Estimating Flash Flood Discharge in Arid Environments Using InSAR Coherence: A Case Study of the Ze’elim Fan, Dead Sea

Flash flood disasters have increased by more than 50% in the first 20 years of the 21st century compared to the last 20 years of the 20th century. Monitoring and understanding flood events might lead to better mitigation of this natural hazard. Using SAR and SAR interferometry (InSAR) proved to be a useful tool for mapping flooded areas due to the lower backscatter or decorrelation of the SAR signal in an open-water environment. In Arid regiem, flash flood water is rapidly drained by evaporation or percolation, often before the satellite image is acquired. To overcome this challenge, we propose in this study to use the InSAR coherency loss, created by surface changes during a flash-flood, to map the runoff path and utilize it to quantify peak discharge (Q_max).

We focus on the Ze’elim alluvial fan along the western shore of the Dead Sea, Israel, an arid area affected by seasonal flash floods a few days a year. We use 34 interferograms of X-band (COSMO-SkyMed/TerraSAR-X) SAR data, covering 25 runoff events between 2017 and 2021, and upstream hydrological gauge data. To consider the natural decorrelation processes, we calculate a normalized coherence (ϒ_n) term, using the average coherence of the study area and the average coherence of a stable reference area, identified by differential LiDAR measurements.

We find a strong correlation between g_n and the logarithm of the peak discharge (Q_max). However, the method is limited by a minimal peak discharge—where energy is too low to change the surface—and maximal total water volume—where decorrelation is saturated. The method may provide tools for reconstructing runoff data in arid areas where historical SAR data is available, and for monitoring in difficult access areas or where hydrological stations are sparse or damaged.