The Impact of Extreme Rainstorms on Escarpment Morphology in Arid Areas: Insights From the Central Negev Desert

Abstract

The impact of climate on topography, which is a theme in landscape evolution studies, has been demonstrated, mostly, at mountain range scales and across climate zones. However, in drylands, spatiotemporal discontinuities of rainfall and the crucial role of extreme rainstorms raise questions and challenges in identifying climate properties that govern surface processes. Here, we combine methods to examine hyperarid escarpment sensitivity to storm‐scale forcing. Using a high‐resolution DEM and field measurements, we analyzed the topography of a 40‐km‐long escarpment in the Negev desert (Israel). We also used rainfall intensity data from a convection‐permitting numerical weather model for storm‐scale statistical analysis. We conduct hydrological simulations of synthetic rainstorms revealing the frequency of sediment mobilization along the sub‐cliff slopes. Results show that cliff gradients along the hyperarid escarpment increase systematically from the wetter (90 mm yr‐1) southwestern to the drier (45 mm yr‐1) northeastern sides. Also, sub‐cliff slopes at the southwestern study site are longer and associated with milder gradients and coarser sediments. Storm‐scale statistical analysis reveals a trend of increasing extreme (>50 years return‐period) intensities toward the northeast site, opposite to the trend in mean annual rainfall. Hydrological simulations based on these statistics indicate a higher frequency of sediment mobilization in the northeast, which can explain the pronounced topographic differences between the sites. The variations in landscape and rainstorms properties across a relatively short distance highlight the sensitivity of arid landforms to extreme events.

Impacts of Rainstorm Intensity and Temporal Pattern on Caprock Cliff Persistence and Hillslope Morphology in Drylands

“…For tuning the fragmentation module to arid environments rate, we calibrate ΔF against surface clast measurements from dated arid alluvial surfaces (Figure S2 in Supporting Information S1). Infiltration parameters for the debris and lower layers were defined based on infiltration measurements and soil texture in the central Negev (Shmilovitz et al, 2023) and we assumed that no water infiltrates into the caprock. Table 1 summarizes the model parameters used in all simulations.…”

“…The aim of this set of numerical experiments is to quantitatively investigate whether measured rainstorm properties and grain size of the cliff-derived debris could explain the observed topographic difference. In this experiments set, we conduct two simulations, each with RIF and CGS based on field measurements and local storm-scale rainfall statistics (Shmilovitz et al, 2023). All other parameters and initial conditions remained unchanged.…”

“…As described above, we run the model with an event-based approach, in which short-duration rainstorms are simulated in a series, one by one. We define a reference storm, termed hereafter the "base-storm" (Figure 3), which is a 30-min-long storm that corresponds to a 100-year return period in the central Negev (Shmilovitz et al, 2023). The storm consists of three 10-min intensity bins and the maximal intensity for 10, 20, and 30 min represents the 100-year intensity (for those durations) as computed by an intensity-duration-frequency (IDF) curve (Figure S1 in Supporting Information S1).…”

“…The climate in the area is hyperarid (mean annual rainfall is 40-90 mm) along the escarpment and precipitation is characterized by discrete showers with extremely high inter-seasonal and annual variability. Between the southwest and northeast ends of Ramon escarpment northern rim (SWS and NES, respectively), there are pronounced differences in the morphology of the cliffs and slopes (Shmilovitz et al, 2023); the sub-cliff slopes at the SWS are longer and associated with milder gradients relative to shorter and dissected sub-cliff slopes of the NES with more debris-free sub-vertical bedrock patches. These differences were attributed to differences in the frequency of sediment mobilization driven by spatial variability in extreme rainstorm properties and grain size of cliff-derived sediment (Shmilovitz et al, 2023).…”

“…Between the southwest and northeast ends of Ramon escarpment northern rim (SWS and NES, respectively), there are pronounced differences in the morphology of the cliffs and slopes (Shmilovitz et al, 2023); the sub-cliff slopes at the SWS are longer and associated with milder gradients relative to shorter and dissected sub-cliff slopes of the NES with more debris-free sub-vertical bedrock patches. These differences were attributed to differences in the frequency of sediment mobilization driven by spatial variability in extreme rainstorm properties and grain size of cliff-derived sediment (Shmilovitz et al, 2023). The aim of this set of numerical experiments is to quantitatively investigate whether measured rainstorm properties and grain size of the cliff-derived debris could explain the observed topographic difference.…”