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Internationale Vereinigung für theoretische undangewandte Limnologie: Verhandlungen

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Delineation of flooded area and floodedvegetation in Balbina Reservoir (Amazonas, Brazil)with synthetic aperture radar

John M. Melack & Yong Wang

To cite this article: John M. Melack & Yong Wang (1998) Delineation of flooded area and floodedvegetation in Balbina Reservoir (Amazonas, Brazil) with synthetic aperture radar, InternationaleVereinigung für theoretische und angewandte Limnologie: Verhandlungen, 26:5, 2374-2377, DOI:10.1080/03680770.1995.11901175

To link to this article: https://doi.org/10.1080/03680770.1995.11901175

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Verh. lnternat. Verein. Limnol. 2374-2377 Stuttgart, J uni 1998

Delineation of flooded area and flooded vegetation in Balbina Reservoir (Amazonas, Brazíl) with synthetic aperture radar

John M. Melack and Yong Wang

Introduction

Throughout the tropics, construction of dams has formed many artificial lakes, sevcral covering large areas (BEADLE 1981, SERRUYA & PoLLINGHER 1983). Typically, thcse reservoirs have short rete.ntion ti~cs and wide fluctuations in water level. Dunng thc ftrst few years aftcr filling, many experience hypolimnetic deoxygenation as drowned vegetation decays, rapid growth of floating macrophytes, and changes m fishcs and other aquatic organisms as habitats shift from riverine to lacustrine. In regions whcre forests are flooded by the rising waters, extensive areas of submerged and emergcnt trees can offer habitat for a rich biota near the surface (PETR 1970) and create a strongly reducing environment at depth.

Since thc damming of the Suriname Rivcr to form Lake Brokopondo in 1964 (HEIDE 1982), several other large hydroelcctric rescrvoirs have been built within tropical forests of South America, including Curuá-Una, Samuel, Tucuruí and Balbina in Brazil QUNK et al. 1981, FEARNSIDE 1989, TuNDISI ct al. 1991, PETRERE & RrBEIRO 1994) and Guri in Vene­zuela (GoNZALEZ et al. 1991). Many more are planncd. Seldom is m ue h of the forest cleared before filling the reservoirs, and extensivc areas of mostly dead, submerged and emergent trees are crcated. De­lineating the area of floodcd forest and its changes through time is of considerable ecological impor­tance and is logistically difficult.

Synthetic aperture radar (SAR) sensors provide imagery especially suitable for examination of inun­dation and of aquatic vegetation in the humid tropics (MELACK et al. 1994, HEss et al. 1995). SARs can ac­quire data day or night and through cloud cover; their longer wavelengths can penetrate canopies and detect flooding beneath forests or aquatic macro­phytes. Smooth water surfaces produce strong spec­ular reflections to the side-looking SAR. Fundamen­tais of SAR as an ccological technique are reviewed in HESS & MELACK (1994) and KASISCHKE & MELACK (1995).

Study site and methods

Balbina Reservoir was formed behind the Balbina hydroelectric dam (closed in Octobcr 1987), built on

the Uatumã River to supply power to Manaus, Ama­zonas, Brazil (FEARNSIDE 1989). Average depth of the reservoir, when full, is estimated to be 7.4 m, and hy­draulic residencc time is about 14 months. Before closure of the dam, abou t SO km2 near the dam was cleared of vegetation; the remainder of an estimated flooded area o f 2360 km2 w as left forested. The actual area flooded an d the extent o f possible aquatic ha b i­tats, i.c. open water, flooded forest or aquatic macro­phytes, remains a debated issue.

Japan's Earth Resources Satellite QERS-1) was launchcd in February 1992 into a sun-synchronous, ncar circular and near polar orbit. On-board the sat­ellite are an optical sensor system and a synthctic aperture radar systcm. The SAR is an L-band (1275 MHz) HH (horizontal transmission and horizontal recciving) system operating at an incidencc angle of 35". Thc ground resolution of JERS-1 imagery is 18 m, and its swath width is 75 km (NrsHIDAI 1993).

JERS-1 SAR imagery, provided from the National Aerospacc Dcvelopment Agency of Japan, was ob­taincd from the Jet Propulsion Laboratory QPL). The imagery, acquired in October 1993, was re­sampled from 18 m by 18 m to 12.5 m by 12.5 m (full resolution), and calibrated by JPL (A. FREEMAN pers. comm., 1994). Using algorithms provided by M. AL­VES at JPL (pers. comm., 1994), we converted pixels from digital numbers to radar backscatter coeffi­cients, and uscd the backscatter images in the analy­ses. We averaged the backscatter coefficient images with a 4 by 4 window to reduce spcckle and process­ingtime.

