Flood-drought cycle of Tonle Sap and Mekong Delta area observed by DMSP-SSM/I

A method for the linear estimation of water surface area within a land domain in the tropical region of Tonle Sap and the Mekong Delta is studied using special sensor microwave/imager (SSM/I) data. The coefficient of correlation between the water area estimated by SSM/I and that estimated by AVHRR falls within a range of 0.97-0.99, when 37 GHz horizontal polarization (H-pol) brightness temperature TB data is applied to the SSM/I data. This implies that observation by SSM/I has practical applications in flood monitoring in tropical regions. We have also verified a method for linear estimation of water area within a pixel, utilizing the difference of emissivity between SSM/I 37 GHz H-pol and vertical polarization (V-pol) data. This estimation shows good secondary correlation with the first method. Values of water area obtained in the dry season appear somewhat larger than those from the NOAA/AVHRR data. Applying the first methodology, a transition of water cover area over Tonle Sap and Mekong Delta was plotted from May 1997 to April 1999. The transition patterns in both watersheds synchronize well with the transition patterns of precipitation in the region. A phase shift of flood-drought cycles of Tonle Sap and Mekong watersheds can be observed. Flooding in the Mekong Delta watershed begins to subside one and a half months after that in the Tonle Sap watershed starts. Fluctuation of the water area of the Mekong Delta synchronizes with that of the water level in the Mekong Delta.