidence angle depends on the frequency 
of the microwave source. The small wavelengths are more 
sensitive to changes in the incidence angles. The effect of the 
incidence angle also depends on the polarization of the 
microwave source and the canopy features. For crops and 
shrubs, the L band backscattering is not affected by the 
incidence angle. According to Holmes (1992) an increase in 
incidence angle from 0° to 90° has little effect on the 
penetration depth of HH polarized radiation due to its low 
attenuation in crops. As some groups of macrophytes present 
crop-like canopies, one can assume that, in the L band, HH 
polarization, the incidence angle is not a key factor. 
Several recent studies (Novo et al. , 1993; Novo et al. , 1995; 
Costa, 1996; Noernberg, 1995) have reported the use of C band 
SAR data for mapping different genus of aquatic vegetation in 
the Tucuruí reservoir. According to the authors airborne C 
band SAR images provided good discrimination among those 
genus characterized by differences in height and biomass such 
as Salvinia sp. and Scirpus sp.. The discrimination among 
genus with small differences in height and biomass such as 
Salvinia sp. and Pistia sp. was not good even when 
multipolarization and multiviewing data were used (Costa, 
1995). 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996