Analysis of lengths, water areas and volumes of the Three Gorges Reservoir at different water levels using Landsat images and SRTM DEM data

The impoundment and operation of the Three Gorges Reservoir (TGR) are geophysical “controlled experiments”. However, detailed procedures, data source and uncertainties on TGR's basic parameters, such as lengths, water areas and volumes, were not available to the science community, and are provided in this study by using Landsat images, SRTM DEM data and in situ measurements. Reservoir lengths are between 650 km and 700 km at the water level of 175 m above sea level (a.s.l.). The natural (but near maximum) water area of the Yangtze River within the reservoir (at the full range of 699 km) based on Landsat images was 489 km2 in August, 2002, before the impoundment. At 175 m a.s.l., the total water area mapped by Landsat images (929 km2) is ∼10% less than those derived from SRTM DEM data (1006 km2) and reported by Three Gorges Corporation (TGC) (1040 km2). SRTM DEM data could classify 90–92% of these areas as water areas, while the total areas from Landsat, SRTM and TGC have over 96% agreements at water levels ≤170 m. The larger estimate of water area by DEM data from both TGC and SRTM at 175 m could be partially explained by the construction of levees. The total static volumes derived from SRTM DEM data above 70 m have a linear relationship (R2 = 0.99, Mean Absolute Difference = 0.43 km3) with those reported by the Information Center of Water Resource (ICWR), Ministry of Water Resource, China. The analysis also suggests that the ICWR-reported water volumes in the reservoir neglect the wedge storage in a flooding peak.