Simulating dryland cliffs evolution in response to extreme rainstorms

Cliff bands are common in drylands and their evolution is often influenced by hydrogeomorphic processes. It has been previously suggested that cliff retreat patterns and morphology are affected by the properties and frequency-magnitude relations of rainstorms. However, basic questions on this topic persist because landscape evolution models typically do not account for the surface processes like runoff generation and sediment transport that occur under short-duration (sub-hourly) intense rainfall. Here we test the hypothesis that changes in rainstorm properties can systematically alter cliff retreat patterns and morphology. We developed a novel numerical model that simulates the response of cliffs and associated sub-cliff slopes to various rainstorm regimes to (1) identify dominant cliff morphologies, and (2) examine if extreme rainstorm properties are encoded in the topography. The new model utilizes the Landlab modeling toolkit and includes an explicit novel representation of surface processes that occur during short-duration rainstorms, including cliff-weathering, infiltration, runoff generation, clast fragmentation, and size-dependent sediment transport. Using a suite of numerical experiments, we vary model parameters and rainfall types and simulate changes in cliff retreat patterns and morphology. Our model results agree well with analytical predictions for cliff morphology under a control case of no transport on the sub-cliff slope, indicating a good representation of processes. Furthermore, sensitivity analyses on cases where sediment transport is explicitly included show that cliff evolution is highly dependent on both the grain size of sediment derived from the cliff and the rainfall intensities. These two factors can alter retreat patterns and determine whether and how fast the cliff can be buried under its own sediment. Numerical experiments based on rainfall and field measurements from the central Negev desert (eastern Mediterranean) demonstrate that including the dynamics of high-intensity rainfall and sediment grain size can help explain observed topographic trends. In addition, for a given imposed storm depth, we find that the rainstorm intensities pattern strongly influences both the cliff retreat and its morphology. Short rainstorms with higher intensities are much more erosive than longer storms with lower intensities. This latter case frequently triggers cliff burying. Taken as a whole, our results demonstrate that cliff evolution and morphology are significantly affected by storm-scale sediment transport dynamics and thus highlight the importance of incorporating high-resolution rainfall forcing into landscape evolution models of dryland landforms.