Water volume variations in Lake Izabal (Guatemala) from in situ measurements and ENVISAT Radar Altimeter (RA-2) and Advanced Synthetic Aperture Radar (ASAR) data products

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Abstract
The water storage variations in lakes affect their physical, chemical and biological processes. Besides, the water masses of these waterbodies reflect the balance of the rainfall and evaporation with surface and ground waters. The lake’s water volume is estimated combining water level variations with accurate bathymetry and shore topography maps. Lake Izabal is the largest waterbody of Guatemala (approximately 673.29 km2). Its water volume has been estimated in the past but the volume variations are still unknown. The lake water level variations are monitored in situ since 2004, but regrettably accurate information about the bathymetry and shore topography is not available. The main objective of this study was to make a first estimate of the Lake Izabal water volume variations. To do this, we combined level variations and inundated area variations. The lack of accurate bathymetry and topography maps was overcame by using inundated area variations in the assumption that every level change reflects an inundated area response, depending on bathymetry and shore topography. The level variations were estimated from an in situ moored gauged in the lake and from the ENVISAT Radar Altimeter (RA-2). The inundated area variations were obtained using 12 ENVISAT Advanced Synthetic Aperture Radar (ASAR) images. Prior to the area estimates the lake’s shoreline was extracted making use of a chain of existing image processing algorithms. The correlation analysis between in situ lake levels and inundated area variations yielded a correlation coefficient of 0.9. The volume variations of the lake were then estimated on the dates of the acquired SAR images. Then, a group of rating curves relating level, area and volume were developed. In order to extend the area and volume estimates to the whole study time period (February 2003 to December 2006) the lake levels from the RA-2 dataset were entered to the rating curves. The estimated water volume variations of Lake Izabal range between 8271.2 × 106 m3 (17th December of 2005) and 9018.15 × 106 m3 (15th July of 2006) in agreement with the most recent estimation of the Lake Izabal water volume (8300 × 106 m3). Regarding the inundated area variations, they range between 672.44 × 106 m2 (17th December of 2005) and 677.2 × 106 m2 (15th July of 2006) in agreement with the Guatemalan government information (673.29 km2). The water volume, inundated area and water level fluctuations of the Lake Izabal show a strong seasonal signal with high stages during the rainy season (May–November) and low stages during the dry seasons (December–April). These results give valuable information for the Lake Izabal management and strengthen the idea of using remote sensing as a powerful and cheap complementary tool for hydrologic purposes.