Inundation and water level dynamics of the Mahakam cascade lakes from satellite radar and on-ground observations

Lakes in the middle Mahakam area are part of an ecologically important wetland in East Kalimantan. Information on lake inundation dynamics is required as baseline data for integrated assessment of flood dynamics, ecological processes, and vulnerability required in planning, designing, and operating land management policies and mitigation measures. Radar remote sensing is well-suitable for identification, mapping, and measurement of tropical wetlands, for its cloud unimpeded sensing and night and day operation. We aim to gain insight into inundation dynamics of the Mahakam cascade lakes (Jempang, Semayang, Melintang) from a series of radar images and on-ground lake depth and water level measurements. We collected a series of PALSAR (Phased Array L-band Synthetic Aperture Radar) images in the years 2007—2010. Water level measurements were carried out at the lakes using pressure transducer from April 2008 through August 2009. The amplitude of lake water level variation exceeds 3.5 m. Bathymetry measurements were carried out using a single beam echo-sounder. Variation of lake extent was obtained by evaluating radar backscatter values against a threshold for open water area. From statistics of radar backscatter over regions covering the permanently inundated river and lakes, we obtained a threshold value of -25.1 to -11.2 dB for open water flooding detection. The lake extent map obtained from PALSAR image series showed there is a large variation in inundation area of the Mahakam lakes. Such a large variation is attributed to their nature as floodplain lakes.