A decision tree model (CLARK & PREGIBON 1992) was used to classify the SAR images ona pixel by pi­xel basis into four classes: water and river, aquatic macrophytes, flooded forests, and upland forests. Decision tree classifiers are hierarchical, multistage classifiers. The dccision rules are determined from training data within thc images. Because a singlc vari­able (L-band, HH backscattering coefficient) was used hcre, the result is analogous to density-slicing the image, using the decision tree model to optimally locate the thresholds between classcs. Ecological conditions in the reservoir were based on observa­tions in the vicinity of the dam made by l. Walker and M. V. B. QuEIROZ (Instituto Nacional de Pes­quisas da Amazonia (INPA), Manaus, Brazil), and

0368-0770/98/0026-2374 $ 1.00 © 1998 E. Schweizerbart'schc Verlagsbuchhandlung, D-70176 Stuttgart

J. M. Melack & Y. Wang, Delineation of flooded area 2375

Fig. 1. Balbina Reservoir made from a mosaic of JERS-1 SAR images (Oct. 1993). Arrow points north, and se al e har d en o tes 20 k m.

observations in the western region of the reservoir by B. NELSON (INPA). Especially noteworthy ob­servations are that almost all the trees standing in the reservoir were dead and many had lost most branch­es. Considerable area among the islands and flooded trees near the dam was covered by floating mats of Eichhornia in the early 1990s. Hills in the vicinity of the reservoir rise abou t 100 m above the watcr.

Results and discussion

Four JERS-1 SAR images cover almost the whole Balbina Reservoir; their image identifica­tions are 414-303, 414-304, 415-303 and 416-303, where 414, 415 or 416 indicate the number of the satellite's orbit. Scenes 414-303 and 304 were acquired on l October, scene 415-303 on 2 October, and scene 416-303 on 3 October 1993. We registered the four images by using ground control points, extracted areas showing the reservoir, and made a mosaic of the images (Fig. 1). In this grey scale image, dark areas have low radar backscatter and bright areas have high backscatter. The white areas are flooded forests, black areas are rivers and open water, and grey areas indicate mostly upland forests. The Balbina dam is in the south-east corner. Linear features to the south-west are open fields and secondary forests.

Figure 2 is an enlarged view of the area near the dam, in which we can distinguish flooded

Skm

Fig. 2. JERS-1 SAR image of area near the Balbina dam.

forests (white), upland forests (light grey), riv­ers and open water (black), and aquatic macro­phytes (dark grey). This figure also contains hills in the south-east and south-west. These topographic features could affect our image analyses; white areas could be classified as flooded forests, and black areas as water. To minimize or eliminate these effects, we have masked out hilly areas.

We identified training areas in the SAR image, extracted the backscatter coefficient of each pixel in these areas, and used the backscat-

2376 Methods

Fig. 3. Decision tree classifier for the area near the Balbina dam, see text for explanation.

water macrophytes upland r~· flooded forests

Skm

Fig. 4. Classified JERS-1 SAR image for the area near the Balbina dam.

ter coefficients in the decision tree model. We created four decision tree classifiers, one for each scene. Figure 3 is an example of the classi­fier for image 414-304. Ellipses are interme­diate nodes and show decision rules; rectangles are terminal nodes showing results. Backscatter coefficients are ona decibel (dB) scale. Figure 4 is a classified image for the area near the dam, as shown in Fig. 2. Black triangles in the south­east and south-west corners were hilly areas masked out during the analyses.

Using the decision trees for the four images, we have classified the SAR image into open wa­ter and rivers, aquatic macrophytes, upland for­ests, and flooded forests. Areas of open water and rivers, aquatic macrophytes and flooded forests are shown in Table l. Adding these areas, we estimate the total area of the Balbina Reservoir as 2407 km2• Our calculation is dose to 2360 km2

, the official area of the reservoir at the 50 m water level, and provides the first in­dependent confirmation of the impoundment's

Table 1. Total areas (km2) of open water and rivers, aquatic macrophytes and flooded forests in the Bal­bina Reservoir, derived from JERS-1 SAR data.

open water and rivers macrophytes flooded forests

Total

699 144

1564 2407

true size. Furthermore, our analysis shows the large a~ea of emergent forest remaining in the reserv01r.

The repetitive, all weather coverage offered by satellite-borne SARs provides a valuable source of information on the extent of inunda­tion and the temporal changes of the aquatic habitats in existing or newly formed reservoirs. Such information should aid both ecological and hydrological studies of tropical reservoirs, especially those in forested ecosystems.

Acknowledgements

We thank l. WALKER, M. V. B. QuEIROZ and B. NEL­soN for ecological information on Balbina Resevoir, B. FoRSBERG for key references, A. FREEMAN for JERS-1 SAR data, and L. HEss, S. FILOSO and S. MAciNTYRE for comments on our manuscript. Fi­nancial support was provided by the NASA SIR­C/X-SAR program through JPL contract 958469 to J. M. MELACK.

References

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FEARNSIDE, P. M., 1989: Brazil's Balbina dam: Envi­ronment versus the legacy of the Pharaohs in Amazonia. - Environmental Management 13: 401-423.

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HEIDE, J. VAN DER, 1982: Lake Brokopondo- Filling phase limnology of a man-made lake in the hu­mid tropics. - Ph.D. Thesis. Free University of Amsterdam, The Netherlands.

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JuNK, W. J., RoBERTSON, B. A., DARWICH, A. J. & VJEIRA, I., 1981: Investigações limnológicas e ic­tiológicas em Curuá-Una, a primeira represa hidrelétrica na Amazônica Central. - Acta Amazônica 11: 689-716.

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Authors' addresses:

J. M. MELACK, Department of Ecology, Evolu­tion and Marine Biology and Institute for Com­putational Earth System Science, University of California, Santa Barbara, CA 93106 USA. YoNG WANG, Department of Geography, East Carolina University, Greenville, NC 27858 USA.