Landscape Transformations After an Extreme Rain Event in a Hyperarid Area: Water Influx and Geological Material Dependencies

The long-term effects of rain on desert landscapes are well established, and major trends are broadly understood, such as mass transfer of sediments and pedogenic transformations. However, on the short-term scale there are still many effects that are poorly documented, such as the timing and style of transformation of various geological materials, and how variations in the amount of rainwater determine the transformations. A rain event that occurred between January and February of 2019 in the northern Atacama Desert creates an excellent case study to assess using remote sensing the different evolutionary paths of several geological materials. In this work we applied InSAR using ISCE 2.5 to generate 78 decorrelation maps from radar images taken between December 2018 to November 2019, complemented by Landsat 8 OLI scenes. We focus on a suite of surface materials across a range of rain amounts: three salt flats (Llamara, Carcote and Ascotan), two gravel deposits (one covered by a thick gypsic soil and the other covered by a thin layer of eolian sand), and a mudflow. Our results indicate that there are three different tempos of changes: 1) sharp and permanent, 2) progressive, and 3) delayed. Abrupt and permanent change corresponding to January - February rain characterized the mudflow (20 - 60 mm rain), the gravel deposits (20 - 60 mm), and Carcote salt flat (>185 mm), although the magnitude of average decorrelation differed markedly among those materials. Progressive change occurred at the Ascotan salt flat (>185 mm), due to continued surface water influx for a period longer than the rain event. Delayed change was detected in a small area of Llamara salt flat (20 mm), where response starting five months after the rain was detected by InSAR but not by Landsat. We hypothesize that Llamara's InSAR decorrelation is due to change of surface roughness that affected the dielectric constant. Two months later, InSAR registers a return to normal. The onset of the InSAR response was synchronous with an increase of pond areas and so is the fading of the InSAR signal with the decrease of pond areas. We hypothesize that shallow groundwater was flowing laterally toward the Llamara salt flat during the half year between the rain event and the InSAR responses, due to piston-driven lateral flow in response to groundwater infiltration during the rain event.