## Materials and Methods
### Ground and Geological Data  
Field observations, morphological analysis, and geological data over the Salar de Aguas Calientes Sur were conducted on 3 April 2018. Salt crust and brine samples were collected jointly with Centro de Informacion de Recursos Naturales and Servicio Nacional de Geologia y Mineria [4] for geochemistry characterization. The geochemistry of water and salt crusts was not extensively measured but is generally known from other contemporary measurements [20]. Morphological and mineralogical features were extensive over the surface of the salar. Field observations showed highly heterogeneous pan crust environments that could be grouped into three different crust types (Figures 3 and 4). Figure 3: Distribution of the salt pan crusts modified from a technical report together with Centro de Informacion de Recursos Naturales [4] and the corresponding author. The map is an original contribution of this paper. Red boxes indicate regions of interest. The first is a hard crust, Hard-pan crust 1, Figure 4a formed primarily by gypsum, halite and detrital particles. It is characterized by an irregular concave shape, uplifted rims, and salt enrichments crystalized as granular forms and thin sheets covering the gypsum pan over the borders indicating that most of the time remains not flooded [21]. These salts with granular form, indicate that they were formed from evaporation and rise of brines by capillarity [8]. Blowouts wind erosion on this crust is reflected by the development of wind deposits on the margins of the salars, also, deflation surfaces are characterized by low roughness and undulations similar to sabkhas formation. In an advanced stage of corrosion, by windblown surface in the presence of water, the remnants of the former crust being characterized by rounded cuspate forms described as pseudobarchans [2]. The second crust type is referred to as Hard-pan crust 2, and it is distributed along the east edge of the salt flat, formed by gypsum and halite containing cavities that indicate dissolution of salts due  
Figure 4: Types of salt crust observed in the field on 3 April 2018. (**a**) Upper panel: Hard-pan crust 1, mixture of salts and sediments (rough surface), (**b**) Center panel: Hard-pan crust 2 (gypsum and halite), (**c**) Bottom panel: Soft pan crust with contents of organic matter and thrust polygons by interaction with water. to infiltration and percolation of water causing loss of stiffness. The surface is somewhat soft, as can be noted from the footprint tracks leftover as seen in Figure 4b. In the west, a salt crust with soft consistency is distributed Figure 4c. Almost flat with mud-crack polygons that contain a mixture of humid salts, primary halite, and less gypsum. The polygons indicate water loss after flooding events [22]. Field observations indicated that because of the soft and flat crust remained flooded, its elevation is lesser than the other salt pan crusts. Towards the north and south of the salt flat, perennial ponds have been observed. The permanent inflow prevents the dry out entirely by evaporation. On the other hand, the water dissolves solutes, old evaporites, and clastic sediments. Salar pools appear to concentrate following structural patterns within the salar with northeast and north-south orientation and have been carved out as blowouts by wind erosion, and later filled with water as either shallow saline, thermal-spring or groundwater seeped upward from below. The deposition of halite and gypsum crystals through evaporation on the surface of the salt flat, together with the movement of surface water and the effect of sandstorm activity, plays an essential role in the temporal variability of the backscattered power on the surface of the salar. For the selection of the regions of interest (ROIs) and backscattering coefficients (\(\sigma^{0}\)) retrieval, thirteen regions of interest were chosen (Table 2), being distributed over three types of salt crusts considered representative over the surface of the salar (Figure 3).