# Inundation Extent Mapping by Synthetic Aperture Radar: A Review
## 2 Techniques to Produce SAR Inundation Maps  
### Principles  
Change in surface roughness is the key to detecting inundation using SAR data. Where the ground surface is covered with water, its low roughness exhibits almost ideal reflective scattering, in strong contrast to the scattering of natural surfaces in dry conditions. Consequently, different scattering mechanisms can cause two opposing changes in the total backscattering intensity: dampening and enhancement. In the case of an open flood where water is not obscured by vegetation or buildings, dampening occurs because most of the scattering intensity is concentrated in the forward direction, whereas the back direction is very weak. In vegetated areas, different scattering mechanisms can happen, depending on the vegetation structure and submerge status. In fully submerged locations, scattering dampening occurs as in an open flood, while scattering enhancement is observed in partially or unsubmerged locations because the dihedral scattering dominates. The backscattering components of vegetation (with a canopy layer) are derived in the enhanced Michigan microwave canopy scattering (enhanced MIMICS) model by [PERSON] et al. [29] from the first-order approximation of the vectorized radiative transfer (VRT) equation [30; 31]. As shown in Figure 1, the vegetation-ground system can be conceptualized by three layers from top to bottom: the canopy layer (C), the trunk layer (T), and the rough ground surface (G). Term 1 (G-C-G) in the figure, for instance, refers to the wave scattered by the ground surface after it penetrates the canopy, which is then scattered by the canopy back to the ground and is finally scattered back to the sensor after penetrating the canopy. Term 7 (direct scattering) indicates that the scattering process on the ground surface only occurs once between the two penetrations of the canopy. A trunk is usually modeled by a cylindrical structure whose length is much greater than its diameter. Term 4a (T-G-C) in Figure 1 represents the scattering wave that consecutively penetrates the canopy layer first, is scattered forward by the trunk, is scattered back in the antenna's direction, and, finally, penetrates the trunk and canopy layer. Term 4b (C-G-T) represents the scattering wave in the reverse order; under physical optical approximation, its forward scattering (in all azimuth directions)dominates. As a result, in 4a and 4b, only the forward scattering from the trunk is retained. If G is rough, the ground scattering energy is distributed to the upper hemisphere, and the forward direction is not very strong. When flooded, the ground surface is close to ideally smooth, and the forward scattering is significantly enhanced, which, in turn, significantly enhances 4a and 4b. Another principle is to utilize interferometric SAR (InSAR) formed from repeated passes to detect non-obstructed flooded areas by identifying the loss of coherence over water surfaces [32; 33; 34; 35]. However, coherence may only be used as the primary indicator where the background exhibits strong coherence, such as in highly developed urban areas whereas can only be used as a complementary source because the surface of a water body is not the only kind of area that loses coherence--the background is full of low-coherence areas, such as vegetation, shadow [36]. Similarly, utilizing differential InSAR (DInSAR) pairs from repeated tracks to detect inundation depth may only be feasible over background areas of strong coherence, which is not the case of beneath-vegetated inundation or non-obstructed inundation in natural areas. Both non-obstructed flood and beneath-vegetation inundation can occur in natural areas; in urban areas, both non-obstructed-flood dampening effects and L-shaped-caused enhancement can be observed. A horizontal smooth inundated surface together with a vertical building can form an L-shaped corner reflector [37], which can be a strong scatterer when illuminated by the radar antenna.