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

Hillslope topographic change in response to climate and climate change is a key aspect of landscape evolution. The impact of short-duration rainstorms on hillslope evolution in arid regions is persistently questioned but often not directly examined in landscape evolution studies, which are commonly based on mean climate proxies. This study focuses on hillslope surface processes responding to rainstorms in the driest regions of Earth. We present a numerical model for arid, rocky hillslopes with lithology of a softer rock layer capped by a cliff-forming resistant layer. By representing the combined action of bedrock and clast weathering, cliff-debris ravel, and runoff-driven erosion, the model can reproduce commonly observed cliff-profile morphology. Numerical experiments with a fixed base level were used to test hillslope response to cliff-debris grain size, rainstorm intensities, and alternation between rainstorm patterns. The persistence of vertical cliffs and the pattern of sediment sorting depend on rainstorm intensities and the size of cliff debris. Numerical experiments confirm that these two variables could have driven the landscape in the Negev Desert (Israel) toward an observed spatial contrast in topographic form over the past 10(5)-10(6) years. For a given total storm rain depth, short-duration higher-intensity rainstorms are more erosive, resulting in greater cliff retreat distances relative to longer, low-intensity storms. Temporal alternation between rainstorm regimes produces hillslope profiles similar to those previously attributed to Quaternary oscillations in the mean climate. We suggest that arid hillslopes may undergo considerable geomorphic transitions solely by alternating intra-storm patterns regardless of rainfall amounts.