remote sensing Article Long-Term Change of the Secchi Disk Depth in Lake Maninjau, Indonesia Shown by Landsat TM and ETM+ Data Fajar Setiawan 1,2, Bunkei Matsushita 3,*, Rossi Hamzah 1, Dalin Jiang 1 and Takehiko Fukushima 4 1 Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japan;
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[email protected] (D.J.) 2 Research Center for Limnology, Indonesian Institute of Sciences (LIPI), Cibinong Science Center, Bogor 16911, Indonesia 3 Faculty of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japan 4 Kasumigaura Environmental Science Center, 1853 Okijuku-machi, Tsuchiura, Ibaraki 300-0023, Japan;
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[email protected]; Tel.: +81-29-853-7190 Received: 31 October 2019; Accepted: 1 December 2019; Published: 3 December 2019 Abstract: Most of the lakes in Indonesia are facing environmental problems such as eutrophication, sedimentation, and depletion of dissolved oxygen. The water quality data for supporting lake management in Indonesia are very limited due to financial constraints. To address this issue, satellite data are often used to retrieve water quality data. Here, we developed an empirical model for estimating the Secchi disk depth (SD) from Landsat TM/ETM+ data by using data collected from nine Indonesian lakes/reservoirs (SD values 0.5–18.6 m). We made two efforts to improve the robustness of the developed model. First, we carried out an image preprocessing series of steps (i.e., removing contaminated water pixels, filtering images, and mitigating atmospheric effects) before the Landsat data were used.