The invention discloses a method for automatically extracting a flood
risk area based on an SAR image.The method comprises the steps: calculating the
water body probability of two time phase SAR images before and after a flood event through employing a C
wave band backscattering coefficient sigma0, and expressing a flood
risk area through employing the increment of the
water body probability. The method is a rapid, convenient and nearly real-time flood monitoring method, can effectively extract the newly added submerging area (flood
risk area) before and after the rainfall event, and comprises the following steps: acquiring two time phase images before and after a flood event, and forming a sigma0 set; generating a frequency
histogram of sigma0, assuming that sigma0 accords with
Gaussian mixture distribution, and then filtering outliers; performing unsupervised classification on the downsampled sigma0 set, and calculating a
prior probability; carrying out iterative calculation on the downsampled sigma0 set by utilizing an expectation maximization (EM)
algorithm and the
prior probability, calculating a distribution curve parameter, and calculating a
water body probability based on sigma0 before downsampling; and extracting a flood risk area by using the water body probability increment of the two time phase images.