Tonlé Sap, Cambodia

Tonlé Sap is visible in the center of this false color corrected reflectance (Bands M11-I2-I1) image captured on Dec. 23, 2025, during the dry season, by the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument aboard the joint NASA/NOAA NOAA-20 platform. Dark blue/black areas show areas covered in water.

Tonlé Sap is the largest freshwater lake in Southeast Asia, located in northwest Cambodia. Fed by rivers and rainfall, the size and volume of the lake varies throughout the year. In the dry season (November to March), the minimum area is about 2,500-3,000 square kilometers (970 - 1,160 square miles) with a volume of 1 cubic kilometer (0.24 mi³). In the rainy season (May - October), the lake swells and can reach a maximum area of 16,000 square kilometers (6,200 square miles) and the volume can reach about 80 cubic kilometers (19 mi³).

The map above shows the false color corrected reflectance image on the left side and the Flood (3-day Window) layer on the right side. Swipe the center bar back and forth to compare the dark blue/black areas that represent water in the left image with areas classified as surface water (in cyan), recurring flood (in yellow), and flood (in red) in the right image.

The Flood layer provides a daily global map of flooding and accumulates water detections for all observations (from the NOAA-20 and NOAA-21 platforms) over the 3 day compositing period, and if the total exceeds the required threshold, the pixel is marked as water. These pixels are then classified as surface water (displayed in cyan), recurring flood (yellow), or flood (red), based on separate reference water and recurring flood layers. The recurring flood class was introduced on December 10, 2025, and helps discriminate unusual flooding from recurring flooding, which is defined as flooding which has been observed in the same location in roughly 1/3 of the years in the reprocessed 22 year MODIS flood product archive (2003-2024).

This comparison shows how the flood layer differs before and after the recurring flood class was introduced on Dec. 10, 2025. On the left side is the flood layer from Dec. 9, 2025, which shows the main portion of the lake in cyan, with much of the surrounding area in red to indicate the flood class. The right side shows the flood layer from Dec. 23 2025, where the recurring flood class has been introduced. The updated flood layer demonstrates that most flooding is occurring in regularly flooded areas and that the flooded areas are not unusual. Learn more in the MODIS/VIIRS NRT Global Flood User Guide. 

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