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We used strong isolated double bounce reflections that appear along forest-water boundaries in JERS -1 SAR images of the Amazon River basin to represent small , narrow rivers that appear indistinct in the original images as bright lines in processed images reduced to 1/256 size. Since the brightness seemed to correlate with the width/water level at the point of the stream, we examined seasonal changes in the brightness of several catchment areas along the Madeira River between 1993 and 1997 and found that the total brightness of a tributary system correlated to the length of its main tributary, a hydrological parameter that can be used to calculate discharge. A mosaic of such images can be used as a quick look map of streams and flows in remote and often dangerous tropical forests. We then explored possibilities for estimating discharge from SAR data. 和 文概 要:ア マ ゾ ン河流 域 のJERS-1SAR画 像 の森 林 と水 との境 界 に沿 っ て現 れ る孤 立 した強 い2回 反射 を用 いて,原 画像 の1/256の サ イズの 画像 に原 画像 で はは っ き りしなか った小 河 川 を 明 る い線 と して 表 し た。 この画像 では広 い水 面は 暗 い領 域 として実 サ イズ で表現 され る。この 明 るさが川 の各 地点 の幅/水 位 に関 係 して い る よ うに見 え た の で,1993年 か ら1997年 の デ ー タ を用 いて,Madeira河 流域 の い くつ かの小 支 流 域 の川 の明 るさの季節 変化 を調 べ た ところ,そ の明 る さの総和 は,流 出量 を求 め るの に使 われ る水 文 学的 パ ラメー タの一 つ で あ る支 流域 の主 流の長 さに比例 して い るこ とが分 か っ た。 したが って,こ れ らの画像 の モ ザ イ クは,小 さなサ イ ズで広大 な領 域 をカバ ー で き る の で,流 路 の変化 しや す い人 里離 れ た熱 帯雨 林地 域 の 小 河川 とその水 量分布 の クイ ックル ッ ク画像 として実 用 的 な価 値 があ る と考 え る。 画像 か らの流 出量推 定 の 可能 性 につ いて も言及 した。 1 . Introduction Satellite-based regular monitoring of spatial dis tribution of streams and amount of flow over tropi cal forests provides important information to pre serve their fragile ecosystems because those forests tend to be extensive, remote, uninhabited, and often dangerous. L-band space borne Synthetic Aperture Radars (SAR) provide one of the best options for observing the water surfaces in such areas because they are not adversely affected by the frequent cloudcover (Ulaby et at., 1986) that characterize such areas. Although many studies have been conducted with regard to L-band SAR detection of large flooded areas in natural forests (Krohn et at., 1983 ; Obmsby and Blanchard, 1985; Inhoff et at., 1987 ; Hess et at., 1990 ; Hess et at., 1995 ; Rosenqvist and Birkett, 2002; Rosenqvist et at., 2002), studies have not been sufficient with regard to the gradual increases of water in small rivers that eventually cause the *RSデ ー タ ・ア ナ リ シ ス:RSDataAnalysis **芝 浦 工 業大 学:ShibauraInstituteofTechnology ***立 正 大 学:RisshoUniv .,****東 京 大 学:Univ.ofTokyo 「写 真 測 量 と リモ ー トセ ン シ ン グ 」VOL.44,NO.5,2005 floods. One of the reasons for this lack of study lies in the difficulty of detecting narrow rivers from SAR images. In a previous paper (Sakurai-Amano et al., 2004), we noted that the appearances of water surfaces change as their widths change. Briefly, wide rivers appear as long, dark objects, but as they become narrower, they turn into continuous dark lines, and then into a series of dark linear blobs. The blobs become lighter as the river becomes even narrower and they eventually dissolve into the background. Along dark objects, we observe intermittently a series of very bright targets. These targets represent double bounce reflections between water surfaces and tree trunks and/or large branches. Similar reflections are observed in inundated forests. The reflections can be used to distinguish dry areas from inundated areas because the intensities of such reflections are correlated with the dielectric constant of under-canopy ground (Richards et al., 1987a; Richards et al., 1987b). We combined both the darkness of the water surfaces and the average intensities of the strong reflections to determine the amount of water flowing in narrow rivers. This paper describes a method for emphasizing double-bounce reflections along forestwater boundaries to visually represent narrow rivers as bright lines in a muchreduced image. We showed that the brightness of those rivers is correlated with the amount of flow. In discussion, we correlated the total brightness of such rivers to their mainstream lengths, a hydrological parameter that can be used to calculate discharge. We also discussed the possibility of estimating discharge from those data briefly.

Visualization of Narrow Rivers within Forested Areas Using JERS-1 SAR Images of the Amazon River Basin